Flats - What am I doing wrong?

[Budgie1]

I'm after some help with taking Flats for calibration.

At the end of a session I leave the DSLR connected to the scope and leave the focus as it was set. I bring the scope into the house and put a single layer of white T-shirt over the end of the scope, make sure there are no creases, and then point the scope at a white LED light in the ceiling and take the flats using AV mode on the camera.

I've also tried pointing at the sky but have come across the same issue.

When you look at the single flat I've attached you'll see that it's darker at the top of the frame. When the frames are stacked into a Master (attached from the same session as the single frame) and put through DSS with the lights, darks & bias it creates a light area at the top of the finished image, when processed.

Am I doing something to cause this when I take the Flats (LED light too bright.) or is this caused by something in the focal train like an alignment issue?

Thanks for any advice.

IMG_0723.TIF MasterFlat_ISO800.tif

[Rustang]

Flats are a thing to master, I thought I had it right until recently. You need to try and avoid any unwanted stray light coming into the light path and i think the way your doing it may not be good for that. The way Ive got my best results recently is by using an LED drawing light pad from Amazon pressed right up against the scope, they are pretty cheap. Place a piece of paper over that and Ive also cut the same aperture of the scope out of foam which then sits around the end to help block any unwanted light. The light needs to dead even for Flats to work and also the correct exposure. There are more knowledgeable people that will come along to help but that's what Ive learnt recently. You can also try the computer screen option with a blank word document on the screen but the best flats Ive achieved to date is with the light pad. I was also told this recently in regards to LED's that they refresh many times per second so any capture shorter than the refresh rate and the light will be uneven so you will need to bring the exposure down to around a second.

You will find a recent good flat of mine to go by below!

 

Edited October 12, 2020 by Rustang

[endless-sky]

It might be the shadow caused by the shutter. How long are your exposures for flats? I read somewhere the advice when using a DSLR: to avoid shadows caused by the shutter, you should avoid short exposures. I use a DSLR as well, and I double the layers of cloth, to make sure my exposures are at least higher than 1 second. They seem to work fine, not causing any shadow issue (that then would translate in lighter parts on the calibrated images).

[Budgie1]

I think I may have found the answer in reading your thread @Rustang . There's mention of the shadow from the flip mirror on one of your flats and @endlessky has mentioned it as well.

I've checked the RAW flats and the exposure is 1/1000 @ 800 ISO. I put one layer of T-shirt over the scope and let AV mode choose the exposure setting.

I've just run some tests and with double layer t-shirt and the scope pointing at a blank Word document on my screen it's exposing for 1/30 and the image is "flat"!   I did try longer exposures using Tv mode and then selected the exposure but 1 second is too long and it just white-outs the image with no dust or video nasties showing at all. Using AV with a double t-shirt layer seems to have done the trick.

Thanks both. 👍

[Rustang]

No problem, it takes a bit of learning to get them done properly 👍

[endless-sky]

Glad I could be of help and glad that it worked!