Star Elongation Issue | Altair 80 EDT PRO

[JamesAstro2002]

Hi there, 

I've recently obtained an 80mm EDT Pro Altair refractor and am getting some star elongation in 3 corners. I have adjusted backfocus to be around the 55mm mark, using a mix of spacers and T2 delrin rings. The center and top left look to be perfectly round. The other 3 corners are elongated all in the same direction, as opposed to the normal radial pattern seen when the sensor is too close to the field flattener. The top right perhaps seems to be the worst.

I'm wondering whether this is a mix of tilt and backfocus? Wasn't sure if it would be sensor tilt on my 1600mm pro, as before getting this scope I was using a 200pds and managed to get perfectly round stars across all corners, which would make it seem to me the sensor is not an issue.

I've attached some images below from aberration inspections.

If anyone has some input, would be really appreciated!

[ONIKKINEN]

Your tilt measures 10%, which is basically nothing at all. Off axis aberration is also around 10% which is very good. My advice would be to leave it as it is and enjoy a working telescope and not try and mix something that is not clearly broken.

The right side has barely perceptible tilt that blurXterminator would definitely get rid of completely. Alternatively binning or resampling a little bit would probably hide that too.

[JamesAstro2002]

Thanks for the input. 

The thing is, I paid a decent chunk of change to get this triplet and I was hoping to get better results with it. My old bargain basement 200pds at this point is giving better results than something 3 times the cost. Also, I have plans for mosaics, so ideally, I'd need the stars at the corners to be somewhat better.

The previous owner of the scope was using a similar configuration as mine and had perfect stars all across the frame, so I know an improvement is possible.

[tooth_dr]

11 minutes ago, JamesAstro2002 said:

Also, I have plans for mosaics, so ideally, I'd need the stars at the corners to be somewhat better.

I agree with the above about leaving well  alone. BXT will tidy things up if required.  If you are inclined you could build a tilt jig and test your currently spacer/adapter configuration.
 

I also do mosaics and APP has zero issues aligning and producing great results even with dodgy stars.  It managed to align my F2.8 newt stars and they were far from pretty. 

[Anthonyexmouth]

What is off axis abberation? 

[ONIKKINEN]

6 hours ago, JamesAstro2002 said:

Thanks for the input. 

The thing is, I paid a decent chunk of change to get this triplet and I was hoping to get better results with it. My old bargain basement 200pds at this point is giving better results than something 3 times the cost. Also, I have plans for mosaics, so ideally, I'd need the stars at the corners to be somewhat better.

The previous owner of the scope was using a similar configuration as mine and had perfect stars all across the frame, so I know an improvement is possible.

Thing is, very small amounts of tilt like shown here are very difficult to iron out. You would first have to figure out which part of the system the tilt comes from - The lens cell, the focuser, the adapters, or the sensor. Very likely a mix of many or all of those, and its not straight forward at all to figure out which of those is the issue. Very easy to go on a wild goose chase and introduce more issues.

There was someone here who built a laser alignment-thingy-doodad-device that determined tilt on a sensor, but cant find the thread now since the forum has some search function issues. The thread was most likely in the DIY section so some manual searching would probably uncover it.

Trying different camera orientations will also give more clues on where the tilt originates from. If you rotate the imaging train and the tilt stays on the right side, then it is somewhere in the imaging train, if it moves corners then the tilt probably originates from before the imaging train. All a bit of guesswork to be honest, i would be very pleased with what you already have although i am used to some really cursed stars from a newtonian.