Is this field curvature, or something else?

[upahill]

I have a TS Optics 65mm Quad. Looking at some of the subs from last night im noticing a distinct egg shape blur that seems to always point toward the centre. Stars in the centre are fairly round.

I thought this scope was pretty well corrected for this? Or am I missing something stupid?

I have cropped the corners and centre of a single 300s sub.

Any opinions or diagnosis appreciated.

[Vixen4eva]

Sell it to me ?

Alan

[Davey-T]

IIRC there's a couple of Astronomy Shed YT videos on fettling this scope, will have a quick search.

Dave

There you go

 

[upahill]

12 minutes ago, Vixen4eva said:

Sell it to me ?

Alan

Don't get me wrong, I still love it

12 minutes ago, Davey-T said:

IIRC there's a couple of Astronomy Shed YT videos on fettling this scope, will have a quick search.

Dave

There you go

 

Thanks, ill give it a watch.

[vlaiv]

I would say tilt + spacing. There seems to be a bit of tilt, as four corners don't exhibit the same amount of aberration.

You need to get the spacing right on such scopes to get completely flat field.

[Vixen4eva]

Mine was a peach sadly i had to sell it..........when funds allow i`ll be searching for a replacement!!

Alan

[MarsG76]

Looks like distortion possibly due to, as mentioned by vlav, spacing.

[vlaiv]

14 minutes ago, MarsG76 said:

Looks like distortion possibly due to, as mentioned by vlav, spacing.

I need to correct myself on that one - totally slipped my mind - quadruplet scopes don't have corrective element inside focuser like add on field flattener - it is positioned inside the tube, and correct spacing should be made in factory - by looking at posted video of review of scope (second part) - it looks like there is no way to adjust corrector to focal plane spacing - it would involve sliding it up/down the tube.

But sensor tilt is still an option that should be considered.

[upahill]

9 hours ago, vlaiv said:

I need to correct myself on that one - totally slipped my mind - quadruplet scopes don't have corrective element inside focuser like add on field flattener - it is positioned inside the tube, and correct spacing should be made in factory - by looking at posted video of review of scope (second part) - it looks like there is no way to adjust corrector to focal plane spacing - it would involve sliding it up/down the tube.

But sensor tilt is still an option that should be considered.

The sensor tilt is a possibility - both due to self astro modding the camera, and its very easy to not lock down the t-ring evenly in the focuser. But that should correct the inconsistency in aberration, but not the aberration itself right? It will still be present?

The scope was second hand so I can't guarantee the spacing hasnt been adjusted since a factory setup. I also couldnt find any examples on astrobin of this scope performing the same way. I believe the front cell is threaded in so possible perhaps that someone has adjusted it slightly in the past.

[vlaiv]

3 hours ago, upahill said:

But that should correct the inconsistency in aberration, but not the aberration itself right? It will still be present?

Well, try to remove tilt first, it might remove aberration completely if it is the only reason for it.

You can do some tests prior to doing anything to get the idea if it is due to tilt only. Tilt creates uneven focus across the surface of sensor, and if it is down to tilt only, you should be able to focus on all parts of sensor - just not simultaneously.

If you can get sharp stars in each corner by altering focus while other parts of sensor have some sort of aberration - then it should be tilt.

If you can't get sharp focus in one of the corners even with careful focusing, then we are looking at different kind of issue - like distancing problem, maybe even collimation issue - you'll probably need to open the scope and check for pinch or just resettle lenses - like part two of above video explains.