Great trick about the OAG simulator on Stellarium. I didn't know it, and just received my ZWO OAG. Last night was trying to understand why I couldn't find any star in the ASI 120Mini guiding. I will retry tonight with your helpful video
Thank you, James, for all your work in providing this information. I also watched your video of the ZWO OAG. I have been racking my brain looking at OAG's, and cameras for both my Celestron 9.25 and my Astro-Tech 111mm EDT refractor. You have demonstrated exactly what I need for each telescope and provided more setup details that any sight I have researched. Thanks again, Sincerely, Jim.
Thanks for all the in depth information! Love it! Most highly intelligent astrophotographers are not big on making videos. Your videos are rare in this instant gratification society we live in.
Great video and info. Thanks James. I just picked up a used Edge 8” along with the Celestron OAG and 174mm. Your videos have been a big help and saved a great deal of time. I’ve said it before but I love your approach to these implementations.
Thanks! Now that nebula season is ending, I'm about to dive head-first into the troubled waters of high-focal-length-OAG galaxy imaging. I'm sure there will be lots of additional lessons. Thanks for watching!
Nice explanation. I'm dealing with some bad mirror flop on my SCT and decided that is it time to move toward an OAG setup. This video clearly explains what I need! FYI, mirror flop is probably the #1 reason to upgrade to OAG from guidescope particular when near meridian. I've had good tracking at 0.5"RMS with star trails after 3min exposures near meridian due to that effect.
Excellent video, describing the principles for calculating the optical distance to the focal plane for the off axis guider OAG (15:10). I just want to point out one mistake (in mho) made by the author. The distance to the main camera should be measured from the flat (most distant) edge of the prism to the sensor surface of the main camera. That is the distance that should be equal to the distance between the center of the Prism to the sensor surface of the guiding camera. This comes from the fact that the light that impacts the flat face will come out exactly from the 45 degrees face.
Hi Julio. I disagree with your measurement comment. Here's why: Let's say the light hits the top of the flat portion of the prism. The light passes through and hits the angled surface 12.5 mm farther back and bounces up to the guide sensor. In this case, the distance from the back of the prism (closest to the imaging sensor) should equal the distance from the top of the prism to the guide sensor. Alternatively, let's consider the light that hits the bottom of the vertical surface of the prism. Here, the light immediately hits the angled surface and reflects up to the guide sensor (including 12.5 mm inside the prism). Now, the distance from the bottom of the prism to the guide sensor must equal the distance from vertical surface of the prism to the imaging sensor. This path length is 12.5 mm longer for both the guide and the imaging sensors than the case above, so the lengths are still equal. Finally, consider the case I show in the video. The light passes through the front vertical surface of the prism, travels 6.25 mm where it hits the angled surface and goes up to the guide sensor (including a vertical 6.25 mm inside the prism). The distance from the center of the angled surface up to the guide sensor must equal the distance from that same point (i.e., at the center of the prism) to the imaging sensor.
Very good James, I've got an oag in my wishlist for use with my C11 to see if I can improve guiding. Had a recent issues with images not stacking and suspect poor guiding is causing this with long exposures over around 3 minutes length.
I hope it’s useful for you, Dave. I’m curious why images won’t stack. Are the star shapes badly elongated? If so guiding may be the issue. A 3-min exposure is not that “long” but my C9.25 is temperamental so I can only imagine how a C11 will perform. What mount are you using and what does PHD report as the RMS error in each axis?
@@Aero19612 I am using a CGX and PHD was reporting around 0.5 and 0.7px if I remember correctly, the individual subs didn't look too bad, I had previously stacked worse images using an 8" Newtonian and DSLR. You can see the subs on my channel the 2 dumbbell videos. I tried with and without a UHC filter. I had tweaked DSS settings as well to try and detect more stars and just had no joy. Maybe combination of slightly out of focus and slight elongation was enough to cause it to fail. I had seen a few other posts where mismatch is focal length or F# will cause guiding issues. I was using a 90/500 guide scope and even tried a 2x Barlow on to bring back the ratios, but no joy.
this was great, thank you for the in depth info. Im cramming a ton of info before I buy my first setup and OAG vs Scope was a question I had. Srsly helpful. Subscribed
Something I did to ensure the guide camera is always set to correct distance when inserted in the OAG. I made a spacer ring the correct length , that fits around the guide camera barrel. The spacer is the correct length to make sure the guide camera is always at correct distance. I 3D printed a cylinder but you could also use a wood dowel rod or piece of PVC pipe cut to correct length.
