Hello Gary. Thanks for your video. I used this to do photometry for the first time. I could not work out Muniwin or AIP4Win or MetroPSF packages...But is it better to use a mono camera with a V filter. Why did you not use that instead of a colour camera?
Interesting overview of your workflow! Very well demonstrated! I learned some practical tricks from you, although I have done photometry for more than 20 years.. There is however something I'd encourage to test. Namely, in CMOS cameras that "gain" is changing how many electrons form one ADU (digital signal unit) on image and it does not change your input signal - the amount of photons. In some sense, the "sweet spot" is often when 1 photon (which frees 1 electron in detector material) is creating 1 ADU. For those ZWO CMOS cameras, it should be around gain 120 or so. Around that gain is also some sensor work regime change, where camera read-out noise jumps suddenly, lower gains have much higher read-out noise. The linearity is (sometimes much) better at lower gain and it is possible to collect more signal (always think of photons!) into a pixel before signal will be distorted by non-linearity effects or filling pixel. At gain 0 or 1, you can collect the largest amount of photons into a pixel before saturation/non-linearities kick in. If your target is bright, you have a lot of signal and the read-out noise from the camera (which is highest at gain 0) is very small compared to the signal from the star itself. You want to collect as much signal as possible with not too short exposure times (8" telescope - I'd aim 20+ seconds to avoid scintillation, google for "gcx scint.txt"). If your target is rather faint, better increase exposure time to collect more photons AND possibly adjust gain a bit higher than 120 so that read-out noise is low[est/er] but the stellar signal stays still in linear ADU range. Your approach of 20...30 kADU is a good and safe rule of thumb from that perspective. But I'd really suggest to measure that limit by yourself. SharpCap has fortunately a nice camera testing tools built-in, so you can estimate what are your good and trustworthy limits of linear ADU range at different gain values. I'd suggest to select just very few, maybe even 2-3 fixed gain values to work with. A'la gain 0/1 for bright object, gain 120 for slightly faint ones and a bit higher gain (below 200?) for really faint targets. When analysing the linearity, it is a good idea to fit those points which look like forming a line and then subtract the fitted regression value from every measured point (actually: 100% * (measured value - fitted value)/fitted value) - that will give a graph of residuals, which usually tells much more detailed story about actual linearity of selected working regime. To have the best match between dark frames and photometric data, you really should take them not just at the same temperature but also definitely at the same gain. Dark frame depends on the temperature exponentially. So if you can afford cooling your camera down to -10 C or even lower, you'll get better results especially when exposure times start to be in several tens of seconds and especially when your targets are relatively faint ones. The amount of dark signal drops ~2x per each 6 degrees or so, so going from -5 to -17 would give ~4x smaller dark signal and also lower dark noise (which is one thing why you would like to keep your dark signal low).
Dear Gary: Thanks for the great presentation. I appreciate the quite detailed manner in which you stepped through the process. I assume that you used the SharpCap Sequence tool during post processing to avoid leaving something out by mistake. It did not appear that SharpCap was itself involved with the post-processing otherwise. With your Master Dark and Master Flat that you used during image capture, I assume that you had previously created those for a given setup. However, you changed the gain after checking the ADU level. Did that not necessitate a different dark file? Or did you just think that it would be a trivial difference? At the end, you showed a very nice light curve for V576 PEG. Does AIJ create a report suitable for submission to AAVSO, or do you have to do something else to get into their format? I also assume that you would report this sequence as "TG". Do you do a transform to get it into Johnson V? Thanks again. Rick
Sorry for the delay in responding, Rick. We moved house recently and still trying to get back into the swing of things. I do use the SC sequencer to guide me through setup, imaging, and post-processing. Often it's just text prompts to say "do this" but it keeps me on track. Yes, I did change the gain, and ideally should have redone the master dark. However, at these relatively low gains, hot pixels are not much of an issue. AIJ does not produce a report suitable for AAVSO submission, but it's straightforward to cut-and-paste the results into an Excel spreadsheet that can be submitted via WebObs. I will be producing a video on this soon. Yes, I report as TG, but it is possible to transform to Johnson V. However, I don't do this as it is currently not accepted practice.
Rick, V5.1 of AIJ now includes a macro for generating AAVSO WebObs-compatible files. It's a great addition to the program. It can be found by selecting the Create AAVSO Variable Star Report.... option on the File Menu of the Multi-plot Main window.
