Excellent video, and very interesting to see the results! The ASI1600 is one of the better behaved CMOS cameras. The 183MM and 294MC would be different stories I believe. Both have significant amp glow, that does not scale well with optimized bias-subtracted darks (the 1600 also has amp glow, but much less. It would be interesting to see the level of amp glow in a bias subtracted 200s dark and a 50s bias subtracted dark that has been stretched to 200s). The ASI294MC Pro also doesn't seem to like short exposures in general! Flat frames are recommended to be 2-3 seconds long from what I've read, and bias frames avoided. I personally only use dark frames, as they contain the bias signal... You could choose to not calibrate the flats, which will work well on dithered sequences, but could definitely cause issues on non-dithered sequences, as the flats can reinject some FPN. Aaargh, things get so complicated in this hobby, glad you're here to investigate everything!!!
Thanks for the info on other camera "personalities." Seems like a good argument to only work with one camera. The 1600 certainly has amp glow I can see in my dark frames, but maybe not as bad as other cameras. And that's a good point that amp glow may/will scale differently. So many variables, so little time.
THANK you for doing this video: I also use the 1600mm pro and am revisiting my imaging workflow and was wondering whether I should take the time to do dark flats or not. I suspected there would be little to no difference and am glad someone took the time to do a comparison. Great video, thank you.
Thanks for watching! I'm glad you found it useful. I'm starting to form library of darks for the range of typical exposures I use for my T-shirt flats just in case I want to switch to dark-flat calibration down the road.
Interesting James. I was reading that BIAS frames are better/applicable for CCD cameras than for CMOS? The difference between dew shield and no dew shield, may just be that they are taken at a different time? I’m not saying that you are not fastidious in your preparation. Simon
Hey Simon. I "hear rumors" about bias frames as they relate to CCD and CMOS. I'm no camera sensor expert, that's for sure and I don't even own nor have ever used a CCD camera. I'm not sure what the source of the statement is. I'm sure there's some logic behind it. I DO know that my CMOS ASI1600 produces a significant bias frame (aka read noise) compared to the cooled dark frame. So something has to be done with the read noise. Now, if you use dark-flats to calibrate your flat frames and you have not removed the bias from the dark-flat (which you should not do), then you're OK because the read noise is in the dark-flat. So, in that sense, you're not using a separate bias frame, but you are accounting for the read noise. Same goes for calibrating your light frames. If you use a Master Dark frame and do not subtract out bias from the dark, that's ok because the read noise is in the Master Dark. The "alternate" flat frame calibration method I mention in the video uses a Master Bias frame and a Master Dark frame WITH THE BIAS SUBTRACTED OUT [I'm not yelling ;-) ] and the Master Dark is "optimized" for the flat frame exposure time. That works as well (see my next video on the topic of "why we calibrate flat frames"). So, in that case, you do need a separate bias frame. But, if you can calibrate your flats with dark-flats, then by all means do so and never take a Master Bias frame. That said, I don't know why everything I just wrote doesn't also apply to CCDs. But then, I'm not camera sensor expert. Sorry for the long reply. Time of day could indeed have an effect on with/without dew shield results. I don't believe that was the case however. I recently experienced another case of mis-matched dew shields (been doing some studies on dew shield length of late). I once again found that when I took light frames with a 3" dew shield and used flats taken with the 8" dew shield (too lazy), those darn artifacts appeared again. My take-away was/is: If you take lights with a dew shield, then take flats with that dew shield. Maybe the results would be different if I used a light panel (which I don't have). As far as not being fastidious...I have a video discussing what happened one night when I forgot to turn the lawn sprinkler system off before leaving my telescope outside. Not fastidious? Guilty, as charged.
@@Aero19612 You are fastidious with your analysis... perhaps the dew shield "masks" any stray (neighbours) light catching "motes" so that they "show up"? My book on AP is by someone who likes to use a CCD - they are expensive. So the CMOS vs CCD may be a "dig" at CMOS. For me, CMOS is affordable. I have ASI294MC as an "intro" camera. Simon
I’m sure the dew shield is providing some degree of stray light reduction. That’s why you want to take flats with the same (or no) dew shield you used to take the light frames. CCD cameras are very expensive. I’m glad CMOS technology is taking over so we can afford this hobby.
Thanks for your videos. Always a pleasure to watch. I have been knocking around the idea of using dark-flats for quite awhile now but still haven't bothered to change my process. I am puzzled why your dark frames are only 50s. I thought that darks were supposed to be the same exposure time as the light frames. No? I see why you used the 50s dark (maybe I don't) but I guess what I am asking is, why wouldn't that skew the result from actually having used the proper 200s dark? Thank you again for taking the time to produce these wonderful videos. Your channels is a favorite....