I forgot to mention, nice video on the setup of the OAG! I am using the celestron OAG on my celestron 9.25 SCT scope, Been very happy with its operation. The large pick off mirror I think is a big plus over some of the other units. Also a nice assortment of adapters to use the OAG with other accessories. Another accessory I 3D printed for the OAG. a 2” filter holder that fits inside the barrel of the male m42 t-thread adapter . Allows me to use any standard 2” mounted filter without adding any back focus distance to my optical path. Take care
I can't tell you how helpful your videos have been for me. I have a C9.25 with a woefully overmatched AVX mount. I have athe same focuser and OAG. I have been attempting unguided imaging but have grown weary of throwing out >50% of my images (especially since it was 14°F last night). I have finally reached focus with my ASI120MC-S only to find that your recommendation for a higher sensitivity guide camera is spot on. I bit the bullet and ordered the ASI174MM mini (you should have a sponsor affiliate link). As I have said before, the methodically researched and presented material of your videos makes a retired aerospace engineer feel like there is a "Taylor made" resource that combines all the attributes which work best for me. Thanks again. Bob
Thanks, Bob! Aerospace engineers rule. You have a challenging setup. I find my CGEM to be challenging enough as it is. When I started out, I used a Celestron NexImage 5 (kind of like the ASI120MC) as a guide camera paired with my guide scope. That color camera was not nearly sensitive enough and I had to adjust the guide scope rings to search for a guide star. What a mess! When I got the ASI120MM, that all went away. But when I switched to the OAG, I knew I wanted to maximize sensor size and sensitivity. The 174 is expensive, but its large pixels and large sensor size are exactly what I was looking for. Glad you took the jump. That'll remove at least some of the frustration.
Hi James.. Excellent video! Thanks for sharing it! Regarding the OAG setup in Stellarium, could you please explain how you got the first value: Prism/CCD distance (mm)?? Thanks and stay safe and healthy!
Hey Diego, The prism/CCD distance is the radial distance from the centerline of the telescope (also the center of your imaging sensor) to the bottom of the prism. Unfortunately, this is not a direct measurement. Here's what I did: Using calipers, I measured the diameter of the inner opening in the OAG body (call this "D"). Then I measured the from the bottom of the prism to the inner edge of the OAG body opening (directly below the prism, call this "d"). The number you want = d - D/2. Good luck! I hope this provides some clarification. And, indeed, stay healthy!
Aloha James, Great video, I have watched it several times. I finally received my Celestron OAG and ASI290 guide camera, to use on a C8 Edge at F/7. Had a quick super easy question! Have you notice any problems with your OAG helical focuser? The reason I ask, is because mine doesn't seem to work very good (I guess you could say it's slipping). If I adjust the focuser so it's in the middle (of that 8mm travel range), insert the small guide camera about half way, tighten the screws, and then try adjusting focus by rotating the helical focuser, the camera doesn't move up or down. I can see the helical focuser moving up and down, but the camera doesn't want to move with it. It only seems to work when the camera is inserted a small amount (about 4 to 8 mm). Thanks again for taking the time to do all your great videos!!
Wow, John, I have not seen or heard about that problem. I'd definitely contact Celestron. Maybe they'll have a good comment. I believe mine works fine. I hardly ever touch it since I got it focused. Next time I have to play with it, I'll pay more attention and let you know if I notice something. Thanks for checking in, John, and good luck with the OAG!
@@Aero19612 Aloha James, I got everything out last night, and was able to achieve focus and guiding. It could be that helical focuser is a little sensitive to over tightening the thumbscrews. It's not that hard to just manually slide the camera up or down. Later on, maybe I can find a separate low profile helical focuser to thread on there I got another problem in that I noticed a little delamination on the prism and I think that was creating some funny reflection patterns on the sub frames (but that might be because I had lowered the prism a little too much [had a small shadow]). I raised the prism back up and hopefully that weird reflection will go away!! Thanks again and will be watching your other digital angle and Stellarium videos again, they have really helped me out a lot! John
Trying! Started two targets (NGC 3359 and NGC 5907) back in March. I've had so few days of good weather that I still haven't finished. Last night was so windy, I ended up throwing out all of the data. Maybe I'll get two more days this week and finish. Thanks for watching, Matteo, and stay tuned!
Well, it's tough to say. You certainly need to balance the scope. In a more recent video, I show this scope with the OAG and the counter weights are about half way up the counter weight bar instead of at the bottom. I'd estimate that I can increase the current payload weight by 30% and still be able to balance it. Thanks for watching, David!
