SharpCap Forums

Hi, I am using the latest version of SharpCap Pro
Using a asi2600mc ( colour camera and Skywatcher Esprit 150 )
I am just looking for a little bit more advice and clarification on capturing flats if possible please.

So the asi2600mc is the colour camera version
I plan to use like stacking and the exact results from the smart histogram, so possibly 70 second + exposures.

So in the capture flats drop down box,
Am i correct in saying that:
I should click the box for Capture and subtract bias frames to correct black level offset in flat?
I should click the box for Create monochrome flat?
I should click the box for Apply new flat when capture is complete?

For the flats histogram, i should be looking at getting the white line of the histogram at around the 50% mark and also get the blue line as close to the 20% as possible and the red line at around 80%
In other words, get the cloured lines at around 20% and 80% but not over?

The last question is gain, i know i should be keeping everything the same, but lets say i am planning to take the lights at gain 100 as per the recommended results setting from the smart histogram,
Would i be better off taking flats at gain 0 rather than gain 100 or just keep everything at gain 100?

Is it best to try and get the flats exposures 1 second + rather than say 0.5 seconds to get a better flat result?

Sorry for the multiple long drawn out questions, jut trying to get a real good understanding on how i should be taking these flats, I think i have been playing around with flats for too long now not fully understanding what i should be doing.

Any advice on these questions would be greatly appreciated,
Thanks.

Hi,

the options that you quoted for capturing flats (use bias, monochrome, apply) are usually the best options. If you intend to take flats with long exposures (more than a couple of seconds) then the option to use dark flats may be better than the bias option.

For the histogram, it sounds about right. The main goal is to keep the curves out of the 90-100% range and out of the 0-10% range (0 to 20% if you can manage it). Keep as much as possible in the middle of the histogram.

There's no harm in taking flats at lower gain, in fact there is often an advantage, since by reducing the gain you need to increase the exposure to keep the brightness right, which captures more photons per flat frame and means the individual flats are less noisy. If you have high gain, noisy, individual flats then you need to use a larger number of them, which takes longer to capture and process.

cheers,

Robin

Thanks for the reply and your advice on this, very much appreciated.
So basically if i am reading this correct, if am to be say taking 70 second images, I am better of to leave the box labeled Capture and subtract bias frames to correct black level offset in flat unchecked, is this correct?
And then just use the darks ( i did forget to mention that i was also looking at capturing darks as well as the flats )

It is interesting to hear that i could take the flats at a different gain to the lights and darks.
It might be worth while trying it out with both gains to see the effect it has.
I know if you are stacking the frames say in ASTAP, it doesn't like you having different gain settings and reports error while stacking them.

But would like to try it out with SharpCap Pro live stacking to see how things go.

Very much appreciated.

Hi,

sounds like we have crossed wires somewhere...

You cannot use your dark frames (70 second exposure) to substitute for flat dark frames or bias frames - that's because the exposure will completely fail to match the flat frames by a big margin.

To get good flat correction for deep sky, you *must* do one of the following

* Use the bias frames option in the SharpCap capture flat tool
* Use the dark flat frames option in the tool
* (If you like doing things the hard way) create a separate dark frame at the same gain/exposure you intend to use for your flats and select it before using the capture flat tool. To be honest, this option is only theoretical - the same results will be obtained using the dark flat option above with less effort.

There is good reason to take your flats at maybe 0.1s to 2s exposure with minimum gain rather than shorter exposures at higher gain - that is the requirement to use less flat frames for the same level of noise reduction in the master flat.

Does Astap really worry about gain differences between flats and lights? That doesn't seem quite right... It should check that lights and darks have the same exposure and gain and that flats and dark flats also match each other, but there is no need for all four to be the same.

cheers,

Robin

Thanks again for putting up with all these questions, it is really appreciated.
I did give it a try last night and results were to be honest terrible.
I am doing 111 second exposures as per the brain function results, also clicked apply to add all the results for the imaging run

So when i did the capture flat frames, I unchecked the box labeled: Capture and Subtract bias frames to correct for back level offset in flat
Should i have done this or not?
I added the master flat frame to the Apply flat section under the processing tab

I then did a capture dark frame, I assumed this was a dark frame that was the same exposures as the lights (111 second exposures )
Or should this of been dark flats? Same exposure as the flats?

I added these to the Subtract dark section in the processing tab.
I let it stack for a bout an hour and the result was a poor grainy image even after playing around with the options.

Did i get the incorrect darks?

Just trying to work trough where i may have gone wrong.
Any advice is appreciated.

Hi,

the best procedure is

* Capture the flat frame first. Leave the 'subtract bias' box ticked - if you untick this and do not take care of the subtraction some other way then you will get poor results

* Once the flat is captured and ready, set the 111s exposure and gain that you need to use for imaging, then capture dark frames

* Once the dark is captured, make sure the dark and flat are both selected and you can start capturing.

Hope this helps,

Robin

cheers, Very much appreciated.
T can see where i have made the error now.
I will put it back into practice next time the clouds are free.
Very much appreciated.