Off centered vignetting in flat frame

[Gabrielwiklund]

Hi all, first post here

I tried taking flatframes with my new camera and got this:

before collimation 50% histogram.fits

So i followed Astro Babys collimation guide, despite being collimated i noticed that using a Howie Glatter Laser in the Baader Clicklock the laser beam did not hit the primary center spot. So i assumed that the focuser was tilted (the laser is collimated), i added 2 washers under each of the 2 upper screws that hold the focuser to the tube which almost put the laser in the center spot and then took new flat frames, this is what i got:

after collimation 50% histogram.fits

Much better... the question i have now is if this is good enough? If not how would i notice it in the scopes performance?

 

Setup:

200PDS

533MC Pro

ES HR CC

 

Clear skies

Gabriel

 

Edited August 8, 2023 by Gabrielwiklund

[ONIKKINEN]

Both look very good to me, but more importantly you should probably not focus on how the flats look when it comes to collimation, at least not make any hasty conclusions from them. The real issues make themselves known when you take actual images (and measure star sizes and so on). You could be in perfect collimation but still have an off-center fully illuminated circle because of tiny issues somewhere along the way (such as secondary not perfectly under the focuser, the mirror not exactly at the center of the tube, the focuser being slightly skewed and so on, list is endless and not very helpful). So my mantra is to not worry about funny looking flats if the images themselves are all ok,

Having said that, you have a relatively small sensor camera so slight collimation issues will be more difficult to notice and if you're lucky you cant notice them at all. With larger sensors even very small adjustments can completely ruin one of the image corners but i think you will have a relatively easy time here.

[Gabrielwiklund]

9 hours ago, ONIKKINEN said:

Both look very good to me, but more importantly you should probably not focus on how the flats look when it comes to collimation, at least not make any hasty conclusions from them. The real issues make themselves known when you take actual images (and measure star sizes and so on). You could be in perfect collimation but still have an off-center fully illuminated circle because of tiny issues somewhere along the way (such as secondary not perfectly under the focuser, the mirror not exactly at the center of the tube, the focuser being slightly skewed and so on, list is endless and not very helpful). So my mantra is to not worry about funny looking flats if the images themselves are all ok,

Having said that, you have a relatively small sensor camera so slight collimation issues will be more difficult to notice and if you're lucky you cant notice them at all. With larger sensors even very small adjustments can completely ruin one of the image corners but i think you will have a relatively easy time here.

Thank you, very helpful answer! Just what i needed.
Yesterday i tried taking flats with the camera rotated to 360, 90, 180, and 270 degrees. It turns out that the vignetting is centered when the camera is rotated 270 degrees.

 

[windjammer]

On my 150mm refractor I use the flat vignetting pattern to collimate.  Obviously the scope is not a newt, so a bit different to you, but with a cheshire I could collimate the focusser but still get asymmetric star flares in the images.  Aiming for a centred round vignetted flat pattern made a huge improvement, so that is now my goto collimation method.  Something you might play with if necessary?

[ollypenrice]

It be best just to post a stretched JPEG directly on here. Not everyone has fast internet and some folks don't like downloading for security reasons.

I've made flats from dozens of instruments over the years and I don't recall ever getting perfectly symmetrical vignetting.

Olly