Really odd star shapes….

[Stuart1971]

Hi all, 

I have a Tak FSQ85 scope being used with a QHY268c camera, and am getting some odd star shapes just at the top of my images, both right and left corners, they look almost like thick crosses, or diffraction spikes…

now on the bottom left and right corners I get a slight star elongation, so would these two things be a sign of an issue such as sensor tilt or such like…?

image below, of top left corner…

 

[ONIKKINEN]

If you didn't mention that it was a refractor i would have assumed the picture is from an out of collimation newtonian. Looks very similar to that.

Could the scope be out of collimation? Dont know how common that is with good scopes like a takahashi though.

[Stuart1971]

9 minutes ago, ONIKKINEN said:

If you didn't mention that it was a refractor i would have assumed the picture is from an out of collimation newtonian. Looks very similar to that.

Could the scope be out of collimation? Dont know how common that is with good scopes like a takahashi though.

It’s only in the top right and left corners though, surely it would show all over if a collimation issue….🤔

[Stefek]

I had something very similar with SW Esprit. Happened suddenly when the temperature outside dropped below -5C. When I tried without focal reducer stars were looking OK, so I somewhat attribute it to the focal reducer. Did not try again at "normal" temperatures yet to confirm 100% that low temperature induces some issues with focal reducer. 

[ONIKKINEN]

16 minutes ago, Stuart1971 said:

It’s only in the top right and left corners though, surely it would show all over if a collimation issue….🤔

Actually i think thats common, at least when my scope was out of collimation (still is sometimes 😅). One side of the image was always better or less apparently bad than the other. If all of the field of view is bad then the scope would be nowhere near collimation, like my first scope, an Astromaster 130 was.

[michael8554]

Hi Stuart,

A good description, but we need a jpeg of the whole of a short exposure sub, stretched if necessary, to best judge the issue.

And/or crops of all four corners.

Michael

[newbie alert]

What's the other corners look like? 

[Adam J]

On 28/04/2022 at 18:39, Stuart1971 said:

Hi all, 

I have a Tak FSQ85 scope being used with a QHY268c camera, and am getting some odd star shapes just at the top of my images, both right and left corners, they look almost like thick crosses, or diffraction spikes…

now on the bottom left and right corners I get a slight star elongation, so would these two things be a sign of an issue such as sensor tilt or such like…?

image below, of top left corner…

 

Dont know if you got this sorted but if not then may I take a look at the centre of the field?

Adam

[Stuart1971]

3 hours ago, Adam J said:

Dont know if you got this sorted but if not then may I take a look at the centre of the field?

Adam

the centre of the field is perfect….

Yes got it sorted, it is because of my small pixel camera, the tak optices were designed for larger pixel cameras, and so when the new CMOS cameras came forward, there were issues with astigmatisms with these optics, with small pixels and larger sensors APS-C and above, so tak brought out an extra corrector / flattener to correct this, it is now supplied with any new FSQ85 but not with mine, so I have purchased this now…it’s a common issue I found out after some investigation…👍🏼

Edited May 10, 2022 by Stuart1971

[michael8554]

35 minutes ago, Stuart1971 said:

it is because of my small pixel camera, the tak optices were designed for larger pixel cameras, and so when the new CMOS cameras came forward, there were issues with astigmatisms with these optics, with small pixels and larger sensors APS-C and above,

I can believe a new CC/FR might have cured the problem, but pixel size affecting coma/pinched optics/astigmatism ?

Michael

[Stuart1971]

11 minutes ago, michael8554 said:

I can believe a new CC/FR might have cured the problem, but pixel size affecting coma/pinched optics/astigmatism ?

Michael

it is not a CC/FR it’s a field flattener….

Well that’s up to you to believe or not, but it’s true, do some research and you will see, also before I bought the 268 I had an older SX M25c with 8.5 micron pixels, and every image were perfect from edge to edge…so that proved it to me…look on CN there is loads of threads about it…and a few on here too…👍🏼

Edited May 10, 2022 by Stuart1971

[michael8554]

Going to 3.76um pixels from 8.5um pixels means the poor star shapes are now being fully displayed, instead of being inside one 8.5um pixel.

So an undersampled image scale of 3.9arcsec/pixel is hiding the problem, not fixing it.

Michael

[Stuart1971]

5 hours ago, michael8554 said:

Going to 3.76um pixels from 8.5um pixels means the poor star shapes are now being fully displayed, instead of being inside one 8.5um pixel.

So an undersampled image scale of 3.9arcsec/pixel is hiding the problem, not fixing it.

Michael

I did not say it fixed it, I said it was not evident in those images, and they were perfect from edge to edge….do you have one of these scopes…??
so tak have produced this flattener to supposedly fix the issue, that we have with smaller pixel cameras, so I am happy, and am not arguing the point with you…👍🏼