Sir, you do such a great job at explaining things. I’m lost when it comes to RMS value. I get the lower the better. But when going from say a 240mm guide scope with a zwo120mini to an OAG. should the RMS decrease substantially. Is the same rms valve for both mean something different. If your bored this old man could use a lesson. Also rms arc seconds vs like .6” the ASIAIR shows. I’m just confused
Hi Anthony, You probably won't see a big change in the guiding RMS value. There are other factors. The mount has a limit for what it can achieve, and the atmosphere has a limit on any given night for what it will give you. You may simply be running into these limits. A more expensive mount can give you some benefit (mostly by reducing stretched stars). I don't have an ASIAIR, but I assume it's showing you the guide error in arc-sec RMS. 0.6" RMS is pretty good (I get about 0.53" RMS 80% of the time with my EQ6-R). Hope this helped a little!
@@Aero19612 yes. Thank you. I guess i confused with the images I see when others are using phd2 non Asiair. The numbers are .89 ( .6”). What is the first number. :).
Amazing content. Does space after a reducer matter or just distance from prism. I just attached my zwo to SW 120 and I have existing spacers after stock reducer. I am hoping the stock spacers are okay here and I don't need to subtract the width of the prism oag? Also I copied your idea on helical focuserr!
Hey Marty. Reducer or OAG? A field flattener requires a specific backfocus length (often 55 mm). A flattener is often combined with a focal reducer. If it's just a reducer, then I don't think backfocus spacing matters; however, the total length from the lens to the camera sensor probably does (check with manufacturer). For the OAG, the specific spacing behind the OAG is not critical; however, the distance from the center of the prism to the imaging sensor must be the same as the distance from the center axis of the OTA "up" to the guide sensor. So you may have to play with the spacing to get those distances to be equal. Good luck!
Thanks James! Yeah, I did some searching after temporary fitting the OAG and I do need to worry about back focus - for the Esprit 120 it requires 75mm so I'll redo it close and then take some test images. Thanks for your reply! I think I have the OAG rough set up - cross fingers.
That's great news! It's getting to be that time of year to switch back to the SCT--just a few more projects to finish with the 250mm and 700mm refractors then it's on to galaxy season. The OAG with the wider-field refractors is a no-brainer. If there's also no issue with the SCT, then I'm a happy camper. Thanks for watching!
Thank you for the informative videos. I have a Celestron C8 SCT and I would like to use the Celestron OAG. Would you still recommend the ZWO ASI174MM as a guide camera? I have been researching this myself and was also drawn to it due to the large sensor size. Does it get fully illuminated from the prism? I wonder if the ZWO ASI290MM is adequate for the task as it is perhaps only 2/3rds the cost of the ASI174MM.
Hi David. Thanks for watching. I have the ASI290 (I use it with the smaller ZWO OAG prism on my ED102) and the ASI174. The much larger area is why I bought the 174. The full sensor should be illuminated by the Celestron OAG prism if the camera's sensor long dimension is oriented normal to the axis of the telescope. I installed mine with the long dimension of the sensor parallel to the telescope axis trying to maximize the "donut" of guide stars, but the inner mirror support tube blocks the outer 25%-30% of the sensor in that orientation (see my "First Use of the Celestron OAG" video). It's more expensive, but I like having the larger sensor.
Great video. I've been using a Celestron OAG with my C8 + 6.3 reducer + DSLR. The optimum back focus for the reducer is 105 mm but using the parts included with the OAG I can only get to 130. Are you using a reducer in your setup?
Hey Greg. I never used my DSLR with my OAG. I can get to about 105 mm behind the focal reducer with my ASI1600+Filter Wheel. Let's see: you have 44 mm for your DSLR + 11 mm for the T-ring (leaves you with a 42 mm inner thread. That leaves us with 50 mm to get to the Focal Reducer. You'll have to find a configuration that leaves out the SCT Adapter (33 mm). So let's put on the part with the external M42 thread (about 12.25 mm, goes into the T-ring), then the OAG body (30 mm), and then the part with the internal M48 thread (about 4.25 mm). So now we have a gap of 3.5 mm. Using this approach, you need an adapter with an internal SCT thread and an external M48 thread that's about 3.5 mm thick. Check out: agenaastro.com/blue-fireball-sct-female-m48-male-thread-adapter-c-04.html It's about 7 mm thick (optical), but you might be able to focus (still about 3.5 mm too long). An alternate approach: replace the OAG part with the external M42 thread with the part that has the internal M42 thread. Then you'll need M42/M42 gender changer like this (I got this part with my ZWO filter wheel): www.highpointscientific.com/zwo-t2-to-t2-adapter?ZWO-T2-T2&gclid=EAIaIQobChMI0YHzy6al6QIV0cDACh3xJw1PEAQYAyABEgIUi_D_BwE It's about 2 mm thick. That should get you to about 102.5 mm -- too short by 2.5 mm. Check my numbers! Good Luck!
