HowTo: Checking the Optics on your GSO Dob

iceman (Mike) · #1 25-05-2005, 06:12 AM

Hey guys.

After all the discussion of GSO Dobs and their optics lately, in particular the astigmatism caused by pinched optics, Geoff (starkler) has kindly written a how-to guide for checking and correcting those problems.

With all the GSO Dob owners on this forum (most as a direct result of advice given here), I strongly urge all of you to follow the directions given here to check your optics. There's been several owners who have done the fix and have much better views as a result.

You need to know how to collimate, and you'll have to do some DIY things like taking out your primary and secondary mirror. If this is pretty scary for you, I suggest you find another forum member who lives near you to help you with the process so that you gain confidence to do it yourself next time you need to.

Anyway, you can find the article in the IceInSpace How-To section, or by clicking on this link:

Checking the Optics on your GSO Dob

Thanks to Geoff for submitting the guide, and please if you want to try this and are having trouble with courage, please post and we'll help you through it!

ving (David) · #2 25-05-2005, 09:25 AM

great stuff. I'll check my optics when i get a clear night to see if i need to do this

slice of heaven · #3 25-05-2005, 10:23 AM

Well done Starkler!!
That'll end some frustrations for a lot of gs owners.
It's a necessary excercise to ensure your getting the most out of your scope.

Starkler (Geoff) · #4 25-05-2005, 11:16 AM

Indeed it is slice. My experience at the Iceinspace vic starparty is that all the GS dobs that I looked through can benefit by following the how-to.

Mike i have picked up a small error. I wrote to remove the rear mirror cell to "loosen the six screws". Could you please edit from "loosen" to "remove" ?

cahullian · #5 25-05-2005, 11:59 AM

Wow looks like scary stuff but I wanna do it he he he.
Great how to and in plain english too. Very nice. So if and when I stuff up I know who to blame

Starkler (Geoff) · #6 25-05-2005, 07:03 PM

Quote:

Originally posted by cahullian
Wow looks like scary stuff but I wanna do it he he he.
Great how to and in plain english too. Very nice. So if and when I stuff up I know who to blame

I knew I forgot something ! The legal disclaimer

It shouldnt really be a scary process. The only damage thats likely is through carelessness, dropping tools on the mirror or over tightening the secondary holder to the spider.
If you lay the tube down before doing any work on it you wont drop anything on the primary mirror.

I would be interested to hear back from GSO dob owners to see what percentage of scopes are having this problem too.

iceman (Mike) · #7 25-05-2005, 09:10 PM

cahullian, just make sure you come to the next IceInSpace star party, or an NAS meeting.. there'll be guys there who will be more than willing to show you the ropes.

slice of heaven · #8 25-05-2005, 09:19 PM

Starkler re feedback
No problems on either mirror on mine, only the foam in the secondary holder.

ving (David) · #9 25-05-2005, 10:14 PM

Quote:

Originally posted by Starkler
I would be interested to hear back from GSO dob owners to see what percentage of scopes are having this problem too.

ok just did it. it was a really easy task guys

my scope was ok. the primary wasnt in too tight and the secondary almost fell out (in a controlled way of course.

thanks for the write-up. very useful

oh, this is my GSO feedback

ving (David) · #10 25-05-2005, 10:22 PM

just a small sugestion that our starkler failed to put in his write-up... and this ones important!!!

before you disassemble your scope lock up any cats you have

I had my OTA on the floor when i took the primary out and the cat decided to climb inside the tube!
lil' mongrel!
if I didnt love him!!!

ving (David) · #11 26-05-2005, 09:00 AM

oh... I'll test tonight if the weather holds out

ausastronomer (John Bambury) · #12 29-05-2005, 04:17 PM

Geoff,

Very well written.

I would like to add something further.

