GS dob optics - Page 2

Dave47tuc (David) · #21 10-08-2005, 06:23 PM

My GS Dob, 10” works very well now. Yes it had a few issues re mirror clamp to tight but over all it was good.

I have flocked the tube which I think has helped. Collimation is very important.
But most of all seeing has the most effect on Planetary views with collimation.

I use to have a 6” Achromat and my 10” is better than that. Eyepieces make a big difference also. Cheapo EP’s are no good. Stick with good quality EP’s and a good Barlow and when conditions as in seeing are on your side you will get nice planetary views.

Cheers.

Starkler (Geoff) · #22 10-08-2005, 07:05 PM

Quote:

Originally Posted by Dave47tuc

My GS Dob, 10” works very well now. Yes it had a few issues re mirror clamp to tight but over all it was good.

Dave I recall when we had our two scopes together at the Vic meet and Jupiter was looking much softer and fuzzier in yours than in mine.
The same for MiGs scope, although his was worse.

For many members who have GS dobs, it is their first scope and Im just concerned that some, maybe many, arent getting the best out of their scope and may not know what to realistically expect performance wise, and consequently be missing out on what they paid for.

MiG · #23 11-08-2005, 12:02 AM

Quote:

Originally Posted by Starkler

I havent heard back about Migs scope yet.

I replied in your first thread and I replied to your PM. Is it because I clicked "Cancel" when it asked "Request read receipt for this message?". I assumed that if I am asked a question and one button is "Yes", then the other one is a poorly worded no.

Anyway, my secondary was pinched. The astigmatism is better now, but I think I still have a smidgen because I didn't sand enough.

RAJAH235 · #24 11-08-2005, 01:54 AM

Has anyone posted pics of the rear cell collimation/lock screw assy. & 2ndary holder assy.
Would be interested to see them both. Seems rather a complicated set-up IMHO...
BC. Could your loose 2ndary screws be the cause of you having to collimate @ each set-up? Sounds a bit suss... Not sure if it was your prob????? Someone mentioned it a while ago.

Anyway, can't vote here as I have a Meade.
Regards, L.

janoskiss (Steve H) · #25 11-08-2005, 03:33 AM

Done the works tonight. Refitted secondary (seemed fine & loose in the holder to begin with), removed and refitted primary. I even removed the metal backing plate though I have been reluctant to do so since I got the scope, thinking it must be there for a reason. But I convinced myself that it's only cosmetic. I rechecked my earlier centre-spot job too, which was spot on

(sub-mm precision).

Then I carefully collimated the scope checked each step with laser and Cheshire. Both tools tell me that it's spot on. So the scope is as good as I can make it.

Spent the last couple of hours dodging clouds and checking out the fruits of my labour. Had the ED80 out as well for side by side comparison.

Result: marginal or no improvement. Star = fuzzball problem remains.

The ED80 focuses stars much more sharply. It's dimmer too, though. But because of the larger aperture of the Dob, the diffracted image of a star (the central Airy disk) should be smaller in the big scope than in the ED80 (2.5 times smaller at the same magnification). But instead, its much bigger and less well defined.

One thing that remained after the overhaul: Slightly defocused stars are not round but kind of sausage shaped one side of focus. After defocusing more, the image becomes circular again (once I can see the diffraction rings).

Hope I can get a look at Jupiter tomorrow, and then I'll cast my vote in the poll. But since I'm not entirely happy with the star images, next thing I'll try is to mask the mirror down by 10%, to 180mm.

iceman (Mike) · #26 11-08-2005, 06:24 AM

Interesting Steve.. what was the seeing like? If all else fails, i'm sure at the StarCamp in October there'll be plenty of tinkering going on and we can hopefully fix the unhappiness from yourself and BD.

slice of heaven · #27 11-08-2005, 10:22 AM

Quote from Starkler
"For many members who have GS dobs, it is their first scope and Im just concerned that some, maybe many, arent getting the best out of their scope and may not know what to realistically expect performance wise, and consequently be missing out on what they paid for."