I have nextstar 8se and avx mount and I am using single uv/ir cut filter? I was able to achieve 110 mm back focus! Is that enough or it need to be 105 exactly! Thanks in advance.
It may be close enough. Give it a try and see if you can focus. If the test images look OK, then you're good to go. If not, you'll have to play with different spacers to get the correct back focus. You find these at various astrophotography websites. Good luck!
Question. In your "First Use" video you find that the outer edge of your guide camera is not illuminated because of upstream the upstream diameter restriction. Would it make sense to change the guide camera orientation to short axis in the radial direction? Thanks again. Bob
Yep. I meant to tell you that in my reply. I recorded that video before I tested it in use. I had hoped to maximize the area of the annular field of view of the guide camera, but, alas, I found the inner tube clipped outer region of the sensor. So, yes, it is better to rotate the guide camera 90 deg (short side of sensor parallel to telescope axis) so that you can make more effective use of the sensor area. I have not rotated mine because (a) I'm lazy and don't want to refocus and (b) the pre-planning procedure is working well. I'll rotate at some point.
Hi James. Thanks for the reply. I wasn't sure if rotating the guide camera would push the field of view out and it would be a toss up. I am just fixing some drawings so that I can take measurements to use in Stellarium for M81 and M101. I still don't have my ASI174 yet, so I am doubtful that I will see any guide stars with my ASI120MC. I hope to give it a try on Thursday. Clear skies and thanks for all your help. Bob
Hey, Dennis. Not sure I follow. There's not setting for "guide stars" per se. For me, a guide star is one (or more) star I can see through the OAG. I use the Ocular option in the upper right of the Stellarium window and turn on the OAG in the settings area (I show that in the video). I have since downloaded additional star catalogs for Stellarium. You can do that by going to the Configuration/Extras window. Down at the bottom, it shows you the additional star catalogs you install. Try me again, if that didn't answer your question.
@@Aero19612 Sorry, I asked that while half asleep. I guess how do you determine in the stars that show up on Stellarium which ones will show up in the guider? I've found that my Stellarium tends to show more stars than my guide camera picks up. I think there's a magnitude filter--but it'll vary depending on f/# and guidecam sensitive I think. Just food for thought, I suppose. No worries!
@@DennisCarmody Yes, I have downloaded all of the star catalogs so I have very faint stars available in Stellarium. I find that I can see and guide on stars of magnitude 11ish or brighter with my ASI174. You might do better with a more sensitive camera or worse with a less sensitive camera.
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Great trick about the OAG simulator on Stellarium. I didn't know it, and just received my ZWO OAG. Last night was trying to understand why I couldn't find any star in the ASI 120Mini guiding.
I will retry tonight with your helpful video
Good luck, Kyredor! I'd be lost without the Stellarium OAG FoV. A must for planning in my opinion. Thanks for watching!
Thank you, James, for all your work in providing this information. I also watched your video of the ZWO OAG. I have been racking my brain looking at OAG's, and cameras for both my Celestron 9.25 and my Astro-Tech 111mm EDT refractor. You have demonstrated exactly what I need for each telescope and provided more setup details that any sight I have researched. Thanks again, Sincerely, Jim.
Great, Jim! I'm glad there is something useful in there. Thanks for watching.
Thanks for all the in depth information! Love it! Most highly intelligent astrophotographers are not big on making videos. Your videos are rare in this instant gratification society we live in.
Thanks Anthony! It's also why I have 6 subscribers.
@@Aero19612 My grandfather used to call TV the boob tube for a reason. No difference here.
Great video and info. Thanks James.
I just picked up a used Edge 8” along with the Celestron OAG and 174mm. Your videos have been a big help and saved a great deal of time. I’ve said it before but I love your approach to these implementations.
Congratulations, Joe! The 174 is a great match for the Celestron OAG. Let me know how things go. Clear Skies!
This one of the best presentations of how to use OAG I have seen. Thank You Jim :-)
Thanks! Took me a while to finally jump in, but I'm sold now. Thanks for watching!
