Mirror astigmatism

[iceman]

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. :poke: :D

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]

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

[Dave47tuc]

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.
:confused:

I aslo thought that my collmation was not right. As out of focus stars before flocking were, gibbous on one side of focus:confuse3:

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.:D

[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 :scared2:

[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]

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]

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

[ausastronomer]

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]

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]

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.

[iceman]

You're right about that John! But tonight might be clear enough to give it a try!

My foam was also lopsided, too much on one side.. so I tore bits off it until it was even and only occupying the middle dugout.

[Starkler]

It appears that a large proportion of GSO dobs are shipped from the factory suffering from astigmatism due to either or both secondary and primary mirrors being flexed by being too tight in their holders.

Mine, Dave47Tuc, Icemans and Migs GSO dobs all had this issue.

If you dont get nice circular rings on a startest, and nice tight round stars when in focus its well worth the effort of checking the the mounting of both mirrors and ensuring they are not pinched in their holders.

Im a bit dissapointed Bintel didnt pick these issues up for Mike and Dave.

Mike I'll get a howto on this out this weekend if Im not made to work again.

[iceman]

I agree, that's what we were supposedly paying the extra for.

Sounds great Geoff, that'd be excellent.

[slice of heaven]

Yet mine was basically ok.
I'll agree Bintel should have fixed that area .It's a simple task.

[iceman]

Well I finally got to send some starlight down the tube and test out the optics.

From a 5 minute test, it's a great improvement. I noticed it instantly, most noticeably on the moons of Jupiter. Last weekend, Louie was saying he was seeing double of the moons.. i wasn't seeing double, but there was a lot of flaring off the moons and it was impossible to bring them to sharp focus without comet like trails or flaring.. and it wasn't coma - was using Louie's 7.5mm Tak LE and it exhibited it in the middle of the field as well.

Tonight, I was able to bring the moons to crisp sharp dots without flaring off the side. Using my 9mm GSO plossl, defocused inside and outside revealed showed virtually the same shape.. collimation was a tiny bit off but not by much. Splitting Alpha Centauri and Alpha Crux was very easy, with bright straight diffraction spikes and no flaring.

Very happy..

Thanks again to Louie, George and John for the help and advice.

[atalas]

Thats good to hear Ice, and I did have a few beers on the night!


Louie :P

[atalas]

In defence of BINTEL you only find these things under the stars and If the ones they looked through were good you might not know of this problem. Im shaw they would like to know about It for future sales .


Louie:)

[slice of heaven]

Thats a fair enough statement Atalas.
The QC in the factory is really at fault not Bintel. I had no real issue with my optics straight out of the box once it was collimated.
So its a bit of a hit or miss affair.

Slice

[rumples riot]

Glad you got it fixed Mike, maybe your planetary images will have a corresponding improvement. Are you doing any Jupiter imaging in the near future?

[iceman]

I'm hoping so too.

As soon as the clouds clear for long enough.. :rain:

My platform will hopefully be arriving in the next 2-3 weeks as well.. can't wait!

[atalas]

Wow so that means more clouds and rain ! darn It.
I can't wait to see the platform in action Ice.

Louie:D

[ausastronomer]

Slice,

One thing I have learned about GS is that they are reasonably quick to address issues with their scopes. This is evidenced by the number of design, manufacturing and specification changes that have been made in the 5 years these scopes have been imported into Aus for. The overtight secondary holder problem was fixed by GS once they became aware of the problem, unfortunately Mike just happened to buy a scope from 1 of the batches that suffered the problem. Current manufacture GS scopes don't have this problem

CS-John B

[ausastronomer]

Slice,

Sorry, One more thing. The astigmatism issue had nothing to do with QC, it was a design spec fault. All of the secondary holders were specked marginally undersize or the mirrors oversize, 1 of the 2. GS's quality control is very good for a Chinese/Taiwanese mass production organisation, it is way better than Synta and the other Chinese optics manufacturers. I am yet to find a GS dob with a "BAD" mirror after minor issues were resolved. On the other hand some of the GS mirrors are excellent, my own 10"/F5 GS mirror is an absolute cracker.

CS-John B

[atalas]

These GS people seem to be right on to things John ,Its good to hear that they want to give people a good product for there money .
You know yourself John It has been the missing ingredient in all but the higher quality gear. Their optics in these telescopes are the perfect proof. Can't wait to see where these people go over the next few years.

