AST8300 (KAF8300 sensor) flats

[chriscoles]

Hello, I have only just started taking flats. Below is a 5 second flat, using the white t-shirt method using the sky as illumination.

I have no idea if this image is suitable as a flat, and what the pattern means. Also shown is the histogram. Could somebody explain whats going on in the image and whether I should do anything to improve the process?

Many thanks in advance! 

[Xplode]

The pattern probably comes from you taking a screenshot of VNC, it's most likely a smooth gradient without the steps if you look at it locally.
If it works as a flat is something that can only be said for sure by actually calibrating an image with the flat.
It does look weird as a flat thou, but since you haven't said anything about your gear i don't have any info to guess how it should look

[Patrick Chevalley]

The right-left gradient can be a sign of non uniform light illumination of the t-shirt.
The image histogram look good, no problem with that. Try to reduce the histogram range from 35000 to 38000 to better show the dust on the preview.
The banding is probably because of colour reduction by VNC, try to open the image locally without using a remote session.

Patrick

 

[tooth_dr]

It doesnt look right for a flat.  Did you take the flats at night/in the dark?  I think extraneous light is a possible culprit here for the gradient to be running horiztonally.  I tend to shoot my KAF-8300 flats at 25000-28000 ADUs, it may not work for you, but works ok for me.

[chriscoles]

Hi, thanks for the replies.

The image was taken in the daytime and and the brightness should be relatively uniform as it isn't obstructed.

Reading about my camera,  AST8300, i put it in the title btw. IT apparently has a slow shutter speed, could the strange effect be the shutter? It isnt VNC, I just opened it locally.

I might bring the scope indoors and use my monitor as a light panel so I can experiment.

[AngryDonkey]

Hello,

I have the same sensor and usually ensure that the flats are 2s or longer to avoid any issues with the shutter (and that seems to work fine for my camera which is a Moravian 8300).

Mike

Edited February 2, 2020 by AngryDonkey

[chriscoles]

Hi, guys. Sorry but I got a bit confused. Actually VNC did have an impact. This is a screenshot from windows. The image is stretched. 

This is the same 5 second exposure.

 

The second image shows the histogram range from 35000 to 38000 to try and show dust... but i don't see any. Any thoughts?

 

[MarcusH]

The AST8300 has a mechanical shutter so make sure you have a long enough exposure to avoid shadowing from the moving shutter. Actually the shutter on the AST8300 is the eight position in the filter wheel so the shutter really is s-l-o-w (much slower than the mirror in a DSLR). I usually try to keep my exposures between 5 and 10 seconds to avoid any issues (I have the first gen. A-version) but that means you'll need a adjustable light source.

[chriscoles]

4 minutes ago, MarcusH said:

The AST8300 has a mechanical shutter so make sure you have a long enough exposure to avoid shadowing from the moving shutter. Actually the shutter on the AST8300 is the eight position in the filter wheel so the shutter really is s-l-o-w (much slower than the mirror in a DSLR). I usually try to keep my exposures between 5 and 10 seconds to avoid any issues (I have the first gen. A-version) but that means you'll need a adjustable light source.

Thanks, markus, mine is also first gen. Do you have an example flat file you could share so I can compare?

I was thinking of using my PC monitor as an adjustable light source, do you think that would work?

[tooth_dr]

37 minutes ago, chriscoles said:

Hi, thanks for the replies.

The image was taken in the daytime and and the brightness should be relatively uniform as it isn't obstructed.

Reading about my camera,  AST8300, i put it in the title btw. IT apparently has a slow shutter speed, could the strange effect be the shutter? It isnt VNC, I just opened it locally.

I might bring the scope indoors and use my monitor as a light panel so I can experiment.

What scope is it?

[chriscoles]

1 minute ago, tooth_dr said:

What scope is it?

Celestron C11 SCT with 0.5 focal reducer and Celestron OAG

[tooth_dr]

Just now, chriscoles said:

Celestron C11 SCT with 0.5 focal reducer and Celestron OAG

Thanks. I ask because my flats looked like that when light was getting in the rear of my reflector. I only takes flats at night now using a screen.  My pc screen works fine, but still needs 4 thicknesses of t shirt stretched over the scope (F2.8)

[MarcusH]

21 minutes ago, chriscoles said:

Do you have an example flat file you could share so I can compare?

I'm away from home at the moment, but I'll have a look tomorrow to see if I can find some.

 

22 minutes ago, chriscoles said:

I was thinking of using my PC monitor as an adjustable light source, do you think that would work?

That's how I've done it too. But remember that monitors usually adjust the backlight by PWM (i.e. switching it on-off rapidly) to fast for the human eye but sometimes not fast enough for the sensor so keep your exposures long.

 

21 minutes ago, chriscoles said:

Celestron C11 SCT with 0.5 focal reducer and Celestron OAG

Well what do you know, here was I thinking that I'm the only one with an AST8300 paired to a C11 !!  😄

[chriscoles]

2 minutes ago, MarcusH said:

I'm away from home at the moment, but I'll have a look tomorrow to see if I can find some.

 

That's how I've done it too. But remember that monitors usually adjust the backlight by PWM (i.e. switching it on-off rapidly) to fast for the human eye but sometimes not fast enough for the sensor so keep your exposures long.

 

Well what do you know, here was I thinking that I'm the only one with an AST8300 paired to a C11 !!  😄

Thanks! no hurry. I will do some experimenting tomorrow hopefully, providing work hast zapped all my energy. 

Are you getting good results with your setup?

[chriscoles]

16 minutes ago, tooth_dr said:

Thanks. I ask because my flats looked like that when light was getting in the rear of my reflector. I only takes flats at night now using a screen.  My pc screen works fine, but still needs 4 thicknesses of t shirt stretched over the scope (F2.8)

Something to experiment with. Do you know whether tshirts or paper work better? I may have to go cloths shopping 🤣

[tooth_dr]

3 minutes ago, chriscoles said:

Something to experiment with. Do you know whether tshirts or paper work better? I may have to go cloths shopping 🤣

 Both work equally 👍🏼 But A4 won’t be big enough for you. 

[MarcusH]

18 minutes ago, chriscoles said:

Are you getting good results with your setup?

I'm quite happy with my results, but it still is early days for me. It's only my second season (we only have astronomical darkness up here from late August to early April, hence the reference to seasons) doing astrophotography. But the improvement from first season has been substantial. This season I also have tried out my Hyperstar, and while extremely picky about backfocus distance and focusing, it sure does make this scope faaaast (F2).

[chriscoles]

Hey, thanks for everyones help. So 7 sheets of A3 and 10 second exposure. I have a flat!!

Now I just have to take 20 of each filter! 

🤩

 

[MarcusH]

Looks like you finally found your dust bunnies.😁

Couldn't find the single flat subs, but found the master flats made from them. First one C11 / F6.3 / Astrodon L / 20 x 7s / -15C, second one C11 / F6.3 / Astrodon R / 20 x 9s / -15C

Processed as recommended by Inside Pixinsight / Light Vortex Astronomy.

 

[tooth_dr]

Very good Chris. Taking flats in the dark is important for when long exposures are required