Thanks. Have the same OAG coming to hook up to my 11" sct. Your attention to details has helped me even though I don't have all the parts yet.
Great! Thanks for watching and good luck with the OAG.
This is a really fantastic explanation of the use an OAG and some of the challenges of using that system.
Thanks! Now that nebula season is ending, I'm about to dive head-first into the troubled waters of high-focal-length-OAG galaxy imaging. I'm sure there will be lots of additional lessons. Thanks for watching!
Well, it has been over a year, but I finally got my stellarium and scope setup like this. Thank you for all of the help in your video.
Ha! Don't feel bad. It took me 5 years to get this setup like this. Hope it works out for you. Thanks for watching, Pat!
Excellent tutorial James!
Thanks! Hope it helps.
Great tutorial - best I have found on OAG
Thanks! and thanks for watching!
Good information in a well done video. I now have the ASI174 mini and Celestron off-axis guider on order. Taking my hobby to the next level. Thanks.
Great, Ron! They are a great pair. Clear skies!
That was an excellent explanation of a complicated topic. Thank you.
Thanks, Anthony! I'm glad it was useful for you.
Thanks James, great video. Thanks for the Stellarium tips at the end...I learned a few things there!
Nice explanation. I'm dealing with some bad mirror flop on my SCT and decided that is it time to move toward an OAG setup. This video clearly explains what I need! FYI, mirror flop is probably the #1 reason to upgrade to OAG from guidescope particular when near meridian. I've had good tracking at 0.5"RMS with star trails after 3min exposures near meridian due to that effect.
Excellent video, describing the principles for calculating the optical distance to the focal plane for the off axis guider OAG (15:10). I just want to point out one mistake (in mho) made by the author. The distance to the main camera should be measured from the flat (most distant) edge of the prism to the sensor surface of the main camera. That is the distance that should be equal to the distance between the center of the Prism to the sensor surface of the guiding camera. This comes from the fact that the light that impacts the flat face will come out exactly from the 45 degrees face.
Hi Julio. I disagree with your measurement comment. Here's why:
Let's say the light hits the top of the flat portion of the prism. The light passes through and hits the angled surface 12.5 mm farther back and bounces up to the guide sensor. In this case, the distance from the back of the prism (closest to the imaging sensor) should equal the distance from the top of the prism to the guide sensor.
Alternatively, let's consider the light that hits the bottom of the vertical surface of the prism. Here, the light immediately hits the angled surface and reflects up to the guide sensor (including 12.5 mm inside the prism). Now, the distance from the bottom of the prism to the guide sensor must equal the distance from vertical surface of the prism to the imaging sensor. This path length is 12.5 mm longer for both the guide and the imaging sensors than the case above, so the lengths are still equal.
Finally, consider the case I show in the video. The light passes through the front vertical surface of the prism, travels 6.25 mm where it hits the angled surface and goes up to the guide sensor (including a vertical 6.25 mm inside the prism). The distance from the center of the angled surface up to the guide sensor must equal the distance from that same point (i.e., at the center of the prism) to the imaging sensor.
Thanks for the review, I was searching for OAG That covers the ASI 183, this video helped a lot.
Great! Thanks for watching!
Very good James, I've got an oag in my wishlist for use with my C11 to see if I can improve guiding. Had a recent issues with images not stacking and suspect poor guiding is causing this with long exposures over around 3 minutes length.
I hope it’s useful for you, Dave. I’m curious why images won’t stack. Are the star shapes badly elongated? If so guiding may be the issue. A 3-min exposure is not that “long” but my C9.25 is temperamental so I can only imagine how a C11 will perform. What mount are you using and what does PHD report as the RMS error in each axis?
@@Aero19612 I am using a CGX and PHD was reporting around 0.5 and 0.7px if I remember correctly, the individual subs didn't look too bad, I had previously stacked worse images using an 8" Newtonian and DSLR. You can see the subs on my channel the 2 dumbbell videos. I tried with and without a UHC filter. I had tweaked DSS settings as well to try and detect more stars and just had no joy. Maybe combination of slightly out of focus and slight elongation was enough to cause it to fail. I had seen a few other posts where mismatch is focal length or F# will cause guiding issues. I was using a 90/500 guide scope and even tried a 2x Barlow on to bring back the ratios, but no joy.
this was great, thank you for the in depth info. Im cramming a ton of info before I buy my first setup and OAG vs Scope was a question I had. Srsly helpful. Subscribed
You're a smart man, Joe! Do some research and then jump into the hobby. Frustrating, expensive, challenging and very rewarding. Welcome aboard!
