Telescope Back Focus: The Ultimate Guide [TAIC Short]
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- Опубликовано: 4 июл 2024
- What is telescope back focus...why is it important...and how do you set it correctly?
Having the proper back focus between your telescope and camera is critical to successful astro imaging. Although you hear the term “back focus” a lot in astrophotography, people use it to mean different things in different contexts. This can be confusing and frustrating. This video will help you make sense of it all.
TAIC host Alex McConahay walks us through an in-depth back focus tutorial, from the basic physics of telescope optics to the dizzying array of spacers, field flatteners, and focal reducers. He ends with practical advice on how to dial in the back focus for your imaging rig.
Full episode from June 5, 2022: • 2022-06-05 | TAIC 8th ...
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#astrophotography #backfocus
One of the most thorough and sensible videos I've seen on the subject. The topic is confusing enough, with enough bad information out there, that we don't need to add differing definitions to the mix. But even manufacturers don't always seem sure what they mean when they say "back focus". There is no industry standard.
I have a solution.
Back focus: applies to telescopes=the fixed distance from the focuser to the focal plane. Can be changed only by using corrective optics such as a focal extender, or by switching to a focuser with a different profile. Not to be confused with "out focus" or "in focus", which usually refers to the inward/outward travel distance of the focuser drawtube.
Back spacing: applies to corrective optics like flatteners=the required space between the front flange of the corrective optic and the camera sensor. This makes more sense than calling it "back focus", because it is not focus we are concerned with here.
Flange focal distance (FFD): applies to cameras=the distance between the front flange of the camera (where a lens or telescope connects) and the sensor. This is a fixed distance you need to account for when figuring out back spacing.
Currently, many people use the term "back focus" to describe all three of these things, sometimes simultaneously. Using consistent, distinct terminology would clear up a lot of the confusion.
I have been totally ignoring backfocus for the last year because I started off with a cheap doublet scope (because I spent most my money on the mount!) that just had my camera attached to the 1.25" interface at the end of the scope, so to me backfocus was just the same as focus, and I didn't get what all the fuss was about!
But now I have a triplet in the mail, and a field flattener, and I kind of knew that I should start thinking about backfocus like a grown-up, and this video explained it perfectly.
I think the point about "between the sensor and the last optic" finally made it makes sense to me.
Where was this video when I purchased all those shims, M42 and M48 tubes, etc! Pulled my hair out before I figured out this topic. Your video is an excellent explanation of back focus and how to get there. Well done, sir! Well done!
Before I watched this video I was confused as to how to compute the back focus for my image train, now I am very confused, thank you.
I am hoping to get a clear night because I thought I was doing something wrong. I added 10mm to the 55mm recommended backfocus and the stars where still pointing to the center of the image at the edges.
I guess the hotech 1x flattener f6-8 55mm back f was likely for the f8 refractor. Mine is f6.
Fantastic presentation! Early on I know I struggled with the fine focusing on some instruments. And after supporting others with their remote setups I recommend not using nonmetallic spacers long term. The 3D printed spacers are great for determining correct spacing, but they are less likely to handle temperature extremes long term, and may allow the camera, OAG, Filter Wheel, slip or rotate.
Clear dark skies
Very clear illustration and presentation. Just what I wanted! Keep up the great work
Good video on back focus with clear explanations and examples. Thanks for making the video.
Thanks for not oversimplifying things, this may have just helped me figure out the issue with my wcope.
I have found clear liquid silicone grease, I use on my fountain pen's ( Twsbi Brand liquid Grease ). when put on the threads for the joints dose aid in letting you get them apart again "Note only use it in very tiny amounts as you only need it on the threads. I apply it on one part first then join the two threads, this way I can't over do it right off the bat.
Really well done! Thank you for providing this video.
Thank you for this video. A great, easy to follow presentation.
Best explanation I've seen so far. Thanks!
Nicely done. Thanks.
This video is so helpful. Thank you :)
I can't speak to other OAG's but the ZWO OAG and OAG-L prism shaft moves in relation to its distance from the guide camera/sensor. Which is a good thing as the challenging aspect of the ZWO OAGs is to get that distance reduced enough to actually get the camera into focus range. With my current OAG-L and a ZWO 290MM Mini, I had to raise the prism shaft about halfway to the point it was just past flush inside the camera well of the OAG body. Even then it was just enough space to give me a minimum amount of +/- focus range at the bottom of the focuser stroke, after all the ZWO spacer/protective rings had been removed from the mini camera to get a maximum insertion depth.
