Flats issue with larger scope

[Simon Pepper]

Hi all

I am after some advice please (posted this one before never got to the bottom of it)! I decided to put away the widefield scope and use my ES127 to get an image of M13. However the reason I put the scope away has come back to haunt me. I cant get rid of this dark spot from my images and I don't even know what it is. Seems to dark to be a dust moat? Whats really odd with this is I use the same camera, filters and filter wheel on my widefield scope and I do not see this mark. I do shoot using NB on widefield and this is RGB, but clearly its not the filters as its on each one and as I shoot NB same camera it cant be the camera? I have looked down the ES127 and it looks clear. Yes there is some dust on the lens, but nothing that could make this mark? I see the master flats contain the same mark, but no matter what I try they will not remove it from the image. 

 

OTA where I have this: ES127, hotech field flattener (could it be that)?, ZWO1600mm Pro, EFW7x1.25, Baadar RGB filters

OTA where I dont see this: Redcat51, ZWO1600mm Pro, EFW7x1.25, ZWEO NB filters

 

Any ideas on what to try or where to clean even will be awesome. Also on a separate one I am still learning RGB imaging, but when I combine my images they are far from what I would call natural colour in the stars all the starts are one colour! Is this because I do not have matched amount of time on RGB? I had an hour on G and R and only 30 mins on R. (clouds)! This was a quick test to see how the scope performed on M13 and I was planning on a lot longer integration, but its going back in the box if I cant figure this one out!

 

Thanks 

Edited July 5, 2022 by Simon Pepper

[vlaiv]

3 hours ago, Simon Pepper said:

hotech field flattener (could it be that)?

My bet is on this one.

There is calculator that can tell you approximate distance between dust particle and sensor depending on F/ratio of scope, pixel size and diameter of shadow. Do search net for one of those to help you diagnose possible location of dust particle.

As far as calibrating it out - it should calibrate out if you use proper calibration.

Any "shortcut" can cause under / over correction (like using bias instead of proper darks or skipping some calibration files).

Looking at image that you posed - Blue filter seems to have over correction on vignetting as well that is visible (there seems to be circle in background of different intensity?).

What is your calibration workflow?

3 hours ago, Simon Pepper said:

. Also on a separate one I am still learning RGB imaging, but when I combine my images they are far from what I would call natural colour in the stars all the starts are one colour! Is this because I do not have matched amount of time on RGB? I had an hour on G and R and only 30 mins on R. (clouds)! This was a quick test to see how the scope performed on M13 and I was planning on a lot longer integration, but its going back in the box if I cant figure this one out!

Nope.

In order to get proper color in stars - you really need to do full color calibration.

To the first approximation - you can scale R, G and B channels based on photometric measurement of a star of known temperature, but in reality you want to use color correction matrix rather than simple weights.

If you don't want to mess with all of that (and it is a bit more technical) - look into Siril for example, or given that you have PI - there is also "photometric" calibration tool.

These are of questionable quality, but many people use them and will therefore produce "expected" looking image (although often not really correct).

[Simon Pepper]

20 hours ago, vlaiv said:

My bet is on this one.

There is calculator that can tell you approximate distance between dust particle and sensor depending on F/ratio of scope, pixel size and diameter of shadow. Do search net for one of those to help you diagnose possible location of dust particle.

As far as calibrating it out - it should calibrate out if you use proper calibration.

Any "shortcut" can cause under / over correction (like using bias instead of proper darks or skipping some calibration files).

Looking at image that you posed - Blue filter seems to have over correction on vignetting as well that is visible (there seems to be circle in background of different intensity?).

What is your calibration workflow?

Nope.

In order to get proper color in stars - you really need to do full color calibration.

To the first approximation - you can scale R, G and B channels based on photometric measurement of a star of known temperature, but in reality you want to use color correction matrix rather than simple weights.

If you don't want to mess with all of that (and it is a bit more technical) - look into Siril for example, or given that you have PI - there is also "photometric" calibration tool.

These are of questionable quality, but many people use them and will therefore produce "expected" looking image (although often not really correct).

Hi Vlaiv thanks for taking the time here and apologies been a hectic day! I managed to get M13 looking ok colour wise I did the photometric colour calibration in PI and made it a lot bluer, but definitely a lot better just played with curves etc and I think its about right now. So as for the flats my procedure is to use a flat panel with the bigger scope as I don't have a big enough t shirt also I am lazy and take them the next day but I don't move the OTA so pretty sure I can get away with this (no issues with other scope doing this). These particular flats were only 30-60 milliseconds, but the ADU value was around 30,000. I will remove the flattener to see if I can see anything obvious on that that could be causing it. Do you think its a dust moat or something else? 

 

Thanks 

[Simon Pepper]

Well this could be something to do with it.... I am not sure if this came off as I took off the flattener (I don't think so) or if its been sitting in the OTA like this for months if the latter this is very embarrassing. Its a compression ring off the end of the hotech flattener. I have removed the extension tubes and compression rings and have noted a small metal chip on one of the threads of one of the extension tubes its silver so could be reflecting? Also there are 3 small screw ends entering the tube one from the focuser I think and then two further back not sure what they are or whether could be causing any issues I doubt it though I believe they are normal.  

[Elp]

A relatively easy test to do is rotate each item one by one which can be rotated to eliminate each one at a time and see if the spot moves.