Help need for type of distortion

[Andy56]

Hi,

 

Last night, just before the clouds rolled in, I managed to get my 130PDS, flattener, and ASI260MC working together.

The images I took have strange shaped stars.

In my experience this isn't coma, there may be some tilt. There is a possibility that the camera moved, ie rotated but I don't think so because the trails ar not centered.

I've attached an image. It's a single sub of 300secs, background neutralised and stretched; and a 3x3 mosaic from Pixinsight aberration inspector.

I did collimate the scope but I'm not good at it yet.

Could someone take a look and point me it the right direction for investigation.

 

Cheers

Andy

 

Edited April 17 by Andy56
Spelling

[Elp]

I think you might be right, an element of tilt and the kidney beaning is down to the sensor being too far from the flattener, see here:

https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus

[Andy56]

Elp: Many thanks. Never heard of Kidney Beaning before.

According to the documentation I have it should have been correct but possibly not.

I'll look at the link later or tomorrow.

 

Cheers

Andy

[Elp]

It's always useful to have a set of thin delrin type rings and sets of small mm backspacing rings at hand when doing AP, especially if you're concerned with star shapes.

Unfortunately I suspect the QA on such flatteners/correctors is only done by batch sample inspection, and each unit will vary ever so slightly from one to the other and away from nominal spec to within a manufacturing tolerance. It might only be 0.5-1mm or so off.

If there's a filter after the flattener you typically add a 1/3 of the filter thickness to the backspacing spec, most people use around +0.3-1mm.

[Andy56]

Hi Elp,

Just measured the back focus distance using the spacers supplied with the ASI2600MC and it's 55mm as required. ie 16.5mm T2-M48 + 21mm Filter drawer + 17.5mm for camera

Looking at the link you sent it appears the sensor is too far away so I need some -ve spacers.

I'll have to replace the T2-M48 16.5mm spacer with a shorter one and buy some spacers.

May be a question to FLO may help for bits.

The tilt may be the focuser.

Cheers

Andy

[Elp]

FLO sell two sets of Astro Essential m42 ring sets which will be useful, there's a single mm incrementing set and a 5mm incrementing set.

[Vroobel]

Try to not buy cheap spacer sets from Amazon or similar source. Once you screw them together with other your parts you may be unable to unscrew them without workshop tools like a vice and a bigger pliers. I experienced it and the cheap ones are damaged while the quality ones are slightly scratched. I'm curious if I could use e.g. WD-40, but I'm worried that it may affect an optics. 

Edited April 17 by Vroobel

[Elp]

Usually it's the cold which shrinks and binds them, if it happens to me I usually heat the affected connection for a few seconds in front of a heater (without getting vital parts near the heat) and they normally loosen nicely, or if there's time just allow them to come up to room temperature.

Edited April 17 by Elp

[Vroobel]

I already tried this way. It looks like they are machined poorly, like the thread is harsh and blocks when you screw it securely (that means quite strongly). 

[alacant]

8 hours ago, Andy56 said:

strange shaped stars

Hi

Tilt is certainly a contributory factor. When you attach the camera, the focuser axis moves; a well known issue with the budget swds focusers. If you haven't done so already, remove the (7 off and usually damaged) rubber washers in the focuser. Take the slack instead using the push-pull pairs of adjuster screws alone.

You can't properly assess the sensor back spacing until the tilt is fixed and you are certain of collimation.

Cheers and HTH

Edited April 17 by alacant

[Andy56]

Many thanks for all your comments and feedback.

I have found that I can remove the ring from the flattener (TS 1.0x GPU Superflat 4-element 2" Coma Corrector) and this allows me 3mm less back focus. Kidney beans means to much back focus.

I will get a set of "Astrodymium Colour Coded Fine Tuning Spacer Rings for M48 Threads" spacers so I should be able to get the back focus correct and you get more for £1 more than the Astro Essentials.

Alacant: I have serviced the focuser and made a huge improvement by cleaning grease off the tube and retensioning the drive part. It stopped the focuser slipping with the weight of the camera etc. I can't remember where all the rubber rings were. I have the "Tommy Nawratil – www.teleskop-austria.com" manual but is doesn't show all the rubber rings. Do you include the rubber rings under the fixing screws in the 7 you mention?

I ask this before I take it apart.

 

Cheers

Andy 

[alacant]

9 hours ago, Andy56 said:

rubber rings

Three at 120º around base and four in the focuser itself, 1 in each corner labelled -rather hopefully- fixing screw  in the reference you cite. If you're serious about it and you don't like the idea of metal to metal, you may wish to fit a paper gasket instead of the rubber.
HTH

 

Edited April 18 by alacant

[Andy56]

Got my spacers this morning. Another great delivery by FLO. Clouds included as usual. Supposed to have an hour or so clear tomorrow evening to give it a try

Fingers crossed. 

 

[Andy56]

So last night had a few hours clear of clouds but with a nearly full moon.

I took out the 3mm thick metal ring that comes with the TS 1.0x GPU Superflat and replaced it with 2x1.0mm Astrodymium rings.

After some trials and tribulations fighting the software (PHD2/scope/EQMOD connection issues) I managed to get some images of M44 before I got to cold and tired. I didn't use Markarian's Chain again because the moon was very close.

The improvement was substantial but still shows "tilt" as in distance between the sensor and the CC not necessarily between the camera and the mirror.

I'll remove the rubber washers and replace them with thin plastic (4 pint milk bottle plastic makes good shims ) and see how it goes. As I see it this relies on the machining of the focuser and the straightness of the tube so may not work on a given 'scope. I may wait till the summer(?) before attempting this as I can get good results at the moment by using (cheating) BxT. 

Images below:

Single sub, Background neutralised, Gradient Correction Manually stretched, no calibration frames.

 

Single sub, Background neutralised, Gradient Correction Manually stretched, no calibration frames but with the magical BxT

 

Cheers

Andy

[michael8554]

I've used spacers made from translucent milk bottle plastic, and was surprised how much light gets in through the edges on long exposures !

Michael

[Andy56]

Good point! Something opaque then. I have some thin black styrene or just use paper as suggested. 

Cheers

Andy

[michael8554]

Paper is translucent too.

That's why it can be used to attenuate a Flat Panel.

Michael