The Aberration Inspector in the N.I.N.A. add-on Hocus Focus is next level technology (IMO) for determining both real life back focus and sensor tilt in an imaging train.
Wonderfully concise.
Alex - that is brilliant! Thanks so much. Mel from Sydney
Awesome explanation the best of all thank you
Excellent video!
The flat side of the OAG prism should be facing the telescope. At 9.23 your figure shows the angled side facing the optic. Small point for sure. Thanks for making this video. I learned a lot.
Wow!
Rub a pure wax candle on the threads at first connect - careful not to have clumps.
Have you tried antisieze?
ok nice if you see stars. how do you know if you are to far away or to close if your whole image is grainy and blurred to the point of not seeing anything?
If you can't even focus the image then most likely, you have added way to much "back" spacing between your focuser and imaging chip. Some telescopes have very little back focus ie - Just enough for an eyepiece. A low profile focus could help in that case. You need to meticulously review you math...
How about when my Star looks like it has a Spikes on it with one larger spike to lower left. like Astigmatism or something. how can that be fixed
Join a club or a forum. You will not an answer for that Q here ...
I don't understand the use of very thin spacers. Doesn't the focuser make the final adjustment?
Yes, the focuser moves the sensor through the zones of focus or non-focus, thus allowing you to achieve proper focus. But, the point of the spacer is not to adjust FOCUS, but BACKfocus. In this context, BACKfocus means the distance between the last optic (flattener, coma corrector) and the sensor. The last optic and the sensor are joined by firm connections, and do not move relative to each other when one moves the focuser. The only good way to adjust that spacing is to insert or remove spacers. This adjustment allows that last optic to work at its engineered proper distance, and flatten, enlarge, correct, shrink, or whatever else it was supposed to do without distorting the image.
@@TAIC I still don't get it. If I can move the focus back and forth, what is the point to get exact distances with thin spacers? Doesn't the sensor need to be at the focal plane? Everything moves with the focuser.
@@peterleveillee1321 Your brain is stuck on the point that you are moving the focal distance. And you are. The the focal distance is set by the original optic, and adjusted by the focuser. There are, however problems with the original "focal distance." Let us just take one of those problems----that of a curved surface meeting a flat surface (for which you will need a "corrector"). Your focus distance is NOT the same all the way across the image circle in any telescope design. There is a curve to it. It is curved, not flat like the sensor. You cannot display a curved surface on a flat surface. So, if you "focus" on the center of the circle, the outer edges will be slightly out of focus, and display other errors, like coma, and so forth. This is because they have not travelled as far as the rays in the middle of the optic. Yes, opticians try to design their lenses such that all that is minimized. It is as flat as possible. But it is still there. So, back at the end of the system, you put in a "corrector." The corrector lengthens the length of the focal distance on the outer edges of the frame, so that they are as long as those in the central area. It is designed that these corrections bring all the focal distances (from the corrector back to the sensor) to meet when the sensor is at a specific distance. Before this point, the correction will be too much. After, it will be too little. (or maybe it is the other way around?!?). This distance is the "BACKFOCUS." (Not the FOCUS). The point of the spacer is to get just the right distance between the LAST optic and the sensor such that the distance between them is the DESIGNED distance. (The distance where the changes in the corrector bring all the light rays to the proper distance---such that the wavefront is indeed flat.) Once that is done, then any light entering from the main optic will be flattened so that it hits the sensor all at the same distance, same focal distance from the main optic. Whether that sensor is in the right position to have the image itself focused depends now on the MAIN focuser position. So, that spacer just means that all the light coming through it will be focused at the same spot. The main focuser then has to be used to make sure that spot is the right focus spot. (Note that my example is about correctors and flatteners. But the same analysis pretty much applies to reducers.)
In short, moving the sensor closer or further from the correcting optic (with or without spacers) makes it work like it should as a corrector. The Main Focuser just brings the whole unit to where it should be to do the focusing. If you used only the main focuser, you would still have the errors you are trying to correct.
@@TAIC I understand all about the reason for correctors, reducers and field flatterers. For one, on my refractor everything moves in and out with the focuser. What is the point of the very thin spacers? They are like washers. What has to be so precise that the focuser cannot adjust. Get to the point!
Peter, Maybe this will help......stop thinking about the main optic delivering a focal "plane." It delivers a curved surface. The correcting optic changes this curved surface into the plane surface, but at only a specific distance from the secondary optic. The little spacers put the correcting optic THE specific distance it needs to be at to make the curve a plane. Having done that, the main focuser can move that optic and sensor to where the plane comes to focus at the sensor.