Everything said here is, of course, correct. That said, you still may wish to choose the gain differently. That is because the highest dynamic range is not necessarily what you want. Instead, you may want the dynamic range optimally adjusted to the objects of interest in the frame. As an example, if you take a picture of Barnard's loop, you will not care how much Rigel in these frame is overexposed, you want that faint nebula to have some internal contrast. So, do not plainly set the gain to "as high as needed, as low as possible", but to a value that is suitable to (i.e., does not saturate) the brightest object in the image you want with good definition and/or properly reproduced color.
Excellent video. I would like to add that this is all for a "perfect" sensor, in practice there's sensors out there (looking at you, IMX183) that show pretty nasty banding at 0 gain and lots of "amp glow" at high gain. Optimal gain is then found by the same considerations you mention, but excluding those bad extremes.
What if i use an OSC and a Duo Narrowband filter? are the mulpiliers compound?, Secondly thak you very much for taking the time to put together this excellent lecture (and the original part) you are a credit to amatuer astronomy.
Great Videos - I am into EAA, and minimizing sub exposure times plays right into EAA. Question, on the sharpcap tools page, for the pixel size, do I modify that if I am binning? For example, I have a 294MC OSC, which has a size of 4.63um, but at longer focal lengths I bin at 2x2, so do I need to change my pixel size to account for the binning?
Great videos; thank you! Help's a lot for a beginner in using a CMOS-Camera. But there a two questions concerning filters i want to use: i purchased a osc-cmos camera (ASi0171MC Pro) and what to ask you which factors should be applied when using either a LPS-Filter (Hutech IDAS LPS-D2) or a Narrowband-Filter like the 'Altair QuadBand OSC CCD 2" Filter' (FWHM 35nm). As far as i understood the factors for narrowband are for mono-cameras; is that right? So which factors do you suggest for this two filters with my ASI071MC Pro? Thanks a lot
Great work Robin. Finally debunking the myths and taking the guesswork out of sub-exposures settings... I have recently moved from DSLR to colour CMOS, moving from a SCT 2000 to a 478 gt81 and considering the move from ATP to Sharpcap Pro. So this video and the previous one are very helpful to at least reduce the risk of wasting many valuable clear nights of bad data acquisition.
Thankyou for the very useful advice and formula for optimal sub length. One thing I'm unclear on is the issue of thermal noise. I'm upgrading my camera and am considering whether it's worth paying 300 quid for cooling. If I use your formula for image degradation ( total noise without cooling divided by noise due to light polution only) I get values of over 10%. (Bortle 4, f5.3) using the published dark current values for the camera. Am I right in thinking I should go for the cooled version?
These calculated times are recommended maximum times. My recommended sub exposure time worked out to 640s. Yes, my guiding can do 0.8" rms, but there's always a spike here or there, creating a small trail. This means that I end up throwing more away when my exposures are longer. Using shorter exposures, I can capture more total exposure time, which means that I have a different "optimal". I can just accept that I will get more stack noise under urban skies than I would get under a dark sky.
Dr. Glover, thanks for the excellent treatment of this subject. One factor that I also consider, at least intuitively, is the dither delay. To take an extreme example, if one images for 1 hour and compares taking 10 sec subs with a 10 sec dither delay as compared to a 1 hour sub the former would collect half the photons. It might be interesting to see how this effect might push the recommended sub exposure time under certain conditions. By intuition it pushes me to 100-200 sec subs (so I only lose 5-10% of the photons) but I would love to see what the math says. Thanks again for your excellent work.
Basically you are reducing the total exposure time T by some time effectiveness factor - time between subs / (subs time + time between subs) Let's assume this factor is constant. Since this factor is multiplying the T you can treat it as if it was a change in light pollution factor P, which is in the denominator of the formula so you have to take the inverse. If the time effectiveness 50%, (i.e. you spend the same in subs as in between subs) then you have to multiply the optimal subs length from the table by 2. If 25% of the time you are in between subs then multiply by 1.333 If 10% of the time then by 1.111 If 5% of the time then by 1.05 … Re dithering the whole thing becomes more complicated, as it is less obvious is to understand the improvement in quality it gives you.
I have an f/7.5 refractor and a Zwo1600-mmcool , so if I'm understanding correctly the recommendation for my rig would be to set the gain to about 139 (*happens to be UG for this camera but it's also where the graph stats to flatten), and to take about 10-30 sec Lum exposures in my Bortle 5-7 skies. When you said 3x longer for RGB, does that mean I should be taking 90 sec exposures for each of those filters? If I bin 2x2 does that mean I can get away with 45 sec exposures for the RGB filters?
Thank you for a terrific presentation. I learned a lot but admit I am confused on one key point: I am in a Bortle 7 region with a f5.9 scope (430mm / 73mm), 12nm narrowband filter (L-Enhance Pro), color CMOS camera (ASI183MC Pro). Do I multiply that 6.4 value by 3 for the RGB, and then that result by 25 for narrowband? So: 6.4 x 3 = 19.2 x 25 = 480 sec subs? Thank you!
Very nice presentation and graphs, I really appreciate it. I have learned a lot. Feedback: place the microphone out of the way of breath next time to improve the audio, bumbs and thumbs from air noses were really loud
This is super interesting and helpful, thanks! Because I can't be bothered with guiding and have a good mount, I always keep my subs short anyway, so this provides me with theoretical justification ;-) Question, if I am using a narrowband filter (L-Extreme) AND a colour camera, do I multiply by both values?
Wonderful summary of this not so easy to understand the gain problem. On the other hand though ... and it would be interesting to hear your opinion about this: whenever i shoot long exposures (and I am talking 5 to 10 minutes, my mount can handle this easily) the image looks by far better, and I get also very dim targets in a Bortle 6 sky. If I understood your presentation correctly I waste my time by doing this. Note: I use a gain of 200 and offset of 30 with my ZWO ASI 1600MM camera. If you like i can pass you some links to example images.