Yeah, it's confusing. When you calibrate the light (not the flat frames) frames, your gain, temp, and exposure time used for the dark frames should all match what you used on your light frames. I believe the 50s exposure is what I was using to calibrate the flat frames. When calibrating the flat frames, you can use Dark-Flats or "optimized" dark frames (i.e., scaled down from longer exposure used for light frames). When you use the dark-flat approach, your dark-flats should have the same exposure time, temp, and gain as you used for the flat frames for that filter. When calibrating the flat frames using darks, then you can use the same long exposure dark frame you used to calibrate the light frames but you have to click "Optimize" in PixInsight to scale down the noise level to match the flat frame. In my case, I had some shorter duration dark frames at 50s I used to calibrate the flat frames. Geez. I think I made things more confusing...sorry
@@Aero19612 I think you explained it perfectly. When you use the "optimized" dark frames in lieu of the dark-flats, I assume you then still need the bias frames? Thanks for taking the time to answer.
Yeah, I use bias frames when I calibrate the flat frames with the dark frames. I also subtract the bias from the dark frame so that it's just the dark frame with no bias signal. I'm trying to switch to using dark-flats. I have a "library" of dark-flats for typical exposures I end up with when taking flats at dawn with a T-shirt. Thanks for watching!
Very interesting James. I’ve never taken dark flats but will give it a try. I’ve heard that with cmos cameras there’s no need to take bias frames for light calibration. Any thoughts? I always have done but then I’ve used a ccd camera exclusively until getting the ASI1600.
I have not heard that about CMOS cameras before, but that does not sound right. If you use Dark-Flats, there is certainly no need to include a separate Bias frame in the calibration (indeed, it would be "wrong" to do so) because the bias is included in the Dark-Flat. For a cooled camera, like the ASI1600, the background noise of my Master Bias is 295 (out of 65000), but a 200-sec Master Dark (with Bias subtracted) is only 17, a 50-sec Master Dark is 7, so the dark for a 55 ms flat frame would be VERY small indeed. If anything, I would say there's no need to include a Master Dark if you calibrate with a Master Bias and "optimized" Master Dark. That said, as I ramble on here, the background level for the Flat frame is 20,000. So is there any need to calibrate them at all! The Bias and Dark background levels are swamped by the Flat illumination level anyway....Just repeated the comparison I did in the video for the stacked M51 set using non-calibrated flats. Looks identical to the sets with calibrated light frames. So maybe your original question is: Is there any need to calibrate flat frames? Maybe not!
For the darks, was the dew heater on? Or were you just using the shield? As an aside I was reading at PixInsight that dark flats are not needed; if one is using PI
Well, for the darks there is no heated dew strap or dew shield. Neither should have any effect on the darks though. I agree with that last statement--my results show there is no difference. However, if you read my long-winded response to John's question in the comments, I may have talked myself out of calibrating flat frames at all!
Hey, Dan. I'm probably in a Bortle 6 or 7 (northern suburb of Dallas). As you'll see in my other reply, my "finished" M51 isn't. Yet. I'm trying to get about 25 hours of imaging time on it and M106. Maybe next week!
oops. No, I'm not using the focal reducer for M51 and M106. I have used it for M33, M81/M82, and the Leo Triplet. Whenever I need that little bit reduction in focal length. M51 and M106 fit nicely in my C9.25 FoV without the focal reducer.
No. But there's some logic to it if you have a cooled camera. When the sensor is very cold, the dark current noise is very low (represented in the Dark calibration frame). The read noise for some CMOS sensors at some gain levels can be relatively high. On the other hand, it takes just as much effort to make a Bias library as it does a Dark library that includes the Bias. So, I just calibrate with Flats (of course) and a Master Dark (Bias is included in the Dark frame). Thanks for watching!
@@Aero19612 Agree , I’ve always done Flat Darks with my CCD, but I am transitioning to a CMOS camera and have been researching, my plan is to stick with Flat Darks. Thanks
So, I've a white t-shirt screen mounted on a ring that I can fit inside my dew shield... but it sits against my SCT corrector plate.. would I be better off placing the ring near the top (opening) of the dew shield instead?
In my opinion (take it for what it's worth) your t-shirt ring should be placed at the end of your dew shield. But maybe try some flats the way you have it and, if there are artifacts, try my suggestion. Just tell me what your experience is! Thanks for watching!