I've had this feeling for a long time. The unwary wouldn't know what to expect and would be assuming what they see is the best they'll get.And thats a shame, as a well setup newt gives brilliant views.

I was fortunate to have started with a well designed and setup dob. When I bought the gs I already had a benchmark to work towards. I can honestly say the views through the gs are very good, with decent eps, stars are pinpoints, planetary views are great and there's nothing nasty about my optics that I can find.

IF there was I can assure you, I would have posted my concerns.

vindictive666 (John) · #28 11-08-2005, 10:43 AM

at the present iam in the same boat i dont really know what to expect of my dob iether really

as ive followed all the advice that ive been given as far as collimation is concerned.

as ive stated in previous posts that i havent got Physical access to a knowledgeable person As far as collimation is concerned, that can show me in person where ive gone wrong (or not) or possibly take me thru the correct steps (this is just me ) if i could get someone properly show me, i have to be content with the views it shows me

(for the time being

)

i am reasonable sure from the advice that ive been given here that its close

but the doubt still remains

on whether its done right or not

)

anyhoo thats my 2 cents

ving (David) · #29 11-08-2005, 11:59 AM

my GS 8" is my first decent scope. the only others are not really comparable... I have to say tho that it gives veiw that were beyond my expectations. but the collimation has to be pretty good.... maybe we need a thread on wht sorta views n00bs can expect when they but a scope so they arent thinking hubble when they buy their 3" wobblotronic because it has occured to me that alot of people might very well expet that type of performance and be disapointed at a fuzzy blurry mag 11 galaxy

er... if it possible

Thiink · #30 11-08-2005, 01:00 PM

I'm in the same boat as a lot of people here that have posted. I have followed the guides and tried collimating 100 times but am not sure exactly what I should be expecting out of my dob.

My primary had already been checked for pinching and had been adjusted. I have also followed Starkler's guide (although when following it I wasnt 100% sure what people had been sanding in their secondary holder, but mine seemed fine anyway), and it seemed to fix my out-of-focus-stars-looking-oval-shaped problem. Stars can be focused down to small blobs, but my planetry images are still average. I am thinking about taking my scope out of town this weekend to try it in dark skies to see if that makes a difference.

To be honest with you all, I am still not confident about my collimation. I have various methods printed from various websites, but all in all I am still not happy with it. I don't think I 100% understand some of the steps and I think it may be hurting collimation.

ving (David) · #31 11-08-2005, 01:16 PM

you got a local astro soc simon? if you do turn up to one of their veiwing nights and get someone to look at it. that'd be your best bet.

cahullian · #32 11-08-2005, 01:37 PM

The trouble I had with collimation was I always turned the bottom screws in the same direction moving the primary mirror closer to the secondary, this made it hard for me to focus on stars properly. I then decided to lower my primary all the way to the bottom and start again.(be careful not to unscrew too far, I screwed one all the way out and it was abugger to get back in) once I had done that and re-collimated i got a lot clearer view on everything I looked at. I haven't touched my secondary mirror at all (too scared) as like others have mentioned I don't really understand the steps involved. Pinching?? sanding?? may as well be double dutch to me.
A few experienced members John B and George have had a look through my scope and they tell me that the mirror is fine and that the views I get are ok, so I'm happy with that. But I would really love to know what to do with the secondary for my own peace of mind.

Thiink · #33 11-08-2005, 01:58 PM

Quote:

Originally Posted by ving

you got a local astro soc simon? if you do turn up to one of their veiwing nights and get someone to look at it. that'd be your best bet.

