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What's wrong? Triangular stars!


Padraic M
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I recently fitted a mirror baffle to the SW 150PDS Newtonian. It's this one.... https://www.teleskop-express.de/shop/product_info.php/info/p12359

I didn't really have anything particularly wrong with the stars beforehand, but I certainly have now! All stars across the image are Y-shaped. Have I tightened the mirror retaining screws too much? Have I ruined collimation?

The mirror retaining clips were just about visible when the baffle was fitted.

The first image below is out of focus to the long side by 5 auto-focus steps.

image.png.b178843a7a6400085b5f38e4bcf1f03d.png

 

This image is after autofocus is run i.e. the best focus achievable. HFR is 5.87.

image.png.2de48ed6515d240f4b0aab48b17cdd5d.png

Any ideas what I've done wrong/how to fix it?

Here's the full range of focus, from long to short in intervals of 5 autofocus steps.

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The clips are not supposed to hold the mirror tightly, they are just supposed to prevent it from falling out and rattling in there.

You should be able to slide a credit card between the clips and the mirror without much resistance. If you cant, its way too tight.

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Thanks @ONIKKINEN  I tightened the retaining screws finger-tight so there's no way you'd slip a credit card or even a slip of paper between clips and mirror. Sounds like they are over-tightened. I'll strip it down tomorrow and redo.

Pretty amazing that you can warp the mirror so easily!

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Triangular stars are the classic indicator of pinched optics, which you've pretty much confirmed.

Mirrors distort easily. Ralf Ottow has a reflector in which an airtight chamber lies beneath the mirror. He can change the figure of the mirror by reducing the air pressure in the chamber - by sucking on a tube connected to the chamber!  I've seen this done before my own eyes (which were rather wide at the time!)

😁lly

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1 hour ago, ollypenrice said:

Ralf Ottow has a reflector in which an airtight chamber lies beneath the mirror. He can change the figure of the mirror by reducing the air pressure in the chamber - by sucking on a tube connected to the chamber!

I reckon he's not an astrophotographer? 😉

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11 hours ago, Padraic M said:

fitted a mirror baffle

Hi
Recommendation: remove both the baffle and the clips. Seal the mirror with three generous helpings of neutral silicone sealant to coincide with Skywatcher's cork. Problem gone

Although I must say that your stars are certainly on the social-media side of scientific, verging on the artistic even. Post it on Instagram? It could catch on:) 

Cheers and HTH

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2 hours ago, ollypenrice said:

by sucking on a tube connected to the chamber

Thanks @ollypenricebut I'm not sure this vari-focal scope will catch on if you have to suck through a tube to set the focal length. Imaging nights are difficult enough as it is!!! It would certainly limit exposure lengths to about 60 seconds before you collapse from oxygen deprivation.

31 minutes ago, alacant said:

Although I must say that your stars are certainly on the social-media side of scientific, verging on the artistic even. Post it on Instagram? It could catch on:) 

Yes I was very impressed myself! they look very cool. Not sure about Instagram though there are enough alien invasion force theories circulating there already.

Will strip it down today, so I'm ready for the next imaging opportunity (whenever that might be).

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I removed the mirror cell, removed the screws from the mirror clips, and reset them loosely with enough space to slide a business card through. The mirror clips can now be rocked slightly from side to side with no movement in the mirror.

I left the clips and baffle in place as the baffle is supposed to increase contrast by blocking reflections from the edge of the mirror. I'll take a look at the stats when I'm finished this run tonight.

I don't think collimation is 100% - I redid it after fixing the mirror but using a cheap laser.

image.png.f6861bb6836b04801eed043cab80a318.png

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On 01/03/2022 at 11:48, Padraic M said:

Thanks @ollypenricebut I'm not sure this vari-focal scope will catch on if you have to suck through a tube to set the focal length. Imaging nights are difficult enough as it is!!! It would certainly limit exposure lengths to about 60 seconds before you collapse from oxygen deprivation.

 

😁 No, you don't have to keep sucking! You evacuate the air till you have the figure you want then close the valve.  Ralf does not, so far as I know, have any commercial intentions for this instrument, nor does he intend to use the 'sucked tube' as a permanent arrangement!  This was a sneak preview, if you like.

I mentioned it in this thread just to show how easily a mirror distorts - or easily relative to the wavelength of light, at least.

Olly

Edited by ollypenrice
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