Triangular stars!

[Robculm]

Hi all,

Just playing with optimisation at the moment in the brief clear moments... Here's an M31 image from earlier this evening (yes, it's a little too large for my FOV!), no calibration frames, just 30 mins of data, 3min subs.

But more importantly, it quite clearly shows the star shape problem I'm having!

Any suggestions? I'm pretty sure the focus is near optimal & have spent a lot of time on collimation. But can't seem to resolve.

Guiding is 'reasonable', around 0.6" total, fairly equal in both RA / DEC.

Many thanks,

Rob

[Ibbo!]

Hi.

To me it looks like pinched optics .

Check that the mirror clips are not too tight.

[alacant]

Hi

We removed the clips completely and instead used neutral silicone sealant to hold the mirror. 6 generous blobs coinciding with the cork on the mirror cell and a little up the sides coinciding with the mirror clip posts is all you need.  This has the added advantage of cleaning the spider spikes too plus it helps hold collimation between sessions and at all mount angles.

Cheers and HTH

** Important: leave the mirror to settle on a level surface overnight before refitting.

[Robculm]

Thanks Steve, Thanks Alacant.

Considering the thickness, it's hard to imagine huh! But fingers crossed this is the cause. Looking back at my previous images, it does appear to have changed over time, which could tie up with me having taken out & cleaned the mirror on a few occasions (I'm a bit festidious about the mirror cleanliness, despite some peoples suggestion it's not important!) & my recent update putting the longer screws in to push the mirror forwards...

Anyway, it's an easy one to try, so will report back as soon as we get another clear night!

I love the silicone RTV approach Alacant 👍. Have just ordered a tube from Amazon. Will make cleaning the mirror much easier too! 😁

Edited September 30, 2021 by Robculm

[Ibbo!]

Do not overclean the mirror it will do more harm than good.

[alacant]

1 hour ago, Robculm said:

ordered a tube from Amazon

Ah, could you post a reference to what  it's called English-trade-name wise? We get asked loadsa times, but only know the Spanish types.

TIA

[Robculm]

I couldn't guarantee it's a good choice, at least until I've tried it! But the one I've ordered is:

https://www.amazon.co.uk/gp/product/B01C4RYE76/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

 

[Robculm]

Haven't embarked on the silicone exercise yet. But I loosened off the mirror 'clips' & repeated the same image. Definitely an improvement 😀.

Actually my 800D is booked in with Andy Ellis for modification in a couple of weeks, so will have a go at removing the clips completely while it's away & see if it improves things further...

Note, same ISO / exposure, just earlier in the evening, hence the brighter background on the 2nd image!

[alacant]

49 minutes ago, Robculm said:

go at removing the clips completely

+1

But much improved already.

It looks as though you have the focuser protruding too: 'D' shaped stars. 10mm or so cut from the end should do it. Getting there:)

Cheers

[Robculm]

Ha, yes, working on that that one. Have the longer screws (+ washers for the springs rather than replacing them). Will put these in when I silicone the mirror... 🙂

 

[alacant]

4 hours ago, Robculm said:

washers for the springs rather than replacing them

The pds springs are rubbish! We'd recommend replacing them with longer 1.6mm wire versions. 6 off, distributed 3 over the existing adjusters and a further three 'passive' over the lock screws. 

Collimation will then hold both between sessions and at any tube angle without having to resort (hehe!) to the locking action.

Cheers

Edited October 2, 2021 by alacant

[Robculm]

Yes, that's a wise suggestion. I did find this UK company with a good range:

https://www.leespring.co.uk/compression-springs

But I have to admit I got a bit lost browsing the 16,000 options!

Will take another look...

[alacant]

3 minutes ago, Robculm said:

UK company

Do Aliexpress do UK? To Spain, this seller ships from Europe. Just specify Europe and the size you want at the checkout. 

[Robculm]

Mirror clips removed, glued in place with RTV silicone (after replacing the screws with M5x35mm) & blackened around the chamfered edge. Hopefully it's stuck down well! 🤞

New springs (1.6mm wire, 20mm length), washers & lock screws to push the mirror forward so that the focus tube 'inclusion' is reduced...

Also discovered that it's much easier to see if the spider vanes are 'straight' while the primary is removed (i.e. looking up from the 'bottom' of the tube), which they weren't on a couple... and made some adjustment to the secondary centration...

As I mentioned, camera is off with Andy Ellis for modification now, so unfortunately can't test for the moment.

Will post results when it's back & there's a clear night...

[alacant]

14 minutes ago, Robculm said:

New springs

Excellent job. 

Suggestion. Fit an m5 nut and then three more springs to the locking screws; 6 springs in total. 

Cheers