Flat mess :(

[Earl]

A little backgroud to the camera im using.

I use a QHY8L and this has a shutter issue which means subs have to be over 4 seconds.

I have Bias, Dark Flats and Flats.

I calibrate the Dark Flats with the Bias Master

Next I calibrate the Flats with the Bias master and the Dark Flat Master.

Then Integrate the calibrated Flats and I get....

I must be doing something wrong here.

I does look like its going in the right direction though

[Earl]

Should I not use the bias frames twice?

[rfdesigner]

It's best to think about what each image contains:

Dark = dark current + bias (long dark frames)

Flat = bias + flat_signal (assuming it's a short exposure)

Bias = bias (short dark frames)

So

Master_Dark = Dark - Bias

Master_Flat = Flat - Bias

To process image ( subs = dark current + bias + signal * flat )

Image = ( subs - Bias - Master_Dark ) / Master_Flat

Hope this helps

Derek

[Earl]

Cheers for the info Derek, but how do Flat Darks fall into that?

[rfdesigner]

Flat Darks.. Just so I know what you mean.. what do YOU mean by a 'flat dark'?

[Earl]

As my Flats are over 4 seconds I have a set of Darks to match them.

Dark Flats according to DSS

[rfdesigner]

Ok.. I'd treat the 'flat-darks' as bias frames... the dark current in them is going to be all but zero.

In maths:

lets assume dark current is 1e/s and readout noise is 10e.

primary CCD noise = SQRT(readoutnoise^2 + dark_current/s * exposure_time)

So 10e readout noise and 1e/s dark current means your noise will double (in power terms) when the imaging time is 100 seconds. If you have a 50 second imaging time it would increase the 'glow' by only 0.22 magnitudes.. with a 4 second 'flat-dark' your CCD noise is utterly dominated by readout noise.

Don't believe me?.. (and scientifically speaking you shouldn't) take some 4 second darks, average them, then take some 8 second darks average them, then subtract the two images and inspect the result.. you should see very very little.

Derek

[martin_h]

Try just using Darks, Flats, and Bias. Dont do anything to them beforehand, just chuck the lot into DSS with some Lights and see what happens.

If you are going to use Dark Flats dont use Bias.

[Earl]

If you are going to use Dark Flats dont use Bias.

It does look like the images have had Bias added to them.

[Earl]

If I use DSS with just Lights, Flats, Darks and Bias I get this.

DSS was fine when i was using my Canon, but using a CCD im having no joy :|

Stretched using STF in PI and saved as jpg

[Earl]

If I use PI to do all the magic I get this (after DBE) without flats

[martin_h]

If thats the case, it looks like there is something wrong with the flats then.

[rfdesigner]

I use IRIS. Although it includes menu options that 'do it all for you' I prefer to use scripts as I keep control of what it is I'm doing.

I'd suggest doing something similar, I'm sure DSS must be able to run scripts or something.

Derek

(I do similar things at work, I hate kit that tries to be clever.. it frequently gets it wrong and you're left wondering what to do.. much better to build your own automated script then you have it all under your own control and understanding)

[Earl]

If thats the case, it looks like there is something wrong with the flats then.

heh... thats my whole point.

However exactly what is the issue is beyond me this is a single flat debayered grey scaled then stretched

[rfdesigner]

I wouldn't debayer prior to flatting/dark removing etc etc

Try treating it as a B&W camera until you've corrected the entire image.... THEN debayer it. (particularly important for darks.. you will be able to correct hot pixels much better)

Derek

[Earl]

Humm

Is it me or are these big circles reflections also.....

I have come to the conclusion that the flats are scrap.

But that still does not answer why there is all the edge stuff going on which has to be amp glow and other related issues.

I need to sort out this reflection issue, which also showed up in a thread i posted with m45.

[cgarry]

If I use DSS with just Lights, Flats, Darks and Bias I get this.

DSS was fine when i was using my Canon, but using a CCD im having no joy :|

Stretched using STF in PI and saved as jpg

That is interesting, I get exactly the same problem when I use DSS with images from my QHY8L. It used to work just fine with my DSLR though.

I now use Astroart 5 for stacking and it does actually work with images from my QHY8L.

Cheers,

Chris

[martin_h]

How do you take your flats? I have the QHY9 which has a shutter and use over 4 sec exposures.

I would give the sensor a clean and re-do the flats.

[Earl]

I use a lightbox, again use a longer exposure due to the 3.97 issue.

Most advice I have read on flats does not cover the issues i have with the qhy8l.

Im going to strip it all down and have a look to see if i can see whats going on.

when i was using a dslr. switch to AV snap snap done.. :|

[martin_h]

Why dont you drop Mike Wilson a pm...he uses the QHY8l... Stargazers Lounge - View Profile: MikeWilson