GS dob optics

Starkler (Geoff) · #1 10-08-2005, 12:07 AM

Over in the bargain achromat thread there are a few saying that small refractors are beating their GS dobs on planetary views. This shouldnt really be the case.

I want to raise awareness of the fact that many GS dobs leave the factory with the mirrors mounted in a less than optimal way, and at the same time take a bit of a survey.

When we had our Vic meet a few months ago, there were three other GS dobs there, two that I got to look through. Both of them had obvious issues.
If at least two out of three from this small sample had issues,along with mine when I first got it, I wondered how many others out there were like this. Thats what prompted me to put together the GS optics how-to
In the case of dave47tucs scope, his primary was clamped in too tightly.
I havent heard back about Migs scope yet.

How many of you have checked your GS dob ?

Starkler (Geoff) · #2 10-08-2005, 12:10 AM

Sorry El. I had to delete the original post to edit the poll options.

asimov (John) · #3 10-08-2005, 02:05 AM

I'm going to have my 2 cents here if you don't mind ? & sorry if my post has nothing to do with your thread Geoff.

I have Parks optics in my 12.5" newt. Not GS. I also own a 6" achro. Both are collimated VERY well. No pinched optics in the newt. Both scopes throw up very good images IMHO.

In DIRECT comparison between these 2 scopes.....The newt blows the achro out of the water. BUT The achro throws up better contrast on planets, & ONE dso. The orion nebula. I'm not talking resolution or light gathering or image scale or anything else, but contrast.

Seeing I like contrasty views, I like looking at mainly Jupiter & the orion neb with the refractor over the newt. BUT! hehehe..If I want to resolve something such as triton? It's not possible in the achro. It's also not possible to get the image scale on jupiter & saturn & still retain crisp images or to see that fine detail.This is where the reflector comes into it's own.

I think the guys that are saying their getting better planetary views out of their refractors either: their reflectors are out of collimation as you said Geoff, or is it possible their actually saying their getting better CONTRASTY views?

Define the statement/or term: 'better planetary views'

johnno · #4 10-08-2005, 02:05 AM

Hi All,
Geoff,
Although I dont own a Dob,I was very impressed by your article on checking optics,which I feel should be read by all Newtonian Scope owners.
Some very helpful info there for all of us.
Thanks,Regards.John

iceman (Mike) · #5 10-08-2005, 06:18 AM

I did it before the how-to article, thanks to recommendations by Louie, George and John B. I gained a definite improvement.

My primary wasn't pinched, as I'd checked this on a previous mirror-cleaning exercise, but my secondary was definitely pinched, causing flaring around bright stars and the moons of Jupiter.. like little comet tails that wouldn't focus to a point.

Definitely a worthwhile exercise in checking (it's not even that difficult) and can improve your views significantly.

slice of heaven · #6 10-08-2005, 08:13 AM

I polled as no tweaking needed, but the foam in the secondary needed trimming.
Can this be a sticky Mike?
The more people are made aware of this the better.

BC · #7 10-08-2005, 08:21 AM

I am too scared to after stuffing around with the collimation, I'm afraid I'll just wreck the thing entirely. I never could figure out from the doco whether the 3 secondary screws (which sit apparently partly unscrewed) do up against anything or whether you just turn them in and out to the required place. They were very stiff when I first turned them and now turn very easily. Also, does the centre mounting screw do up against anthing or is it simply turned in or out to get the desired location?

slice of heaven · #8 10-08-2005, 08:51 AM

BC
The centre screw only positions the secondary.
The 3 outside screws are for adjusting the angle of the secondary.They should be firm against the secondary holder.
The 4 screws act together to hold position.

BC · #9 10-08-2005, 08:57 AM

Oh.....

Thankyou. I guess that means my secondary set up is all a bit loose. Think it's time I got that longer tube Orion sight/tube cheshire and bit the bullet.

ving (David) · #10 10-08-2005, 09:01 AM

my optics were pretty good to start with, i get great planetary views (seeing permitting). I did do some trimming of the foam behind the secondary tho.

I dont know bout you guys but I alway hear great stuff particularly about the 8" GSO. I know that my primary is great

Starkler (Geoff) · #11 10-08-2005, 09:51 AM

I probably wasnt very clear in the howto on the visual checking part.

In viewing a star the real test is what shape you see when focus is very close to, but not quite at focus.
One can focus in and out to see the rings in a startest, but these rings should become smaller and diminish down to a round dot or small blob as you approach focus. If seeing any other shape there may be a problem with astigmatism.

Starkler (Geoff) · #12 10-08-2005, 09:55 AM

BC,

The centre screw 'pulls' on the secondary holder, and the other three 'push' on the secondary holder. Adjust the centre screw for position toward or away from the primary, and the other three for tilt.
Be careful not to do it up too tightly as the holder is only soft plastic and the screws dig in.

ballaratdragons (Ken) · #13 10-08-2005, 11:11 AM

Geoff,

I collimated till the cows come home, I followed your 'How-To', use a Cheshire, do everything I can and yet I know my views are still fairly average. Objects just never focus.

The star-test gave good results and yet I cannot get sharp crisp focus (maybe the crummy series 500's)

I get very good images of Globulars, but average to poor images of single stars and galaxies!

I can find the galaxies, I just can't see them in focus.

I know this scope can see better than it is at present but I just can't seem to get it to that 'Excellent' point. Very frustrating when I know what I should be seeing!!

ving (David) · #14 10-08-2005, 11:27 AM

big refractors like yours ken will give outstandig views of DSOs like globs. it probably is your EPs that are letting you down tho... I am slowly weeding outmy 500s. got a 6mm left and i am seriously considering getting rid of that soon. at the star party try some other EP of the same FL and see if theres any difference.

ballaratdragons (Ken) · #15 10-08-2005, 11:34 AM

I will be David, in the hope it is only my EP's. BTW my scope is a reflector!! but I know you know that!

ving (David) · #16 10-08-2005, 11:40 AM

oops, my booboo

ving (David) · #17 10-08-2005, 11:45 AM

for planetary viewing (not nebs) you might want to put in a moon filter or an off axis mask... I find viewing jupiter to be very bright and glarey. cutting back the light helps to create more contrast and better viewing for me cause i am not being blinded... some will disagree with this tho.

janoskiss (Steve H) · #18 10-08-2005, 12:01 PM

Quote:

Originally Posted by iceman

My primary wasn't pinched, as I'd checked this on a previous mirror-cleaning exercise, but my secondary was definitely pinched, causing flaring around bright stars and the moons of Jupiter.. like little comet tails that wouldn't focus to a point.

That sounds very familiar! I have not refitted my secondary yet either. Might do it tonight.

slice of heaven · #19 10-08-2005, 01:03 PM

I agree Ving
I like twilight viewing of the brighter solar system objects to subdue the brightness.

janoskiss (Steve H) · #20 10-08-2005, 01:59 PM

Quote:

Originally Posted by asimov

I think the guys that are saying their getting better planetary views out of their refractors either: their reflectors are out of collimation as you said Geoff, or is it possible their actually saying their getting better CONTRASTY views?

Define the statement/or term: 'better planetary views'

I did write before that I thought the refractor had better contrast, but now that I think about it, that's not really what I meant. I guess I meant better sharpness with a definite focus. I can't wait to see how the Dob performs after the overhaul.