Flats for 8" sct + sxv h9 - exposure length

[Demonperformer]

I have been thinking about flats.

I know that when using the canon, I should just set it on automatic, but what to do when I am using the ccd.  It seems to me that for a given scope and camera, the exposure time should be constant.

So, in order not to re-invent the wheel, is there anyone out there who uses a 8" sct with reducer (I will be using f5, but I guess f6.3 will get me in the ball-park) and an SXV H9 camera (it uses the Sony ICX285AL Exview HAD CCD  chip, so I guess any camera using this chip would give about the same results) combo?  If so, what sort of exposures do you use for your flats?

Thanks.

[Chris-A]

It would depend on what light source you are using for your flats presumably. I've always understood it to be whatever exposure gets you to 20k-23k ADUs for the peak of the histogram.

[lensman57]

I have been thinking about flats.

I know that when using the canon, I should just set it on automatic, but what to do when I am using the ccd.  It seems to me that for a given scope and camera, the exposure time should be constant.

So, in order not to re-invent the wheel, is there anyone out there who uses a 8" sct with reducer (I will be using f5, but I guess f6.3 will get me in the ball-park) and an SXV H9 camera (it uses the Sony ICX285AL Exview HAD CCD  chip, so I guess any camera using this chip would give about the same results) combo?  If so, what sort of exposures do you use for your flats?

Thanks.

If your capture software has a histogram then use an exposure that gives you a histogram 1/3 to 2/5 of the way in. The ADU  reading of 20~ 25000 is almost the same thing with the caveat that your cursor must lie on a neutral background for this value to be valid, not so easy if the flat has a lot of uneven expoure and dust bunnies as mine always do. If you do not have a decent EL panel use your laptop or a tablet set to white.

A.G

[MeyGray3833]

Taking good flats can be tricky at first, however; it will click into place when you start geting into it.

Have a look on the net, there is a lot of info about how to do it with various light sources from sky flats to box flats.

The right flat for your set up depends entirley on your set up, so a level of experimentation is required to get the best ones. I use a camera with an 8330 Kodak chip on a Tak TSA 120 with a light box and from L to Ha exposure times vary between 2 to 8 seconds and I take 50 of each to integrate into a master. I aim for just under halfway on the histogram or approximaely 30,000 ADU.

[Demonperformer]

Thanks for the replies.

I see your point about it being dependent upon the light source (fairly obvious I suppose ).  I am thinking probably EL panel, when I can afford one, (1) for ease, and (2) probably more reliable than something I have cobble together myself.

I am trying to get into APT for my capture software.  This is primarily because astrotortilla uses the output.  Not sure if it has an ADU or a histogram - will have to go exploring.

The general concensus (if I am reading these replies properly) is that I will have to experiment with my own setup to see what times are required.  But presumably, once I have worked out a figure for (say) the R filter, it should be the same time each time I use that filter?  Or will I have to experiment every time?

In the back of my memory I think I read somewhere that you should take half your flats with the light source one way round, then rotate it through 180 (or maybe 90?) degrees to take the rest so that any imperfections in the light source are cancelled out.  Do people do this or is that overkill?

Thanks.

[Yoddha]

APT has histogram Go to Tools tab and click on Histogram...

The next version should have a tool for automatic determination of the flat exposure

[Demonperformer]

Thanks, Ivo.  Found it.

Automatic exposure length for flats would be really useful, I reckon.

Thanks.

[glappkaeft]

There are several ways to partially or entirely automating the process of taking flats. Most of them are only useful if you use the more advanced (and pricy) software suites like MaxIm DL or ACP though. If you look around you might be lucky...

[2stroke]

This will help http://astroromp.com/astronomy/shooting_flat_frames.htm

[ollypenrice]

You'll never get a standard exposure time that always works. It's very fickle and changes radically with filters.

Altough the standard advice is 1/3 to 2/3 of the way up the histogram I'd advise going lower than that. Sticking to around 13000 counts gives me the best flats. Some setups I use tolerate much higher values but not all.

Olly