QHY8 Flat values

[blinky]

I am taking 0.5sec images with my QHY8 at the moment and Maxim shows the average as 23,422.152 - all looks good. I also see the max value is 65,535 - I assume this is not meaning that the image is overexposed, rather that a couple of hot pixels are fully illuminated - again not a problem. But..... Why does the minimum show as 0 - I assumed since I have an offset of a little over 100, that the minimum would be that value????

[adamsp123]

My guess Craig, it is over exposed, I just had a look at some of my flats and the max is no more than 40,000.

Because my Atik has a mechanical shutter I need minimum exposures of 4 secs!! I use a dim light and have a white T shirt over the scope.

[blinky]

Interesting Pete - I assume you have a few hot pixels the same as me and you dont get a max value of 65535. Hmm..... I think I will try again drop the exposure further to see what effect it has.

[adamsp123]

Oh I should say I use a dim low energy bulb in a desk lamp illuminating a large-ish oil painting canvas, light to canvas about 5ft away.

Flat attached- the dark areas in the corner are around 32,000

HTH Pete

[blinky]

Hmmm, OK, I am about to nip outside and try smaller subs for the flats.

[roundycat]

you could use a hot pixel filter on a single flat, that will tell you if you have rogue pixels. Passing the cursor over the centre of the flat (Info palette set to Aperture) will give you a good guide as to how bright the average is around the centre. You may need a shorter exposure.

Dennis

[martin_h]

When I shoot my flats (QHY8) I go for aMAX 22,000 using an EL panel and 3 sheets of A4 and get about 2 or 3 secs exposure.

[blinky]

weird.... I lowered the exposure till I got the same sort of readings as you guys and my flat is horrible! It's really fuzzy & blotchy! Not over or underexposed but it looks like certain areas are more sensitive that others.

I think I will got back to the same exposure as this morning as it seems to be better. I would post the flat but it's on the Obsy PC and I forgot to copy the image up and have shut down the obs.

[roundycat]

In a way flats are supposed to be blotchy. What you are probably seeing is the dirt and dust in the optical system, all out of focus. That is what the master flat removes from each light frame.

If you go back to the longer exposure you owe it to your self to ensure that you are not overexposing and thereby rendering all variations in exposure (for whatever reason) equal. Such a flat will not work.

To test your master flat apply it to a single flat, you should get uniform brightish grey everywhere.

Dennis

[blinky]

Never actually thought of applying it to a single frame! Will try that with the new and the old flat.

[adamsp123]

In a way flats are supposed to be blotchy. What you are probably seeing is the dirt and dust in the optical system, all out of focus. That is what the master flat removes from each light frame.

If you go back to the longer exposure you owe it to your self to ensure that you are not overexposing and thereby rendering all variations in exposure (for whatever reason) equal. Such a flat will not work.

To test your master flat apply it to a single flat, you should get uniform brightish grey everywhere.

Dennis

I never thought of doing the flat on flat thingy, well just tried it and "cracking Gromit" a nice even light grey colour all over.