Quadruplet - Flat Field

[Aidan]

Good Evening,

Could i have some opinion on this image please?   I have always seemed to battle with getting my images to have a nice flat field of view without elongated stars in the corners etc.   I downloaded the trial version of CCD Insperctor to see what was going on... i am wondering if i am becoming a too reliant on it where in fact the image is actually quite good.

The attached image is using my newly aquired 65mm Quadruplet scope with an Atik 383 & EFW2 filter wheel.    The scope's focuser has the ability to manage tilt, but i dont want to start messing with that.    If the attached image was one of yours, would you be trying to improve the flatness or just leave it as is and get on with capturing images??

One part of me is saying, it looks ok.. the other after running it though CCDI says start fiddling with it!   I fear i might be chasing somethigng that does not need chasing!

Rgds
Aidan

AA70Q_Atik_1.fit

[RayD]

5 minutes ago, Aidan said:

Good Evening,

Could i have some opinion on this image please?   I have always seemed to battle with getting my images to have a nice flat field of view without elongated stars in the corners etc.   I downloaded the trial version of CCD Insperctor to see what was going on... i am wondering if i am becoming a too reliant on it where in fact the image is actually quite good.

The attached image is using my newly aquired 65mm Quadruplet scope with an Atik 383 & EFW2 filter wheel.    The scope's focuser has the ability to manage tilt, but i dont want to start messing with that.    If the attached image was one of yours, would you be trying to improve the flatness or just leave it as is and get on with capturing images??

One part of me is saying, it looks ok.. the other after running it though CCDI says start fiddling with it!   I fear i might be chasing somethigng that does not need chasing!

Rgds
Aidan

AA70Q_Atik_1.fit

If I were you Aidan I would be absolutely delighted with that.  It's a largish chip in a small 'scope, and short of a tiny bit in the bottom right, that looks really good.

Never try to run a single image of an object through CCDI.  It will give you all sorts of false information and have you chasing your tail.  You need 20 odd short (10 second) subs taken in an even star field to have any chance of useable information.

Take a few more subs, stack and process them and see what you think then.

Don't forget also that you need the focus to be spot on.  How are you achieving focus?

[Aidan]

Many thanks,  that is very encouraging :-)   It also put my suspicion that i was chasing a problem that does not really exist!

I will try your suggestion about multiple images at short exposure to see what comes back in CCDI.

Focus is autofocused using the Pegasus system.    I'd probably say last night was not the best night with layers of mist coming over every once in a while.   If i remember, FWHM on the AF was around .9 to 1.1

[RayD]

3 minutes ago, Aidan said:

Many thanks,  that is very encouraging :-)   It also put my suspicion that i was chasing a problem that does not really exist!

I will try your suggestion about multiple images at short exposure to see what comes back in CCDI.

Focus is autofocused using the Pegasus system.    I'd probably say last night was not the best night with layers of mist coming over every once in a while.   If i remember, FWHM on the AF was around .9 to 1.1

Ok your focus was probably good then.  I'd just try to get some more date, run dither between subs, stack and process and I would wager this to be great.

Have a go with CCDI if you fancy a tinker, but don't take it as gospel.  There is no better evidence than looking at the image itself.

[steppenwolf]

I'd be pretty happy with that field although I rather feel that focus is a little soft. The field is not perfect but this is a large sensor and a wide field of view so I'd say it was as good as you might expect with this combination. I use CCDI myself but I don't let it be the final arbiter of my images, I only use it if I have already found an issue and want to try and quantify it!

[Aidan]

Thanks Steve.

When I first loved the image into CCDI and compared the mk1 eyeball to the 1’s & 0’s I felt straightaway that I might not be trusting my instinct!

and focusing is something I probably need some work on and try some different step sizes.

My other 70mm scope is a different story... that has horrible elongated stars across the bottom of the image which I’m pretty sure is down to focuser sag.  I’ll address that and seek advice in a new thread!

[Sp@ce_d]

I've been tweaking a new Camera/scope setup. I've got CCDI but found the Pixinsight abberation inspector / spotter scripts useful. You probably need a more dense star field to be sure but they look pretty good. Id get a few images going... when the clouds head off

AA70Q_Atik_1_corners_AbberationInspector.fit

AA70Q_Atik_1_corners_AbberationSpotter.fit

[Aidan]

Thanks,

The spotter script in PI is something i'd not come across before.  It sure will come in useful in the future.

Rgds

Aidan

[symmetal]

Hi Aidan, it's easy to spend too much time chasing imperfections than actually taking images and CCDI can be responsible for this. I've had a similar time with my ZS61 and ASI1600 and apart from the last few days there has only been the odd few minutes of stars visible since October so test images were all I could do. Coincidentally, I did post yesterday, an image very similar to yours enquiring how collimation was measured in CCDI particularly with a refractor, as it was always reported as spot on even when the image was slightly off centre. It was concluded (with Ray's help ) that there's not much to measure with a refractor in the way of collimation error.

I noticed with your image that apart from the focus being a bit out, the corner stars tented to smear towards the centre (like back to front coma) rather than the usual elongated corner stars you get with spacing errors. I did run your image through CCDI to check and apart from some tilt it reports the collimation as being noticeably out. I don't know if that is enough to cause the star shapes you're getting and maybe difficulty in attaining a good flat field.

Alan

[Sp@ce_d]

The too close or too far away example half way down in this post is always a good reference..

 

[RayD]

15 hours ago, Aidan said:

Good Evening,

Could i have some opinion on this image please?   I have always seemed to battle with getting my images to have a nice flat field of view without elongated stars in the corners etc.   I downloaded the trial version of CCD Insperctor to see what was going on... i am wondering if i am becoming a too reliant on it where in fact the image is actually quite good.

The attached image is using my newly aquired 65mm Quadruplet scope with an Atik 383 & EFW2 filter wheel.    The scope's focuser has the ability to manage tilt, but i dont want to start messing with that.    If the attached image was one of yours, would you be trying to improve the flatness or just leave it as is and get on with capturing images??

One part of me is saying, it looks ok.. the other after running it though CCDI says start fiddling with it!   I fear i might be chasing somethigng that does not need chasing!

Rgds
Aidan

AA70Q_Atik_1.fit

He has a quad, spacing isn't a issue.  Once focused your spacing is right, that's the point of them.

Please please don't look at one image with nebulosity with CCDI and try to interpret the state of your equipment with it.  You will be chasing something that may not exist.  It absolutely needs an even field of stars to work with any degree of accuracy.  It will not work accurately on the image you have posted.

Getting your focus nice and tight will help flatten things.  As noted earlier by me, and by @steppenwolf, this is a fairly big sensor.  Hoping for pinpoint stars right in to every corner with a mid range OTA with this sensor is a big ask.  What you have is very good.  

Unless you really know what you are doing leave it as it is as adjusting things could just make it worse.