Something I did to ensure the guide camera is always set to correct distance when inserted in the OAG.
I made a spacer ring the correct length , that fits around the guide camera barrel.
The spacer is the correct length to make sure the guide camera is always at correct distance.
I 3D printed a cylinder but you could also use a wood dowel rod or piece of PVC pipe cut to correct length.
Very good idea/solution, Steve! 3D printing is a perfect match with the astrophotography hobby. Thanks for watching!
I forgot to mention, nice video on the setup of the OAG!
I am using the celestron OAG on my celestron 9.25 SCT scope,
Been very happy with its operation. The large pick off mirror I think is a big plus over some of the other units.
Also a nice assortment of adapters to use the OAG with other accessories.
Another accessory I 3D printed for the OAG.
a 2” filter holder that fits inside the barrel of the male m42 t-thread adapter .
Allows me to use any standard 2” mounted filter without adding any back focus distance to my optical path.
Take care
I can't tell you how helpful your videos have been for me. I have a C9.25 with a woefully overmatched AVX mount. I have athe same focuser and OAG. I have been attempting unguided imaging but have grown weary of throwing out >50% of my images (especially since it was 14°F last night). I have finally reached focus with my ASI120MC-S only to find that your recommendation for a higher sensitivity guide camera is spot on. I bit the bullet and ordered the ASI174MM mini (you should have a sponsor affiliate link). As I have said before, the methodically researched and presented material of your videos makes a retired aerospace engineer feel like there is a "Taylor made" resource that combines all the attributes which work best for me. Thanks again. Bob
Thanks, Bob! Aerospace engineers rule. You have a challenging setup. I find my CGEM to be challenging enough as it is. When I started out, I used a Celestron NexImage 5 (kind of like the ASI120MC) as a guide camera paired with my guide scope. That color camera was not nearly sensitive enough and I had to adjust the guide scope rings to search for a guide star. What a mess! When I got the ASI120MM, that all went away. But when I switched to the OAG, I knew I wanted to maximize sensor size and sensitivity. The 174 is expensive, but its large pixels and large sensor size are exactly what I was looking for. Glad you took the jump. That'll remove at least some of the frustration.
Hi James.. Excellent video! Thanks for sharing it! Regarding the OAG setup in Stellarium, could you please explain how you got the first value: Prism/CCD distance (mm)?? Thanks and stay safe and healthy!
Hey Diego,
The prism/CCD distance is the radial distance from the centerline of the telescope (also the center of your imaging sensor) to the bottom of the prism. Unfortunately, this is not a direct measurement. Here's what I did: Using calipers, I measured the diameter of the inner opening in the OAG body (call this "D"). Then I measured the from the bottom of the prism to the inner edge of the OAG body opening (directly below the prism, call this "d"). The number you want = d - D/2. Good luck! I hope this provides some clarification. And, indeed, stay healthy!
@@Aero19612 Thanks James! It's clear now!
very informative and clear explanation... subscribed!
Thanks for watching, Michael! And thanks for subscribing!
Aloha James, Great video, I have watched it several times. I finally received my Celestron OAG and ASI290 guide camera, to use on a C8 Edge at F/7. Had a quick super easy question! Have you notice any problems with your OAG helical focuser? The reason I ask, is because mine doesn't seem to work very good (I guess you could say it's slipping). If I adjust the focuser so it's in the middle (of that 8mm travel range), insert the small guide camera about half way, tighten the screws, and then try adjusting focus by rotating the helical focuser, the camera doesn't move up or down. I can see the helical focuser moving up and down, but the camera doesn't want to move with it. It only seems to work when the camera is inserted a small amount (about 4 to 8 mm). Thanks again for taking the time to do all your great videos!!
Wow, John, I have not seen or heard about that problem. I'd definitely contact Celestron. Maybe they'll have a good comment. I believe mine works fine. I hardly ever touch it since I got it focused. Next time I have to play with it, I'll pay more attention and let you know if I notice something. Thanks for checking in, John, and good luck with the OAG!
@@Aero19612 Aloha James, I got everything out last night, and was able to achieve focus and guiding. It could be that helical focuser is a little sensitive to over tightening the thumbscrews. It's not that hard to just manually slide the camera up or down. Later on, maybe I can find a separate low profile helical focuser to thread on there I got another problem in that I noticed a little delamination on the prism and I think that was creating some funny reflection patterns on the sub frames (but that might be because I had lowered the prism a little too much [had a small shadow]). I raised the prism back up and hopefully that weird reflection will go away!! Thanks again and will be watching your other digital angle and Stellarium videos again, they have really helped me out a lot! John
Great information and thanks for the explanation!