In his previous video I think he explained it. You took 10 minute exposure on gain 10 and it looks great Now take 1 minute exposure at gain 100 and it doesn't look really that great. But by taking 10x1 minute exposures with gain 100 - stacked - it will look pretty damn close (if not better) to your single 10 minute sub exposure. At the end of the day, it really comes down to whether your mount is able to track well for a 10 minute sub and how much hard disk space you got. since 10 x1 minute subs will of course take up more space on your hard drive than 1 single 10 minute sub. There are other reasons people taking shorter and shorter subs, once is because they can , the other is possibly satellite trails and maybe their mount is only reliable for 1 minute or - like me- a 5 minute exposure at my location will saturate my sub too much even at gain 0.
I've looked up the read noise vs gain chart for my camera (1.img-dpreview.com/files/p/TS560x560~forums/59260758/37aa3dc7d9834108a99d18161cf28756) and it shows the read noise increasing proportionally relative to gain. However, there's a dip at 800iso, should I shoot at 800iso or 200iso in order to minimize read noise? Bortle 3 sky, f2 camera lenses (no telescope yet). Thx a lot!
I've been using the SharpCap sensor analysis to find the 'sweet spot" for gain/offset. To me, what I'm looking for is the lowest read noise/deepest well depth mix that I can get. With my sensor that is just past the 'switch spot" noted here where read noise makes a huge step change down and well depth (for my camera) is approx 50% of the original value. I could keep chasing higher gains, but well depth drop off is on a much steeper slope than read noise reduction (relatively speaking), so not worth it to me. I do have sets of calibration frames set for a couple of step changes in gain after that point for dimmer objects should I think it's necessary, but I've not had any issues with the quality of the results I've obtained by prioritizing read noise/well depth as my setting criteria. YMMV
OK, I have QHY128C, readout noise is ~3.5. Bortle 3 sky. According to this optimum sub lenght will be 400sec. As it is color camera, I multiply it with 3. That will be 1200 sec. 20 minutes for a sub!!!
Yes? You have the option to exposure very long and continue to collect signal that is not possible for us with light polluted skies, not a bad problem to have.
Think I have been doing it completly wrong for the last 12 months, trouble is the maths part just does nothing for me its like a complete block. I have always gone unity gain ,subs and doing my subs at Lum 30secs, RGB 120 secs (ed80 and a ZWO ASI 1600 mono cooled) my temp ive always run at -25, thanks for the tutorial
Unity Gain on its own as a term and what it is, not important, but since many camera makes placing the unity gain exactly where the high-conversion mode starts (usually at 10 for QHY, 100 for ZWO ~ so I put my gain to 11 (or 101 for ZWO to be on the safe side) ) . But double check this with your gear, because I have a QHY camera that doesn't switch anywhere to anything, and where the unity gain is indeed irrelevant, because I just set the gain+ exposure based on the subframe histogram I get.
Hello! I have SharpCap v3.2.6482, 64 bit, using with a SVBONY SV205. Under "Camera Controls" the exposure setting is grayed out and I cannot change it. Slider bar does not work either. The window says Exposure 15.6 ms. How can I get the exposure setting to work? Thanks!
@@boaty1968 Mine *is* the Pro version. License is Valid Expiry Date: Friday, May 13, 2022. But I had the "AUTO" box checked. When I unchecked it, the exposure controls work fine. Thanks anyway.
Is the zwo asi 533 MC not cooled camera a good camera? I have been looking around a bit and have noticed the sharp jump in price from uncooked to cooled cameras. Technology costs money. Dr. Glover also said the zwo camera the 183 is very popular. I cannot afford a cooled camera or I should say my wife says that's to much for any camera. Ha ha
Great videos! One question - you mention "for RGB/Color, multiply by 3" - does your tools.sharpcap.co.uk calculator do that if I select "color" or are you saying that I still need to multiply the end result sub formula?
SO i have access to two sites. one bortal 6 and one bortal 2-3. in the better location is says i should take 43 min exposures? 43 mins! surely I must be doing something wrong here..
You are just looking at it the wrong way. You have the OPTION to shoot longer and collect fainter signal that is impossible in your bortle 6. If at a dark sky site you shoot as long as you can before you get saturated stars/tracking issues/potential clouds/etc. This does not mean you are required to shoot 43 minutes, but that you can continue to see benefits for far longer than you could in the city.
The exposure time are much longer than I thought. I watched your presentation. All along, you hinted we were exposing for too long and at the end of the presentation (1:12 in this video) you provide a table that shows I need to expose for time period 3 to 10 times longer than the 4 minutes I normally go with. You confused me.
Be careful with that table, the camera in the example is 50 quantum efficiency (*50QE) and 3.75um pixel size. If you got a newer CMOS camera (made in the last 3 years) I can almost guarantee you, that your QE is above 80% , also check your sensor size and read noise numbers. The best thing you can do is do the sensor analysis in Sharpcap and then turn on histogram, click the brain icon and do an optimal exposure calculation to really see what exposure length you need.
By these calculations, I should only need to expose for 12 seconds with my 533MC-Pro at 100 Gain in a Bortle 6 sky. 1.5e read noise, 5.63e/s/px sky background. This just doesn't seem right to me lol. Maybe I'll have to give it a shot.
12 seconds maybe, but because imagine how many subs you would have to make and how much hard drive space you would need , I would proportionally scale back to more reasonable subs , maybe multiply by 3 , so it's 36 seconds per sub and try with gain 40-50. You can also put in a light pollution filter and that will also help to increase the exposure time, thus decreasing the number of subs to take.