I used several scripts, but the most used was the startup script. It is mainly a series of prompts to make sure I do everything in the correct order: SEQUENCE PROMPT "Has the mount been leveled? Is physical hardware connected? Press OK to Continue." ENABLECONTROLS False PLAY SOUND Alert PROMPT "Are COUNTER BALANCE WEIGHTS in place? Press OK to Continue." ENABLECONTROLS False PROMPT "Is WiFi connected? Press OK to Continue." ENABLECONTROLS False PROMPT "Is PC time correct? Press OK to Continue." ENABLECONTROLS False PROMPT "Switch on Mount? Press OK to Continue." ENABLECONTROLS False PROMPT "Check DM for Comm Port. Press OK to Continue." ENABLECONTROLS False PROMPT "Select Comm Port via Toolbox. Press OK to Continue." ENABLECONTROLS False RUN "C:\Program Files\Ciel\skychart.exe" WITH PARAMS "" WAIT False PROMPT "Connect CdC to Telescope/Unpark Mount/Initiate Tracking. Press OK to Continue." ENABLECONTROLS False PROMPT "Set CdC Observatory/Check Lat/Long. Press OK to Continue." ENABLECONTROLS False PROMPT "Update Comet/Asteroid/Sat Database CdC. Press OK to Continue." ENABLECONTROLS False PROMPT "Check EQMOD Lat/Long Correct. Force Epoch J2000 - Advanced Setting Telescope. Press OK to Continue." ENABLECONTROLS False RUN "C:\Program Files (x86)\PHDGuiding2\phd2.exe" WITH PARAMS "" WAIT False PROMPT "Connect PHD2 to Mount/Check Correct Guidescope Selected. Press OK to Continue." ENABLECONTROLS False CAMERA OPEN "ZWO ASI533MC Pro" LOAD PROFILE "533MC DSO (ZWO ASI533MC Pro)" PROMPT "Select Darks and Flats. Press OK to Continue." ENABLECONTROLS True PROMPT "Focus and Polar Align OTA. Press OK to Continue." ENABLECONTROLS True END SEQUENCE
Thank you, great content
Hello Gary. Thanks for your video. I used this to do photometry for the first time. I could not work out Muniwin or AIP4Win or MetroPSF packages...But is it better to use a mono camera with a V filter. Why did you not use that instead of a colour camera?
Interesting overview of your workflow! Very well demonstrated! I learned some practical tricks from you, although I have done photometry for more than 20 years..
There is however something I'd encourage to test. Namely, in CMOS cameras that "gain" is changing how many electrons form one ADU (digital signal unit) on image and it does not change your input signal - the amount of photons. In some sense, the "sweet spot" is often when 1 photon (which frees 1 electron in detector material) is creating 1 ADU. For those ZWO CMOS cameras, it should be around gain 120 or so. Around that gain is also some sensor work regime change, where camera read-out noise jumps suddenly, lower gains have much higher read-out noise. The linearity is (sometimes much) better at lower gain and it is possible to collect more signal (always think of photons!) into a pixel before signal will be distorted by non-linearity effects or filling pixel. At gain 0 or 1, you can collect the largest amount of photons into a pixel before saturation/non-linearities kick in.
If your target is bright, you have a lot of signal and the read-out noise from the camera (which is highest at gain 0) is very small compared to the signal from the star itself. You want to collect as much signal as possible with not too short exposure times (8" telescope - I'd aim 20+ seconds to avoid scintillation, google for "gcx scint.txt").
If your target is rather faint, better increase exposure time to collect more photons AND possibly adjust gain a bit higher than 120 so that read-out noise is low[est/er] but the stellar signal stays still in linear ADU range. Your approach of 20...30 kADU is a good and safe rule of thumb from that perspective. But I'd really suggest to measure that limit by yourself.
SharpCap has fortunately a nice camera testing tools built-in, so you can estimate what are your good and trustworthy limits of linear ADU range at different gain values. I'd suggest to select just very few, maybe even 2-3 fixed gain values to work with. A'la gain 0/1 for bright object, gain 120 for slightly faint ones and a bit higher gain (below 200?) for really faint targets. When analysing the linearity, it is a good idea to fit those points which look like forming a line and then subtract the fitted regression value from every measured point (actually: 100% * (measured value - fitted value)/fitted value) - that will give a graph of residuals, which usually tells much more detailed story about actual linearity of selected working regime.
To have the best match between dark frames and photometric data, you really should take them not just at the same temperature but also definitely at the same gain. Dark frame depends on the temperature exponentially. So if you can afford cooling your camera down to -10 C or even lower, you'll get better results especially when exposure times start to be in several tens of seconds and especially when your targets are relatively faint ones. The amount of dark signal drops ~2x per each 6 degrees or so, so going from -5 to -17 would give ~4x smaller dark signal and also lower dark noise (which is one thing why you would like to keep your dark signal low).
Yep, thanks for taking the time, it sure helps.
Dear Gary: Thanks for the great presentation. I appreciate the quite detailed manner in which you stepped through the process. I assume that you used the SharpCap Sequence tool during post processing to avoid leaving something out by mistake. It did not appear that SharpCap was itself involved with the post-processing otherwise. With your Master Dark and Master Flat that you used during image capture, I assume that you had previously created those for a given setup. However, you changed the gain after checking the ADU level. Did that not necessitate a different dark file? Or did you just think that it would be a trivial difference? At the end, you showed a very nice light curve for V576 PEG. Does AIJ create a report suitable for submission to AAVSO, or do you have to do something else to get into their format? I also assume that you would report this sequence as "TG". Do you do a transform to get it into Johnson V? Thanks again. Rick
Sorry for the delay in responding, Rick. We moved house recently and still trying to get back into the swing of things. I do use the SC sequencer to guide me through setup, imaging, and post-processing. Often it's just text prompts to say "do this" but it keeps me on track. Yes, I did change the gain, and ideally should have redone the master dark. However, at these relatively low gains, hot pixels are not much of an issue. AIJ does not produce a report suitable for AAVSO submission, but it's straightforward to cut-and-paste the results into an Excel spreadsheet that can be submitted via WebObs. I will be producing a video on this soon. Yes, I report as TG, but it is possible to transform to Johnson V. However, I don't do this as it is currently not accepted practice.
Rick, V5.1 of AIJ now includes a macro for generating AAVSO WebObs-compatible files. It's a great addition to the program. It can be found by selecting the Create AAVSO Variable Star Report.... option on the File Menu of the Multi-plot Main window.
Gary can you share your Sharpcap script?
I used several scripts, but the most used was the startup script. It is mainly a series of prompts to make sure I do everything in the correct order:
SEQUENCE
PROMPT "Has the mount been leveled? Is physical hardware connected? Press OK to Continue." ENABLECONTROLS False
PLAY SOUND Alert
PROMPT "Are COUNTER BALANCE WEIGHTS in place? Press OK to Continue." ENABLECONTROLS False
PROMPT "Is WiFi connected? Press OK to Continue." ENABLECONTROLS False
PROMPT "Is PC time correct? Press OK to Continue." ENABLECONTROLS False
PROMPT "Switch on Mount? Press OK to Continue." ENABLECONTROLS False
PROMPT "Check DM for Comm Port. Press OK to Continue." ENABLECONTROLS False
PROMPT "Select Comm Port via Toolbox. Press OK to Continue." ENABLECONTROLS False
RUN "C:\Program Files\Ciel\skychart.exe" WITH PARAMS "" WAIT False
PROMPT "Connect CdC to Telescope/Unpark Mount/Initiate Tracking. Press OK to Continue." ENABLECONTROLS False
PROMPT "Set CdC Observatory/Check Lat/Long. Press OK to Continue." ENABLECONTROLS False
PROMPT "Update Comet/Asteroid/Sat Database CdC. Press OK to Continue." ENABLECONTROLS False
PROMPT "Check EQMOD Lat/Long Correct. Force Epoch J2000 - Advanced Setting Telescope. Press OK to Continue." ENABLECONTROLS False
RUN "C:\Program Files (x86)\PHDGuiding2\phd2.exe" WITH PARAMS "" WAIT False
PROMPT "Connect PHD2 to Mount/Check Correct Guidescope Selected. Press OK to Continue." ENABLECONTROLS False
CAMERA OPEN "ZWO ASI533MC Pro"
LOAD PROFILE "533MC DSO (ZWO ASI533MC Pro)"
PROMPT "Select Darks and Flats. Press OK to Continue." ENABLECONTROLS True
PROMPT "Focus and Polar Align OTA. Press OK to Continue." ENABLECONTROLS True
END SEQUENCE