Glad you found it useful, Michael!
Great video James. Really really helpful! Did u post some results of the imaging with this set-up? Thanks
Trying! Started two targets (NGC 3359 and NGC 5907) back in March. I've had so few days of good weather that I still haven't finished. Last night was so windy, I ended up throwing out all of the data. Maybe I'll get two more days this week and finish. Thanks for watching, Matteo, and stay tuned!
Such a great video thank you for your effect
Thank you, John! Clear Skies!
Great video James ! Nice tips :)
Interesting points on inertia and payload. Can you estimate the benefit in increased possible payload with an OAG?
Well, it's tough to say. You certainly need to balance the scope. In a more recent video, I show this scope with the OAG and the counter weights are about half way up the counter weight bar instead of at the bottom. I'd estimate that I can increase the current payload weight by 30% and still be able to balance it. Thanks for watching, David!
Sir, you do such a great job at explaining things. I’m lost when it comes to RMS value. I get the lower the better. But when going from say a 240mm guide scope with a zwo120mini to an OAG. should the RMS decrease substantially. Is the same rms valve for both mean something different. If your bored this old man could use a lesson. Also rms arc seconds vs like .6” the ASIAIR shows. I’m just confused
Hi Anthony,
You probably won't see a big change in the guiding RMS value. There are other factors. The mount has a limit for what it can achieve, and the atmosphere has a limit on any given night for what it will give you. You may simply be running into these limits. A more expensive mount can give you some benefit (mostly by reducing stretched stars). I don't have an ASIAIR, but I assume it's showing you the guide error in arc-sec RMS. 0.6" RMS is pretty good (I get about 0.53" RMS 80% of the time with my EQ6-R). Hope this helped a little!
@@Aero19612 yes. Thank you. I guess i confused with the images I see when others are using phd2 non Asiair. The numbers are .89 ( .6”). What is the first number. :).
@@anthonygraham6449 The first number is in pixels. The second number is in arc-sec.
Amazing content. Does space after a reducer matter or just distance from prism. I just attached my zwo to SW 120 and I have existing spacers after stock reducer. I am hoping the stock spacers are okay here and I don't need to subtract the width of the prism oag? Also I copied your idea on helical focuserr!
Hey Marty. Reducer or OAG? A field flattener requires a specific backfocus length (often 55 mm). A flattener is often combined with a focal reducer. If it's just a reducer, then I don't think backfocus spacing matters; however, the total length from the lens to the camera sensor probably does (check with manufacturer). For the OAG, the specific spacing behind the OAG is not critical; however, the distance from the center of the prism to the imaging sensor must be the same as the distance from the center axis of the OTA "up" to the guide sensor. So you may have to play with the spacing to get those distances to be equal. Good luck!
Thanks James! Yeah, I did some searching after temporary fitting the OAG and I do need to worry about back focus - for the Esprit 120 it requires 75mm so I'll redo it close and then take some test images. Thanks for your reply! I think I have the OAG rough set up - cross fingers.
All I've ever used with my 10" SCT is an OAG and I've never once had an issue finding a guide star.
That's great news! It's getting to be that time of year to switch back to the SCT--just a few more projects to finish with the 250mm and 700mm refractors then it's on to galaxy season. The OAG with the wider-field refractors is a no-brainer. If there's also no issue with the SCT, then I'm a happy camper. Thanks for watching!
Thank you for the informative videos. I have a Celestron C8 SCT and I would like to use the Celestron OAG. Would you still recommend the ZWO ASI174MM as a guide camera? I have been researching this myself and was also drawn to it due to the large sensor size. Does it get fully illuminated from the prism? I wonder if the ZWO ASI290MM is adequate for the task as it is perhaps only 2/3rds the cost of the ASI174MM.
Hi David. Thanks for watching. I have the ASI290 (I use it with the smaller ZWO OAG prism on my ED102) and the ASI174. The much larger area is why I bought the 174. The full sensor should be illuminated by the Celestron OAG prism if the camera's sensor long dimension is oriented normal to the axis of the telescope. I installed mine with the long dimension of the sensor parallel to the telescope axis trying to maximize the "donut" of guide stars, but the inner mirror support tube blocks the outer 25%-30% of the sensor in that orientation (see my "First Use of the Celestron OAG" video). It's more expensive, but I like having the larger sensor.