Not that I know of. I might contact the observatory and ask if they know.

ving (David) · #34 11-08-2005, 02:02 PM

i am sure they'd be willing to help

asimov (John) · #35 11-08-2005, 03:14 PM

I find the sight-tube invaluable for lining up the secondary. Before I built the sight-tube, I used to spend hours adjusting the seondary by basically guessing when it was in the centre as viewed down the draw-tube. Therefore I was never confident if it was right or not. Now i'm 100% confident it's right. I'ts just the primary to contend with after the setting up of the secondary. I do this by adjusting the tilt of the primary until the spider looks centred in the primary. Once that's done, you can be sure it's getting close to reasonable collimation. The hard part comes when your out there tweaking the primary using the star-test method.

ving (David) · #36 11-08-2005, 03:36 PM

i rolled up a piece of a4 paper to use as a sight tube last night... worked fine

asimov (John) · #37 11-08-2005, 04:29 PM

Yes Ving, I've done that too. does the trick.

janoskiss (Steve H) · #38 11-08-2005, 04:41 PM

The exact position of the secondary is not important. As long as you can see the entire primary once you're done, it's fine. What is really important is the orientation of the two mirrors.

Centring the reflected image of the spider may not be sufficient for satisfactory collimation. The spider may be slightly (few mm) off centre and it usually cannot be centred without drilling new mount holes in the OTA or bending the spider vanes. First centre the reflection of the centre spot of the primary (line up with X-hairs in sight-tube and/or use laser), then centre the reflection of the Cheshire.

ausastronomer (John Bambury) · #39 12-08-2005, 08:29 AM

Quote:

Originally Posted by slice of heaven

Quote from Starkler
"For many members who have GS dobs, it is their first scope and Im just concerned that some, maybe many, arent getting the best out of their scope and may not know what to realistically expect performance wise, and consequently be missing out on what they paid for."

I've had this feeling for a long time. The unwary wouldn't know what to expect and would be assuming what they see is the best they'll get.And thats a shame, as a well setup newt gives brilliant views.

After some discussion with Geoff I will attempt to post some hints on exactly what you should and should not see at the eyepiece of your dob.

When we are testing the scope here it is not necessary to use magnification as high as you would for a "typical" star test to determine the actual quality of the optics, what we are looking for here are noticeable problems and collimation, so a magnification of between 10X and 20X per inch of aperture is adequate.

Generally it helps to have reasonable seeing conditions so that you can get a nice focus on a star image.

Collimate the scope as best you can with whatever mechanical assistance (laser or cheshire) is at your disposal.

Aim the scope at a fairly bright star, brighter than mag 2 is good, also at a fairly comfortable angle like 60 degrees or so elevation. Altair is presently a good choice.

Defocus the star outside focus until you can comfortably see the shadow of the secondary mirror and also have clearly defined diffraction rings.

You should see the following things:-

1. The entire outside ring of the diffraction pattern should be perfectly circular, not close to circular, if its not there are issues.
2. All of the diffraction rings should be perfectly circular and concentric within each other.
3. The shadow of the secondary mirror should be perfectly centered within the diffraction rings
4. The shadow of the secondary mirror should be perfectly circular.
5. The shadow of the secondary mirror should be evenly blacked out and not shaded more on one side than the other.

If all of the above fit then there should be no major issues and collimation should be close.

Now bring the scope slowly back to focus, as you approach focus the diffraction pattern should reduce evenly and the star should focus down to a really nice sharp point of light.

From the point of focus slowly bring the scope outside focus, a pinhole of light should appear in the middle of the secondary shadow, take a note of how far you moved the focuser before this pinhole appeared. Now move the focuser inside focus, the pinhole of light should appear at the same distance inside focus as it did outside focus. The pinhole should also be consistent on both sides of focus in terms of size and brightness.

If you don't get this pinhole effect as I have described, your collimation is not perfect.

CS-John B

It is normal to see a little movement inside the diffraction pattern, this is caused by air turbulunce.

janoskiss (Steve H) · #40 12-08-2005, 10:37 AM

These tests sound nice easy to do (much easier than a star test at 200+x), Thanks John?

If the info is accurate, can it be appended to the GS Optics Check Howto, or put in some other permanent easy-to-find place?