Dew shield or no dew shield is not an option for those of us with HyperStar - we have to use a dew shield since the HS and camera are located in front of corrector/secondary mirror. Clear Skies & God Bless. Be safe.
The dew shield has so many advantages. Just one big disadvantage (wind) for me. Do you do anything to deal the increased wind sensitivity, or is your mount much better at dealing with it? Thanks for watching!
James Lamb I have been OK so far. My most recent sessions were at targets gradually approaching zenith through the night so the OTA was going a bit vertical thereby getting more stable as the session progressed. My EQ6-R Pro is pretty solid given my EdgeHD 8"...not much of a moment arm being put on the mount since my SCT is so short/small/light compared to a 9.25 or a bigger frac. Love your channel, keep up the awesome job! You are making a difference.
Good. By the way, you're the 2nd or 3rd person who has good things to say about the EQ6-R and/or the AZ-EQ6-R mounts. I'm very interested. Thanks for the kind words!
I upgraded from an Evolution mount to the EQ6-R Pro. It has been the biggest/most significant modification/investment I have made, followed closely by going cooled mono ccd. I am very satisfied with its accuracy and stability for my setups - HyperStar/Guider or Focal Reducer/Motor Focuser/OAG/Filter Tray/CCD. I have heard that the CEM60 is another good option but I am fine with my setup.
Hey Michael. I don't think it's that critical. We will change focus throughout the night because of temperature and, as you say, due to filter changes. By the time you take your flats, the temperature will be even more different. You just don't want to be "way" off. Pleeeease let that be true!
@@Aero19612 As far as I understand, the focus when taking flats is indeed not critical. Any filter vignetting and dust mote sizes are determined by the angle of the incoming light rays (and thus by the focal ratio) and distance to the camera sensor. Theoretically on an SCT, that FR changes slightly as focus changes since the main mirror is moved, but it's so small as to be insignificant for flats (if you're not moving the main mirror by many turns of the knob) :-)
Excellent video, and very interesting to see the results! The ASI1600 is one of the better behaved CMOS cameras. The 183MM and 294MC would be different stories I believe. Both have significant amp glow, that does not scale well with optimized bias-subtracted darks (the 1600 also has amp glow, but much less. It would be interesting to see the level of amp glow in a bias subtracted 200s dark and a 50s bias subtracted dark that has been stretched to 200s). The ASI294MC Pro also doesn't seem to like short exposures in general! Flat frames are recommended to be 2-3 seconds long from what I've read, and bias frames avoided. I personally only use dark frames, as they contain the bias signal... You could choose to not calibrate the flats, which will work well on dithered sequences, but could definitely cause issues on non-dithered sequences, as the flats can reinject some FPN. Aaargh, things get so complicated in this hobby, glad you're here to investigate everything!!!
Thanks for the info on other camera "personalities." Seems like a good argument to only work with one camera. The 1600 certainly has amp glow I can see in my dark frames, but maybe not as bad as other cameras. And that's a good point that amp glow may/will scale differently. So many variables, so little time.
THANK you for doing this video: I also use the 1600mm pro and am revisiting my imaging workflow and was wondering whether I should take the time to do dark flats or not. I suspected there would be little to no difference and am glad someone took the time to do a comparison. Great video, thank you.
Thanks for watching! I'm glad you found it useful. I'm starting to form library of darks for the range of typical exposures I use for my T-shirt flats just in case I want to switch to dark-flat calibration down the road.
Interesting James. I was reading that BIAS frames are better/applicable for CCD cameras than for CMOS? The difference between dew shield and no dew shield, may just be that they are taken at a different time? I’m not saying that you are not fastidious in your preparation. Simon
Hey Simon. I "hear rumors" about bias frames as they relate to CCD and CMOS. I'm no camera sensor expert, that's for sure and I don't even own nor have ever used a CCD camera. I'm not sure what the source of the statement is. I'm sure there's some logic behind it. I DO know that my CMOS ASI1600 produces a significant bias frame (aka read noise) compared to the cooled dark frame. So something has to be done with the read noise. Now, if you use dark-flats to calibrate your flat frames and you have not removed the bias from the dark-flat (which you should not do), then you're OK because the read noise is in the dark-flat. So, in that sense, you're not using a separate bias frame, but you are accounting for the read noise. Same goes for calibrating your light frames. If you use a Master Dark frame and do not subtract out bias from the dark, that's ok because the read noise is in the Master Dark. The "alternate" flat frame calibration method I mention in the video uses a Master Bias frame and a Master Dark frame WITH THE BIAS SUBTRACTED OUT [I'm not yelling ;-) ] and the Master Dark is "optimized" for the flat frame exposure time. That works as well (see my next video on the topic of "why we calibrate flat frames"). So, in that case, you do need a separate bias frame. But, if you can calibrate your flats with dark-flats, then by all means do so and never take a Master Bias frame. That said, I don't know why everything I just wrote doesn't also apply to CCDs. But then, I'm not camera sensor expert. Sorry for the long reply.
Time of day could indeed have an effect on with/without dew shield results. I don't believe that was the case however. I recently experienced another case of mis-matched dew shields (been doing some studies on dew shield length of late). I once again found that when I took light frames with a 3" dew shield and used flats taken with the 8" dew shield (too lazy), those darn artifacts appeared again. My take-away was/is: If you take lights with a dew shield, then take flats with that dew shield. Maybe the results would be different if I used a light panel (which I don't have). As far as not being fastidious...I have a video discussing what happened one night when I forgot to turn the lawn sprinkler system off before leaving my telescope outside. Not fastidious? Guilty, as charged.
@@Aero19612 You are fastidious with your analysis... perhaps the dew shield "masks" any stray (neighbours) light catching "motes" so that they "show up"?
My book on AP is by someone who likes to use a CCD - they are expensive. So the CMOS vs CCD may be a "dig" at CMOS.
For me, CMOS is affordable. I have ASI294MC as an "intro" camera. Simon
I’m sure the dew shield is providing some degree of stray light reduction. That’s why you want to take flats with the same (or no) dew shield you used to take the light frames. CCD cameras are very expensive. I’m glad CMOS technology is taking over so we can afford this hobby.
Thanks for your videos. Always a pleasure to watch. I have been knocking around the idea of using dark-flats for quite awhile now but still haven't bothered to change my process. I am puzzled why your dark frames are only 50s. I thought that darks were supposed to be the same exposure time as the light frames. No? I see why you used the 50s dark (maybe I don't) but I guess what I am asking is, why wouldn't that skew the result from actually having used the proper 200s dark? Thank you again for taking the time to produce these wonderful videos. Your channels is a favorite....
Yeah, it's confusing.
When you calibrate the light (not the flat frames) frames, your gain, temp, and exposure time used for the dark frames should all match what you used on your light frames. I believe the 50s exposure is what I was using to calibrate the flat frames.
When calibrating the flat frames, you can use Dark-Flats or "optimized" dark frames (i.e., scaled down from longer exposure used for light frames). When you use the dark-flat approach, your dark-flats should have the same exposure time, temp, and gain as you used for the flat frames for that filter.
When calibrating the flat frames using darks, then you can use the same long exposure dark frame you used to calibrate the light frames but you have to click "Optimize" in PixInsight to scale down the noise level to match the flat frame. In my case, I had some shorter duration dark frames at 50s I used to calibrate the flat frames.
Geez. I think I made things more confusing...sorry
@@Aero19612 I think you explained it perfectly. When you use the "optimized" dark frames in lieu of the dark-flats, I assume you then still need the bias frames? Thanks for taking the time to answer.
Yeah, I use bias frames when I calibrate the flat frames with the dark frames. I also subtract the bias from the dark frame so that it's just the dark frame with no bias signal. I'm trying to switch to using dark-flats. I have a "library" of dark-flats for typical exposures I end up with when taking flats at dawn with a T-shirt. Thanks for watching!
@@Aero19612 Thank you!
Very interesting James. I’ve never taken dark flats but will give it a try. I’ve heard that with cmos cameras there’s no need to take bias frames for light calibration. Any thoughts? I always have done but then I’ve used a ccd camera exclusively until getting the ASI1600.
I have not heard that about CMOS cameras before, but that does not sound right. If you use Dark-Flats, there is certainly no need to include a separate Bias frame in the calibration (indeed, it would be "wrong" to do so) because the bias is included in the Dark-Flat. For a cooled camera, like the ASI1600, the background noise of my Master Bias is 295 (out of 65000), but a 200-sec Master Dark (with Bias subtracted) is only 17, a 50-sec Master Dark is 7, so the dark for a 55 ms flat frame would be VERY small indeed. If anything, I would say there's no need to include a Master Dark if you calibrate with a Master Bias and "optimized" Master Dark. That said, as I ramble on here, the background level for the Flat frame is 20,000. So is there any need to calibrate them at all! The Bias and Dark background levels are swamped by the Flat illumination level anyway....Just repeated the comparison I did in the video for the stacked M51 set using non-calibrated flats. Looks identical to the sets with calibrated light frames. So maybe your original question is: Is there any need to calibrate flat frames? Maybe not!
For the darks, was the dew heater on? Or were you just using the shield? As an aside I was reading at PixInsight that dark flats are not needed; if one is using PI
Well, for the darks there is no heated dew strap or dew shield. Neither should have any effect on the darks though. I agree with that last statement--my results show there is no difference. However, if you read my long-winded response to John's question in the comments, I may have talked myself out of calibrating flat frames at all!
Did you use a focal reducer. Great video . I’m in the beginning glade I found your videos . What is your light pollution there .
Hey, Dan. I'm probably in a Bortle 6 or 7 (northern suburb of Dallas). As you'll see in my other reply, my "finished" M51 isn't. Yet. I'm trying to get about 25 hours of imaging time on it and M106. Maybe next week!
oops. No, I'm not using the focal reducer for M51 and M106. I have used it for M33, M81/M82, and the Leo Triplet. Whenever I need that little bit reduction in focal length. M51 and M106 fit nicely in my C9.25 FoV without the focal reducer.
Have you ever tried bias only for flat calibration? though I’ve heard this may not be optimum for some CMOS sensors.
No. But there's some logic to it if you have a cooled camera. When the sensor is very cold, the dark current noise is very low (represented in the Dark calibration frame). The read noise for some CMOS sensors at some gain levels can be relatively high. On the other hand, it takes just as much effort to make a Bias library as it does a Dark library that includes the Bias. So, I just calibrate with Flats (of course) and a Master Dark (Bias is included in the Dark frame). Thanks for watching!
@@Aero19612 Agree , I’ve always done Flat Darks with my CCD, but I am transitioning to a CMOS camera and have been researching, my plan is to stick with Flat Darks. Thanks
So, I've a white t-shirt screen mounted on a ring that I can fit inside my dew shield... but it sits against my SCT corrector plate.. would I be better off placing the ring near the top (opening) of the dew shield instead?
In my opinion (take it for what it's worth) your t-shirt ring should be placed at the end of your dew shield. But maybe try some flats the way you have it and, if there are artifacts, try my suggestion. Just tell me what your experience is! Thanks for watching!
I have to use a dew shield to obstruct the light from my neighbour that never closes their curtains. So my dew shield is also a light barrier...
I know what you mean, Peter. Maybe add some back felt lining inside the dew shield to cut down on reflections if that's a problem.
Dew shield or no dew shield is not an option for those of us with HyperStar - we have to use a dew shield since the HS and camera are located in front of corrector/secondary mirror. Clear Skies & God Bless. Be safe.
The dew shield has so many advantages. Just one big disadvantage (wind) for me. Do you do anything to deal the increased wind sensitivity, or is your mount much better at dealing with it? Thanks for watching!
James Lamb I have been OK so far. My most recent sessions were at targets gradually approaching zenith through the night so the OTA was going a bit vertical thereby getting more stable as the session progressed. My EQ6-R Pro is pretty solid given my EdgeHD 8"...not much of a moment arm being put on the mount since my SCT is so short/small/light compared to a 9.25 or a bigger frac. Love your channel, keep up the awesome job! You are making a difference.
Good. By the way, you're the 2nd or 3rd person who has good things to say about the EQ6-R and/or the AZ-EQ6-R mounts. I'm very interested. Thanks for the kind words!
I upgraded from an Evolution mount to the EQ6-R Pro. It has been the biggest/most significant modification/investment I have made, followed closely by going cooled mono ccd. I am very satisfied with its accuracy and stability for my setups - HyperStar/Guider or Focal Reducer/Motor Focuser/OAG/Filter Tray/CCD. I have heard that the CEM60 is another good option but I am fine with my setup.
Great! Thanks for the feedback
Do you know how critical focus is when taking flats? If the focus is different between filters, should the flat focus match?
Hey Michael. I don't think it's that critical. We will change focus throughout the night because of temperature and, as you say, due to filter changes. By the time you take your flats, the temperature will be even more different. You just don't want to be "way" off. Pleeeease let that be true!
@@Aero19612 Thanks James.
@@Aero19612 As far as I understand, the focus when taking flats is indeed not critical. Any filter vignetting and dust mote sizes are determined by the angle of the incoming light rays (and thus by the focal ratio) and distance to the camera sensor. Theoretically on an SCT, that FR changes slightly as focus changes since the main mirror is moved, but it's so small as to be insignificant for flats (if you're not moving the main mirror by many turns of the knob) :-)
Ahh. Thanks for the confirmation, Cuiv. I knew I wasn't prepared to do any more work on flats.
@@CuivTheLazyGeek Thanks Cuiv.
Where’s the finished m51......