On several occasions you mention about laying the OTA flat on the ground, specifically, to remove the primary mirror/cell and to remove the secondary assembly. I don't agree with this, I have pulled many of these scopes apart and always find it easier to work with the OTA sitting on the base. To remove the primary you leave the scope on the base and let the OTA drop to its lowest position where it rests on the cross brace on the base, you then walk around the back of the scope and unscrew the mirror cell and remove it. To remove the secondary, stand at the front of the scope with it still on the base and lower the angle of the OTA so the tube lays horizontally, unscrew the centre screw on the secondary holder and remove the secondary, use your other hand to then lay the scope to its lowest pointing position resting on the cross brace. The only thing you need to remember is do not lift the tube while the primary is removed otherwise it will nosedive and smash into the crossbrace denting the tube, you need to gently lower it down against the crossbrace each time you lift the front of the tube while the primary is removed.

No need to take it off the base at all IMO.

CS-John B

Starkler (Geoff) · #13 29-05-2005, 05:58 PM

Fair comments John.
First time I removed my mirror cell from the ota, it was very tight and needed some persuasion to come out. This was safer with the OTA laying on the floor. I just suggested that as anything dropped wont fall very far and I wanted to minimise the chance of those unfamiliar with their scope to cause damage .

Astroman (Andrew Wall) · #14 29-05-2005, 06:17 PM

Watch out Ving or you will only be able to see NGC 6543.

Or.....

Does this mean your scope is now a CATadioptric?

Sorry for the sick humour

ausastronomer (John Bambury) · #15 29-05-2005, 11:07 PM

Quote:

Originally posted by Starkler
Fair comments John.
First time I removed my mirror cell from the ota, it was very tight and needed some persuasion to come out. This was safer with the OTA laying on the floor. I just suggested that as anything dropped wont fall very far and I wanted to minimise the chance of those unfamiliar with their scope to cause damage .

Geoff,

Thats a very valid point you make.

On my current scope the primary cell comes out very easily so thats not an issue at all. I have however seen several scopes where the primary cell was quite tight to remove, I think maybe the earlier manufactured scopes possibly. On the scopes where the cell is very tight the best option as you say is to lay the OTA down flat, I lay it on a bed or couch and then tap the edge of the mirror cell using a piece of timber and a hammer, just keep tapping gently and get someone else to "catch" the mirror/cell as it comes away from the tube. Second and subsequent removal is usually a lot easier.

CS-John B

cahullian · #16 30-05-2005, 02:50 AM

No worries Mike I tried to get to last weeks meeting but it wasn't to be.
I'll try and make next months.
When I look at a bright star with my 9mm and go out of focus I only get a big fuzzy dounut shape. Is this good or bad? anyone know because I sure as hell dont : )

Gazz

iceman (Mike) · #17 30-05-2005, 06:07 AM

I'm with Geoff, in laying the OTA on the floor - however I lay it on 2 of the foam cutouts that the scope was packed with the box. Stops it from rolling around.
I'd also urge caution in undoing some of the screws that hold the primary in, in that you don't strip the phillips head.. I had one screw that was tight and i've almost stripped the phillips head and it's getting harder and harder to remove that screw each time I take the primary out.

I've replaced it with a different screw for now, that doesn't quite fit the same but it's better than not being able to get the screw out at all

Quote:

When I look at a bright star with my 9mm and go out of focus I only get a big fuzzy dounut shape. Is this good or bad? anyone know because I sure as hell dont

Gazz, big fuzzy donut is fine, but it's the shape of the donut that matters. The dark patch in the middle of your donut is the shadow of your secondary mirror. Is it in the centre of your donut, or off to one side? When you go out of focus the other way, is it in the same spot?

ving (David) · #18 30-05-2005, 09:19 AM

Quote:

Originally posted by Astroman
Watch out Ving or you will only be able to see NGC 6543.

Or.....

Does this mean your scope is now a CATadioptric?

Sorry for the sick humour

good one!

I layed mine in the floor too when removing the primary cell. it was in very tight tho, but I thought it'd have less distance to fall if it should slip out

ihad it in the cradle when taking the seconday out tho... less back breaking

cahullian · #19 30-05-2005, 10:14 AM

Ill check it out tonight Mike but from memory it looked like it was central in the dounut.

Starkler (Geoff) · #20 30-05-2005, 11:47 AM

If it looks like a big fuzzy donut you have de-focused too far.
At closer to focus you should see a series of rings bright and dark.