Great video. I've been using a Celestron OAG with my C8 + 6.3 reducer + DSLR. The optimum back focus for the reducer is 105 mm but using the parts included with the OAG I can only get to 130. Are you using a reducer in your setup?
Hey Greg. I never used my DSLR with my OAG. I can get to about 105 mm behind the focal reducer with my ASI1600+Filter Wheel. Let's see: you have 44 mm for your DSLR + 11 mm for the T-ring (leaves you with a 42 mm inner thread. That leaves us with 50 mm to get to the Focal Reducer. You'll have to find a configuration that leaves out the SCT Adapter (33 mm). So let's put on the part with the external M42 thread (about 12.25 mm, goes into the T-ring), then the OAG body (30 mm), and then the part with the internal M48 thread (about 4.25 mm). So now we have a gap of 3.5 mm. Using this approach, you need an adapter with an internal SCT thread and an external M48 thread that's about 3.5 mm thick. Check out: agenaastro.com/blue-fireball-sct-female-m48-male-thread-adapter-c-04.html
It's about 7 mm thick (optical), but you might be able to focus (still about 3.5 mm too long). An alternate approach: replace the OAG part with the external M42 thread with the part that has the internal M42 thread. Then you'll need M42/M42 gender changer like this (I got this part with my ZWO filter wheel):
www.highpointscientific.com/zwo-t2-to-t2-adapter?ZWO-T2-T2&gclid=EAIaIQobChMI0YHzy6al6QIV0cDACh3xJw1PEAQYAyABEgIUi_D_BwE
It's about 2 mm thick. That should get you to about 102.5 mm -- too short by 2.5 mm. Check my numbers! Good Luck!
How you can achieve 105 mm back focus using the celestron Off-axis guider on focal reducer and asi294mc pro ?
I have nextstar 8se and avx mount and I am using single uv/ir cut filter? I was able to achieve 110 mm back focus! Is that enough or it need to be 105 exactly! Thanks in advance.
It may be close enough. Give it a try and see if you can focus. If the test images look OK, then you're good to go. If not, you'll have to play with different spacers to get the correct back focus. You find these at various astrophotography websites. Good luck!
Question. In your "First Use" video you find that the outer edge of your guide camera is not illuminated because of upstream the upstream diameter restriction. Would it make sense to change the guide camera orientation to short axis in the radial direction? Thanks again. Bob
Yep. I meant to tell you that in my reply. I recorded that video before I tested it in use. I had hoped to maximize the area of the annular field of view of the guide camera, but, alas, I found the inner tube clipped outer region of the sensor. So, yes, it is better to rotate the guide camera 90 deg (short side of sensor parallel to telescope axis) so that you can make more effective use of the sensor area. I have not rotated mine because (a) I'm lazy and don't want to refocus and (b) the pre-planning procedure is working well. I'll rotate at some point.
Hi James. Thanks for the reply. I wasn't sure if rotating the guide camera would push the field of view out and it would be a toss up. I am just fixing some drawings so that I can take measurements to use in Stellarium for M81 and M101. I still don't have my ASI174 yet, so I am doubtful that I will see any guide stars with my ASI120MC. I hope to give it a try on Thursday. Clear skies and thanks for all your help. Bob
James, really good video. You are too technical. Pretty good work!
Agreed! When Excel spreadsheets outnumber astrophotographs, you know you have a problem. Thanks for watching, Ray!
What stellarium setting do you use to see only guide stars?
Hey, Dennis. Not sure I follow. There's not setting for "guide stars" per se. For me, a guide star is one (or more) star I can see through the OAG. I use the Ocular option in the upper right of the Stellarium window and turn on the OAG in the settings area (I show that in the video). I have since downloaded additional star catalogs for Stellarium. You can do that by going to the Configuration/Extras window. Down at the bottom, it shows you the additional star catalogs you install. Try me again, if that didn't answer your question.
@@Aero19612 Sorry, I asked that while half asleep. I guess how do you determine in the stars that show up on Stellarium which ones will show up in the guider? I've found that my Stellarium tends to show more stars than my guide camera picks up. I think there's a magnitude filter--but it'll vary depending on f/# and guidecam sensitive I think. Just food for thought, I suppose. No worries!
@@DennisCarmody Yes, I have downloaded all of the star catalogs so I have very faint stars available in Stellarium. I find that I can see and guide on stars of magnitude 11ish or brighter with my ASI174. You might do better with a more sensitive camera or worse with a less sensitive camera.
Great video
I have 9.25 / 50mm guide scope
120mm asi / eq6 pro
Would that work?