My Flats!

[dalepenkala]

Hello Everyone,
I've done some reading on taking flats. My understanding is it eliminates vignetting dust motts etc.. I've noticed this goofy pattern in all my DSO images just off center to the 5 o'clock position. It has this funny look to it in the images I take. After taking my flats I noticed looking at them I have this goofy pattern in the same location of my flats/master flat that I take in SC. I'm attaching my flat for you to look at to tell me if what im seeing is what a flat should look like and if not what am I seeing in this flat? Every flat I take is like this. You will need to ZOOM in to see this pattern I'm describing but its there.
FYI I take my flats are taken at 300 gain and based on what I have read my white is in the 50-60% range and the other's are within the 20-80%. I'm averaging 30 frames for the master flat.

Thanks!

Dale

[Menno555]

It looks rather sharp, so maybe there is a very faint smudge on the camera protective window? Or maybe a filter that is very close to your camera?
Sometimes those kind of smudges are visually only visible when looking at them at an angle with a bright light.

[dalepenkala]

It looks rather sharp, so maybe there is a very faint smudge on the camera protective window? Or maybe a filter that is very close to your camera?
Sometimes those kind of smudges are visually only visible when looking at them at an angle with a bright light.

Thanks for the comment, I thought the same thing when seen this myself. I looked extremely close and even disassembled my camera to recharge the dececant tablets. Made sure everything was clean and lint free.

What’s really funny is in this same location on ALL my images I get a funny blotchyness to it. Its faint but I see it and it bothers me. A friend of mine mentioned to me he thought maybe I was getting condensation on the camera sensor thus the reason for recharging the dececant tablets. Anyway I’m struggling on this and was hoping for some better help here.

Thanks!
Dale

[oopfan]

Hi Dale,

You mentioned dew. It sounds plausible. Here is an example of severe dew that I experienced with my first cooled camera on a humid summer night last year:

https://astrotuna.com/youve-got-a-dew-m ... t-problem/

My camera has no built-in dew management system, so I crafted a small resistor heater band that I wrapped around the camera's nose with Velcro. I've not had a problem since then. I'm not suggesting that you need to do the same but it is an option. The nice thing about the heater band is it also funnels some warmth to the filter wheel. It's good that you replaced the desiccant tabs.

Good luck,
Brian

PS: You might want to increase the number of Flats from 30 to 50. How do you illuminate? I've got a 71mm refractor so EL panels are convenient.

[dalepenkala]

Hi Dale,

You mentioned dew. It sounds plausible. Here is an example of severe dew that I experienced with my first cooled camera on a humid summer night last year:

https://astrotuna.com/youve-got-a-dew-m ... t-problem/

My camera has no built-in dew management system, so I crafted a small resistor heater band that I wrapped around the camera's nose with Velcro. I've not had a problem since then. I'm not suggesting that you need to do the same but it is an option. The nice thing about the heater band is it also funnels some warmth to the filter wheel. It's good that you replaced the desiccant tabs.

Good luck,
Brian

PS: You might want to increase the number of Flats from 30 to 50. How do you illuminate? I've got a 71mm refractor so EL panels are convenient.

Hello Brian
Thank you for your reply! I will go to 50 instead of 30 and give that a try.
I have another question, I’m using a gain of 300 as this is what I always shoot at. Is it better to shoot with a lower gain or do you do that to get the exposure to 3-5 seconds? I’ve not been able to get them down that slow. About 1/15 - 1/30th is where I’m at with he white in the 50% range and the other histograms within the 20-80% range.
I’m using a brand new bunch of white t-shirts and a clear blue sky. I’m currently using an 8” Newt and as stated doing 30 frames for my master flat.
I don’t know that I really see any difference with and without the flats.
Question: during live stack, if you toggle the flat on and off will that automatically correct with the flat or is that something that it only happens before you start?

Thank you for your reply again! I’m truly in need of some guidance here.

Dale

[admin]

Hi,

it certainly looks like something that is very close to the sensor because of the fine detail that is visible in that area of the flat frame. Dew or Frost would be favourite candidates I think.

On the subject of flat frames whilst life stacking, the flat and dark correction is applied to each new frame before it is added to the stack, so if you start a stack with flat correction turned off and then turn it on after five frames, you will end up with a stack of five frames that were never flat corrected and the rest of the frames will be flat corrected. If the number of frames with flat correction is significantly bigger than the number without then the final image should come out fairly well.

Cheers, Robin

[oopfan]

Hi Dale,

I wish I could help you more. I'm just an old guy who does things "old school". I tried Live Stack once but learned that I like delayed gratification more. Now that's old!

My gut tells me that new desiccant will help.

Brian

[dalepenkala]

Hi,

it certainly looks like something that is very close to the sensor because of the fine detail that is visible in that area of the flat frame. Dew or Frost would be favourite candidates I think.

On the subject of flat frames whilst life stacking, the flat and dark correction is applied to each new frame before it is added to the stack, so if you start a stack with flat correction turned off and then turn it on after five frames, you will end up with a stack of five frames that were never flat corrected and the rest of the frames will be flat corrected. If the number of frames with flat correction is significantly bigger than the number without then the final image should come out fairly well.

Cheers, Robin

Thank you for the info Robin,
I don’t understand how that can be happening as I check for this thru the session but will try to check more throughout my session.
Thank you for the explaination on the way the flats and darks are applied! That answers my question!

Thanks!
Dale

[dalepenkala]

Hi Dale,

I wish I could help you more. I'm just an old guy who does things "old school". I tried Live Stack once but learned that I like delayed gratification more. Now that's old!

My gut tells me that new desiccant will help.

Brian

Just being able to ask question helps me Brian.
Noting wrong with old school.

I will replace my desiccant tablets then. I did recharge them however maybe I should have just replaced them.

Thanks!
Dale

[dalepenkala]

Is there any kind of standard as to what a “good or bad” flat frame should look like?

Dale