Advice on flats please.

[batema]

Hi,

I don't know if this is the right section but I have done the t shirt at dawn for flats and stacked them in DSS to get this result. I have a QHY-9 with gain 12 and offsey 112. Does this master flat look OK??????? I have no idea. I tried to insert a graph I got from canon zoom brouser a but too big.

Mark

[gregbradley]

What was the scope and setup?

It seems very even. Most of my flats show more uneveness no matter what scope. Some scopes seem more even than others and some show round bright circle with darker outside edges (FSQ with reducer for example).

I even had one that showed bright squares in the centre for some reason (filter related?).

How many images, how bright is the exposure (ADU) how did you stack them?

Greg.

[ptc]

Greg: square filters that don't have blackened edges will give clearly visible artifacts in the flats. Most pronounced in the green, clear, [OIII] and sometimes blue filters. Typically not so bad with Red and Halpha

you ought to blacken the edges. See this:

http://www.edmundoptics.com/techSupport/resource_center/downloads/wp-edge-blackened-optics.pdf

ciao
rdc

[batema]

Sorry,

Shot using a William Optics Flt 11o with QHY-9. I took 10 shots at 0.5seconds and then 5 shots at 1 second and combined them. I was watching the graph as the sun came up but wanted to keep it roughly the same. I don't know what ADU means and they were stacked in Deep Sky Stacker using median.

Thanks

Mark

[gregbradley]

Hi Richard,

I typically coat the edges of my felt pen with a black sharpie pen. Not sure if I did for those on the Tak BRC though. Was a while ago. Perhaps they weren't, as you say.

Cheers,

Greg.

[gregbradley]

Shot using a William Optics Flt 11o with QHY-9. I took 10 shots at 0.5seconds and then 5 shots at 1 second and combined them. I was watching the graph as the sun came up but wanted to keep it roughly the same. I don't know what ADU means and they were stacked in Deep Sky Stacker using median.

Thanks

Mark[/QUOTE]

Good point, I am not sure what ADU stands for. The last letter U no doubt stands for "unit". The A and D not sure but it is a measure of the brightness level in the image. So a high ADU would be a bright star and a low ADU would be the dark background of many astro images.

I finally found what the abreviation stands for "analog-digital units".

Basically a measure of how many electrons were in each pixel after an exposure is my limited understanding ie dark areas would be a low number, bright areas would be a high number.

Full well capacity is how many electrons each pixel can hold. Richard here has posted a paper about getting 80% of the full well capacity as your level for flats for minimum noise.

Generally median combine is a common stacking method.

So the ADU number is important as it gives a quantitative measure to expose your flats to. If it were broad daylight then super short flats may be OK but only if your camera has an electronic shutter. I don't see any shutter artifacts in your flats so I assume it does have an eletronic shutter. If it is a mechanical shutter generally you need longer exposure times like 3 seconds to avoid seeing the shutter in the flat.

I do my sky flats at dusk or dawn, usually dusk these days.

My flats do not look like yours and tend to be more uneven and show some minor (or major as with the FSQ106ED and reducer) vignetting depending on which scope and setup.

You should be able to see the light dropoff at the corners of your light exposures and see that the flats would correspond to this. If your lights are very even then flats are almost unnecessary in my opinion for pretty pictures.

Greg.

[ptc]

I didn't find that the sharpie was opaque enough. That was my first choice. I ended up using model car paint and a paintbrush. Tim Khan found a source for the Testors flat black paint in a pen......

;)

[gregbradley]

Hmm, I might have to get one of those. I'll ask him where he got it from.

Cheers,

Greg.

[Paul Haese]

Mark just some observations.

I have my flats dead center of the histogram, so I play around with it until it is quite close to that mark or if over exposing then I have it to the left of dead center.

Secondly, one should see the dust motes in your image if it is correctly exposed. I cannot see any of them in your flat. So again take a look at the exposure.

You are close to getting them right.

[mill]

With the QHY9 you cant take a 0.5 Second flat because of the shutter.
Flats have to be longer than approx 2 seconds so the shutter doesn't show.
Something is wrong here because this flat is way too even.

[batema]

Thanks all. I will need to get a regulator on my flat box and have another go. All good fun.

Mark