Star Shapes

[Darth_Muppet]

I'm currently imaging M101 through my 102mm using the Artemis 285.

Have used a Bhatinov mask, so the focusing is relatively spot on.

Took a 240 sec sub 1x1 binning but my star shapes do not seem to be round.

Anyone know what causes this at all in the above image, they seem almost to be stretched to the right and distorted?

As you can see here:

I took the same image through the WO 70mm and the stars look fine in that sub as below:

[brianb]

If you can rule out guiding errors, and the errors are uniform over the frame, it must be flexure somewhere in the camera mounting on the 102 which is different to the 70 - maybe the focuser. If the errors vary across the frame then maybe you have some aberrations, possibly due to a decentred element in the objective. If this is the case the visual performance of the scope at high magnification will be poor to very poor, even in steady seeing.

[Darth_Muppet]

Well, if it were tracking errors, I've taken longer subs on the 70mm of that object ( 600 secs ) and it's still round stars. When I switch back to the 102 , same settings, I get the star pinching.

If you do look, you can see that in the first image, the distortions are across the entire field.

[themos]

You could try capturing the Airy disc with an artificial star...

[Darth_Muppet]

Still on a learning curve here, how would I go about doing that?

[themos]

I made an artificial star by placing a black plastic disc over a LED torch and making a tiny hole in the plastic with a sewing needle. I put the torch as far away from the scope as I could (about 25 metres but more is better).

For your telescope, N.A.A. Telescope Math Calculator gives an Airy Disc of 2.49 src-seconds and an image scale at prime focus of 5.186 arc-minutes per mm (or 0.311 arc-seconds per micrometer). Your Airy disc would then be (2.49/0.311 = ) 8 microns across. Your camera has a 6.45 micron pixel pitch. It looks like you'll need a Barlow lens to capture the Airy pattern. But you should be able to observe it with a high-power eyepiece. Hopefully you should be able to spot any non-roundness of the Airy disc.

[brianb]

you can see that in the first image, the distortions are across the entire field.

They're not that obvious to me, without the magnified extract - but from what I can see, the effect seems worst in the top left corner and least in the bottom right corner.

Maybe the distortion is there in the other image too, but isn't visible because the focal length is shorter, reducing the effect of any tracking or guiding errors in your rig.

[SteveP]

Darth

I think you can eliminate poor focusing, poor polar alignment and poor guiding. It could be differential flexure but to me, it looks more like your camera isn't orthogonal in the 102. The usual culprits are

1 The focuser itself having a little play, especially if you have it racked fully out to get to focus

2 Adapters/extenders in the imaging train, especially push-fits. It's unusual in screw-ins unless thay have a manufacturing fault.

Have a look at any differences at all between the two imaging trains, including any adapters, to isolate the potential source

If it is flexure, go through the setup in a systematic way and tighten all the fixings

HTH

Steve

[FraserClarke]

I'd agree with Brian that the errors don't look the same across the entire field (which would point to optics rather than tracking).

You could also distinguish tracking errors from optical performance by comparing short exposures (of brighter stars) with long exposures (of fainter stars). If it is tracking/flexure, it should be worse in the longer exposures.

Given the errors are not very big, I would imagine the shorter focal length of the WO could easily hide them...

[themos]

Does anyone know of a recipe that can give you a quantitative view of star non-roundness? We typically get FWHM readings in our software but roundness?

[Darth_Muppet]

Well it would seem that the lens cell may have been affected by the temperature. It was down to -7 and although it shouldn't have made a difference, the contraction was causing a slight restriction on the lens cell. It's gone back now for testing and for the problem to be rectified.

[Tim]

I'd agree with Brian that the errors don't look the same across the entire field (which would point to optics rather than tracking).

You could also distinguish tracking errors from optical performance by comparing short exposures (of brighter stars) with long exposures (of fainter stars). If it is tracking/flexure, it should be worse in the longer exposures.

Given the errors are not very big, I would imagine the shorter focal length of the WO could easily hide them...

this is good advice. Find a bright star, Arcturus, Vega etc and do a series of short exposures with the star placed at various points around the image. If the optical train is suspect, then the errors will show up without complication introduced by guiding or tracking or flex issues.

not many things in astrophotography are as frustrating as odd shaped stars

[FraserClarke]

Does anyone know of a recipe that can give you a quantitative view of star non-roundness? We typically get FWHM readings in our software but roundness?

Ellipticity is the measure of non-roundness -- it's the ratio (to first order) of the long and short axes of an ellipse. A round star has an ellipticity of 0.0. Higher the ellipticity, more elongated the star image is.

[LukeSkywatcher]

It really doesnt look too bad to me in the 1st image but i can see how it would be quite annoying if you are really into imaging.

[ollypenrice]

A few thigs to add;

- You can make an artificial star by glueing a small ballbearing onto a black card and illuminating it with something like a well focused bike headlight. Only the light from the nearest bit of the ball gets into the camera or EP. A point source, more or less. Simple, cheap and I know it works.

- I suspect pinched optics. It is the triangular shape that makes me think this way.

- Certainly try imaging the star on different parts of the chip.

- When all else fails you can write a star rounding action in Ps but all else has not yet failed!!

Olly