Flat Field Exposure Length in Maxim with 314L+?

[AstroManDan]

Hi, I have an Atik 314L+ and am using Maxim DL together with an EL light panel inside perspex.

What length of exposure should I be looking for?

Is it the "Max Pixel" Stats on the Camera Info pane I should be looking at?

Thanks

Daniel

[roundycat]

Whatever exposure gives you about 20,000ADU as an average of the whole image. Use MaxIm's info palette with the readout set to Average. You may need to adjust up or down when you try your master flat if it under or over corrects.

Dennis

[NickK]

I needed to use a cotton t-shirt folded a few times to reduce the luminosity! With it I can get a 1 second exposure at about 1/3 pixel value (i.e. about 20K out of 65K).

[AstroManDan]

Thanks.

In the 'Information' pane in Maxim I can see 'Average' as one of the fields in the window (is that what you mean?), but there are 3 different modes with this in: 'Aperture' - 'Region' - 'Area'

Which one do I choose?

I am using Maxim v5.

[AndyUK]

Hi Dan - I was also taking some test flats over the weekend with my EL Panel, ATIK 314L+ and MN190 .

I'm certainly no expert in MaximDL5, but in the information window, I believe that Area gives you the Average across the whole frame (whereas Aperture gives you the values where the cursor is), but there shouldn't really be all that much difference really...

Obviously everyone's settings will be different, (and I don't know if it wil be of any help) but in order to get figures about 1/3 full ADU (+/-), I got exposures (taken through Artemis Capture) as follows - L=0.03, R=0.16, G=0.09, B=0.055, Ha=3.5. (I haven't tried to stack them yet though to see what the averaged ADU will be).

[davew]

Hi all,

Can I suggest you try this site ? - Sky Flats Assistant MaximDL Plugin His plug in is so easy to use and gives such good results I don't know why no one ever mentions it.

Here's a quick and simple approach. Go into Maxim and take a flat. Open the screen stretch window and switch to " Range ". Now let's imagine the lower figure is 20000 and the higher 25000. That would make the middle around 22500. If it's way above what you want, lower the exposure time and if it's too low, go longer.

Now we will have a base figure to start with.

Open the plug in. Tell it what you want. Say 20 flats, 20000 ADU . Tell it the figure you found for the exposure and let it do it's stuff.

This plug in as you will see was intended for sky flats but is still very good for panels. It will sort all your flats out for you. It works quite well at dawn so long as you're a bit quick off the mark and give it it's best chance.

Does that help ?

Dave.

[davew]

It's easy to follow what Dennis said now. If the flat isn't correct then open the plug in up again and change the ADU setting.

Let's say you tried 20000 and feel you want to try 25000. Just tell the plug in and it will automatically change the exposure length to suit.

With an 8300 chip I aim for around 3 seconds by masking the light with paper. You could go quicker.

Dave.

I've tried 20000 to 30000 and have settled on 25 / 28000. According to a Microscope site I read ( science based ), they said that so long as you don't saturate the chip higher is better. Then you read what gets said here and it doesn't seem to hold true. Try it and see what gives the best results.

Just re read that bit and it sounds like I'm saying people here aren't correct. Not so.

[MartinB]

That looks like a good plug in thanks Dave

You need to stay within the linear response of the chip for flats to work. To work out the max ADUs you need to know the full well depth and the gain of your camera and then look at your chips response curve. This is probably a worthwhile excercise if you have very limited exposure time e.g. when taking sky flats. However, with an EL panel you don't have these constraints so you can stick with a lower value.

20 000 should be fine but less will be also be good. The big issue then is how many flats. If you are using lower ADU levels you just need to collect more flats. I aim for 1 000 000 ADUs worth of flats, so 50 flats at 20 000 ADUs or 100 flats at 10 000 ADUs

[davew]

Ah yes, I forgot to mention the number of flats as against the ADUs !

He's just updated the plug in. I've not used version 3 yet.

Dave.

[roundycat]

sorry Dan - Andy answered the question re which Info setting. It is Area, using anything else will give you greatly varying values.

Martin is following trusted advice with the very high ADU aggregate. The simple reasoning is that a high ADU value, when averaged to make the flat, will provide a master with very low noise.

I found that shooting flats with an ADU of about 9,000 or less and using ten of them in various tests gave me 'quiet' enough flats that I do not worry about their added noise. It is well below shot and readout noise. I do tend to use twenty but the point is a much smaller number of relatively low level flats is not going to destroy your pictures.

Dennis

[AstroManDan]

Just ytring this great plug-in out, and when you said:

"Tell it the figure you found for the exposure and let it do it's stuff"

Which field is this?

Initial Exposure Time

Min Exposure Time

Max Exposure Time

Also if I bin the R,G,B channel images at 2x2, should I also bin the flats at 2x2 as well to match?

Thanks

Daniel