Louie:cool:

[slice of heaven]

Aus
I was thinking that maybe they fixed the problem after I removed my secondary and it slipped out nicely. Then I read Starklers post near the top of this page. His and Icemans scopes were purchased last year ,I'm guessing, Daves and MiGs were purchased April and Feb this year. Mine was purchased in March this year.
Maybe Bintel still had some scopes with pinched secondaries?
But MiGs was purchased from Andrews, as was mine.
So there is still an issue with QC at the factory.
Throw in the fact that these scopes are still nowhere near reasonably collimated from the factory shows their not listening hard enough, if at all. I think after 5 years someone might have mentioned this to them by now.
I have no qualms with their optics,as I've stated before, its the overall package thats a concern. Its great to have decent ap scopes available at a price thats affordable . But if someone buys one that doesnt know how to address the issues then they will be disappointed.
When it comes to tools, instruments and equipment I'm a CRITIC.

Back on subject, I dont think most people take startesting their scopes serious enough.
Regardless of which collimating tool/tools you use the only real test of how good your scope is performing is the startest.
Someone posted a half decent startest diagram(or a link to one)
to show the problems. Its a quick diagnostic check of your optics and it costs absolutely zilch to do. And it takes no time at all to do. It also shows how good your seeing is for the night as well.
The better the seeing ,the better you can assess the optics. Even during poor seeing I believe you would pick up serious faults .
Everyone wants the best out of their scope and the startest is the best method I've found for ensuring this.

Slice

[iceman]

I'm sure the secondary can probably be lined up quite well at the factory, but the primary will never be properly collimated by the time it reaches your door, after travelling from Taiwan, to Sydney, to your door.

Collimation can be thrown out just by transporting to a dark site in the back of the car, along with temperature differences etc, I wouldn't be too hard on GS - I'd expect it to be pure luck if the scope arrived at your door collimated, even if Bintel or someone does it in their shop after receiving shipment.

I agree that a startest can be quite beneficial for fine tuning, but I think that there are some collimation problems that just won't show up in a startest. My opinion is to use tools first to get as accurate as they allow, and then finetune/check with a startest.

That's my opinion though.

[slice of heaven]

I Agree Ice
You cant expect perfect collimation but having to move a secondary 20mm up the scope is not fine tuning.
Yes, you need tools to collimate the scope first but a pinched mirror would have to show up in a startest IF its affecting your image.
Maybe I am Harsh on gs but like I said "I'm a critic " lol

[iceman]

Yeh I agree, there's no reason the secondary shouldn't be very close to spot on.

[fringe_dweller]

Hi again chaps,
I dont know, but just in case anyones still interested in the curved types of spiders used in my pics - heres a great article from
CN http://www.cloudynights.com/item.php?item_id=511
I havent taken many pics thru f6 8" dob to show you quality - but here is a rough single shot of mars from august 03 - taken with a 35mm film camera on a tripod /105mm lens pointing from a distance into a 32mm EP, in the backyard - the film is fuji NPZ 800 so not known for good reds ssoooo - colour is in important in planetary views of course :-)

[fringe_dweller]

re star testing - they arent that many nights in a year that are good enough for a quality star testing as many of you know - so can be hard - specially with tube currents ect. But on the whole you young astronomy enthuisiasts are soooooo lucky today - When i was young (many many moons ago) a quality telescope was like owning a rare maserati! I didnt get to look thru a good one till i was 30! (not to mention the internet for goodness saake)
Same in just about every area EP's everything (dont get me going on music related equipment/help) - you are sooo spoilt - Large apertures for 300 bucks indeed LOL
Cheers
Fringey (wheres my walking frame?) (the generation before me was raised on bread and drippings!)

[[1ponders]]

You took that through afocal :eyepop::jawdrop: That's awesome detail of mars using that technique. :( all that learning for the webcam etc :(

[fringe_dweller]

woops correction mars shot - re film speed it was fuji 400 - 1000/ second, and i was using a 32mm with 2.8 klee barlow - sorry for that

[fringe_dweller]

Hi Paul, wow thankyou :) - but webcams these days blow my mind - Looking at Bird's recent Jupiter and Saturn - I was just shaking my head in disbelief :) Afocal still is up there in full disc moon/solar pics stuff i reckon imo
Wish i was into afocal in 2001 on Mars - that was hot as well - the polar cap was so 3-d before the storms it was unbelievable - we could see the cliffs in 3-D relief quite clearly
Cheers
Kearn

[[1ponders]]

You've got me tempted to try a few film afocals this year when mars get to opposition. Still too early in the morning for me at the moment :)

Sorry getting off topic here. I'll pipe down.

[fringe_dweller]

this is a pic took of the bright moon during the same session as the mars shot - same equipement (very old $100 Pentax MG has only one manual time setting (besides bulb of course) and that is 100th of a sec not 1000th as i said earlier *sigh* And good luck with it mate - its all goood fun :)

BUT We were Disturbed and Outraged to see those damn graffiti artists had got there before us!!!!???
:eyepop: