Stacking help

[Anthonyexmouth]

Am i expecting too much from the stacking process ability to remove dust motes? 

I can see the motes in my flats but they are also there after stacking in the final image, fairly large ones. 

will add a flat and stacked image for anyone whos interested. Stacking in DSS mainly, sometimes APP but still confused about some of the settings there. 

F_2020-03-28_16-59-09_4.5s_294mc__Color_G125_OFS22.fit

Autosave.tif

 

[wimvb]

To see if a master flat works, you can try to calibrate a single flat frame with your master flat and dark/bias. That should give you a perfectly calibrated and flat image with only some noise.

Flat frames need to be calibrated with either bias (ccd, dslr) or darks (cmos), or they won't work properly.

[vlaiv]

I'm failing to download Autosave.tif for some reason, but I wonder

How come that your single flat sub is 22MB while stacked version is 400MB - that is x20 increase in size. Maybe you made mosaic or something?

In any case - from fits header on flat, I see that you are using APT - and it seems that people have issues with APT and automatic calibration frames. I'm not sure why is that, I personally don't use APT so can't tell what might go wrong.

Don't use any automated process or wizard - shoot all your subs on manual settings. Take dark, flats and flat darks (plenty of each) and of course pay attention that darks are matching lights in all settings and that flat darks are matching flats in all settings.

[Anthonyexmouth]

4 hours ago, vlaiv said:

I'm failing to download Autosave.tif for some reason, but I wonder

How come that your single flat sub is 22MB while stacked version is 400MB - that is x20 increase in size. Maybe you made mosaic or something?

In any case - from fits header on flat, I see that you are using APT - and it seems that people have issues with APT and automatic calibration frames. I'm not sure why is that, I personally don't use APT so can't tell what might go wrong.

Don't use any automated process or wizard - shoot all your subs on manual settings. Take dark, flats and flat darks (plenty of each) and of course pay attention that darks are matching lights in all settings and that flat darks are matching flats in all settings.

I was hoping I'd avoided the apt problem by not using the flats aid and just eye balling it and shooting manually. The stack is bigger because I drizzled in DSS to see if the same thing happened. Are you saying I should maybe use a different capture software to get the calibration frames? 

[vlaiv]

Just now, Anthonyexmouth said:

I was hoping I'd avoided the apt problem by not using the flats aid and just eye balling it and shooting manually. The stack is bigger because I drizzled in DSS to see if the same thing happened. Are you saying I should maybe use a different capture software to get the calibration frames? 

If you are already using manual exposure for both flats and use the exact same exposure length and settings for flats and your darks are good (shot at exact same exposure/settings as your lights) - you should be ok calibration wise.

What is left to do is check if you have any light leaks, and make sure you turn of dark optimization in DSS.

Don't use drizzle - there is no point.

If you are using latest version of DSS, make sure that:

a) Dark optimization is turned off (it is turned on by default I believe in latest version)

b) Dark multiplication factor is not applied

Both settings are under Dark

[Anthonyexmouth]

13 minutes ago, vlaiv said:

If you are already using manual exposure for both flats and use the exact same exposure length and settings for flats and your darks are good (shot at exact same exposure/settings as your lights) - you should be ok calibration wise.

What is left to do is check if you have any light leaks, and make sure you turn of dark optimization in DSS.

Don't use drizzle - there is no point.

If you are using latest version of DSS, make sure that:

a) Dark optimization is turned off (it is turned on by default I believe in latest version)

b) Dark multiplication factor is not applied

Both settings are under Dark

Ill check that as soon as I get home.

Did you see the large motes in the flat? What could I be doing wrong that would make them still show in the stacked image? 

I carried out the new set of calibration at night to make sure there was no light leak, put the 2 covers over the pier too in case neighbours turned any lights on. 

[vlaiv]

14 minutes ago, Anthonyexmouth said:

Ill check that as soon as I get home.

Did you see the large motes in the flat? What could I be doing wrong that would make them still show in the stacked image? 

I carried out the new set of calibration at night to make sure there was no light leak, put the 2 covers over the pier too in case neighbours turned any lights on. 

Under or over correction happens when there is some sort of other signal than light in flats and your lights (dark not removed properly, light leak, or some manipulation with dark - like dark optimization).

Under correction is when dust shadows are still seen as being darker. Over correction is when dust shadows and vignetting becomes brighter than it should be.

Under correction happens when "flat is stronger" or "light is weaker".

It is very hard to get weaker light, but it can happen. For example - darks taken when camera was hotter, will create darks with higher value (more dark current buildup due to higher temperature) and you subtract those higher darks from lights - you end up with lower value lights (subtract more than you should - you end up with less).

Other thing that can happen is stronger flats (higher values than needed in flats) - this happens if one uses bias instead of flat darks for example as bias don't contain dark current signal that is present in flat darks hence bias is lower in value and again - you subtract those so it will take away less leaving higher value.

Dark optimization will do that if you don't use bias and have stable bias files (mostly with CCDs and DSLRs - it looks like CMOS astronomy cameras have issues with bias). Dark optimization is multiplying dark to get it to proper value - but if you don't remove bias from darks - you'll multiply bias as well - and that is bad since bias does not depend on either temperature nor on exposure length - and you assume it does and try to compensate for any of those two. This again makes more or less of bias signal and messes up flat calibration as a consequence.

[Anthonyexmouth]

Just gonna give it a go taking flats with SharpCap. It suggests taking monochrome flats. Should I do that or tick the colour box?

[vlaiv]

Take mono flats.

[Anthonyexmouth]

Just been reading the SharpCap docs and does it make a master flat for you? thats kinda the way i'm reading it. Anyway, heres the result. I set it to output a .tiff but still outputs a .png

 

[vlaiv]

How about doing it without "assistant" or "wizard" or "helper" and such?

Just plain old "capture image" and you set exposure and parameters and capture some flats and then repeat with same parameters but this time cover the scope to capture flat darks.

[Anthonyexmouth]

2 minutes ago, vlaiv said:

How about doing it without "assistant" or "wizard" or "helper" and such?

Just plain old "capture image" and you set exposure and parameters and capture some flats and then repeat with same parameters but this time cover the scope to capture flat darks.

I'll have a crack at that when its dark, will be easier to avoid light leak as its a permanent garden setup.