Mirror astigmatism

[iceman (Mike)]

At the starparty on the weekend, Louie and George suggested my mirrors (primary or secondary) have some astigmatism, as a result of mirror clips being too tight and pinching the mirror.

So last night I endeavored to check and/or fix the problem..

John Bambury gave me a call and gave me instructions for the secondary, which was a great help, thanks John. I'm hoping John will do a full writeup of the procedure (with pictures) for correcting astigmatism or pinched mirrors in these GS Newts, for the how-to page of IceInSpace.

Anyway the secondary came out from the spider veins easy enough. After unscrewing the clip holding the secondary it took quite a nudge to loosen it from the mirror, so it was definitely tight. The mirror itself also took quite a bit of work to get it out of the secondary holder.

Once I got it out, I gave the secondary a clean while I was there, as it was quite marked from being dewed up on many occassions. I used some sandpaper around the inside edge of the holder to allow the secondary to fit a bit looser. I also tore off some of the packing in the holder so that it didn't press the mirror out so much.

I packed it back in, put the secondary in and did the clip up much looser, with no pressure on the mirror itself.

I then took the primary out, by undoing the 6 screws around the rear end of the OTA. The mirror cell slid out easily and I carried it all to the kitchen. I checked the mirror clips, and as I remembered, they were quite loose although it could be argued that 1 of them was a tad tight. So anyway I cleaned my primary again while I had it out as it was very dirty from dew and dust. I did the mirror clips up very loosely. They held the mirror, but I was able to wobble the clips side to side easily, and there was no downward pressure on the mirror.

So, i'm ready to collimate and startest it.

I didn't get time to do either of those last night, but will collimate it tonight and do a startest when the clouds clear. Hopefully it's done the job and hopefully my views will improve as a result.

Thanks to Louie, George and John for your assistance!

[slice of heaven]

I just did my secondary and found the mirror slipped out nicely.
I did trim the foam though as it was favouring one side.
I also noticed the bare edge of the secondary mirror was showing so I ran around the edge with a black permanent texta to remove any chance of stray reflections.
Thanks for making me aware of possible problems in this area guys

Slice

[trufflehunter (Wayne Robinson)]

Keep us posted, Mike. It will be interesting to hear of any improvements to your views!

[Dave47tuc (David)]

Hi Mike,
Geoff had suggested mine had the same problems.

As I flocked the tube last Saturday, this gave me time to check the optics.

I was suprised how tight both mirror clamps were.


I aslo thought that my collmation was not right. As out of focus stars before flocking were, gibbous on one side of focus

So on re setting the optics I use Tectron collimating tools.
All is much better.

Stars are nice points and out of focus Stars are how they should be.

Good luck on doing yours, it will be better.

[fringe_dweller]

regarding mirror clasps and pinched optics - My mate uses silicon to embed the secondary onto the holder - no clasps - I havent seen any problems with that method in ten years of usage.
I am taking some shots presently will post em here then.
As a humorous story regarding pinched optics - my first scope nearly 11 years ago was a department store plastic optics coat hanger refractor on a EQ (and then followed by a classic tasco 4.5") - when you did a star test the airy disc was a perfect triangle!
Cheers
Fringey the dark sky hugger

[slice of heaven]

Silicone also works for the primary.
The mirror clips can be bent back off the face the mirror and silicone applied to the sides of the clips to hold the mirror.

[atalas]

Man you would'nt want to slip would you.



Louie

[fringe_dweller]

on my ten - and on lots of (~40 i think) mirrors and scopes he made during his mirror scope making years - he uses the highest grade gaff tape - same as good roadies use
and maybe some silcon as well (I think? - not game to pull it out and look

Have a whinge Fringe

[fringe_dweller]

for the primary i should of said

[slice of heaven]

Geez youve got big eyes Louie

Same as doing anything with optics, take your time and do it with care.

[fringe_dweller]

as promised here is pics - both primarys (and maybe flats as well) - badly need recoating - they are both somewhere between 8 - 10 year old coatings.
the 8 inch has had a few lives and a few owners - but i'm the last - it started off in an equatorial mount and has been re-spydered with the curved vain and modified hence the patched up tube lol - it actually use my mates first mirror ( and second tube he ever used ) he ever made 26 years ago while a kid at uni studying optics amongst other things and only pyrex glass one - he refigured it during his barely recent heydays to his current perfectionist standards - as you can see he is not big on asthetics - doesnt care - just optics matter

[fringe_dweller]

and 210mm

[iceman (Mike)]

Nice idea for the veins!

[fringe_dweller]

Tony is a seriously smart guy - type who does complex maths just for entertainment :p totally unlike me of course , wipes drool off chin

[slice of heaven]

How do you adjust the diagonal on the second one Fringey?

[iceman (Mike)]

Ok i've recollimated.. so far so good.. Now I just need some clear sky to do a star test!

[ausastronomer (John Bambury)]

Originally posted by iceman
Ok i've recollimated.. so far so good.. Now I just need some clear sky to do a star test!

Mike,

Making a learned guess based on the 300 gallons of water per minute running off the roof of my back awning you won't be doin it tonite

Cs-John B

[Starkler (Geoff)]

What I thought was some residual astigmatism from my secondary on a star test, turns out to be my eyes. They have worsened in the last couple of years

[fringe_dweller]

Originally posted by slice of heaven
How do you adjust the diagonal on the second one Fringey?

You dont! LOL naa there is some it is held there with wide bolts and nuts - and the holes in the bit of tin where it attatches to the inside of the tube, sorry i mean Spyder/vain ahem , are oversized so you can wriggle it around a bit and clamped it back down with the bolts - and the rest is sheer brute force - you have to see it done to beleive it! deadly accurate i always star test them ect. They dont go out of collimation really anyway - the strong steel tubes help a fair bit too. And i have traveled with them everywhere - bumpy roads, heaps soakings you name it. Very strong.
I dont know, but Tony might be horrified and even embarrassed? to know that i am talking about and posting rough images of his scopes in rough condition. but he doesnt really put scopes together much anymore, just the odd mirror - (mostly just loves being married and a dad most of all these days, doesnt care a lot for it anyway - the cheap chinese dobs coming out put him just when he got rolling years ago now, and then he injured his shoulder grinding an 18 inch bohemoth ), so I dont know? will tell him i guess.
Cheers
Fringey

[MiG]

At the Vic starparty Starkler (I hope I got that right) suggested I try sanding the secondary holder.
I sanded for what seemed like quite a while and got to the stage of light friction from the holder when moving the mirror. I decided to leave it like this so that the mirror would still be held in place. I also trimmed the foam where it had spread out and was squashed between the holder and mirror. This could be an important step.
I then collimated the secondary a bit diffferently than before.

There was most definitely an improvement. Before it was easy to see the astigmatism when going slightly in and out of focus whereas now the out of focus regions are at least quite close to circular until you get to a size where seeing makes it hard to see what's going on. I don't think that collimation could be responsible for this.


Basically, I got a very noticable improvement from this procedure.