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rmcpb
27-03-2006, 08:19 AM
With all the clouds over the past weeks I have been modding my scope more and more.

A couple of the major ones are:


After having a play with NEWT the secondary supplied with the scope is way too large at 50mm. NEWT suggested a 33mm secondary so a call to Bintel and I settled on a 39mm Discovery secondary with a Novak cell and spider. In it went quite easily except the spider is a bit too wide for the tube so I just extended the round threaded bits at the end out of the vanes through the existing holes in the tube and made some plywood blocks to slip over them outside the tube so I could tighten them up without cutting the spider down. This part worked well BUT when I tried to collimate the secondary the Novak setup is a horror!! The three collimating screws are at the top of the secondary cell, combine this with a four vane spider and one of the screws is right under a vane. After much frustration it was aligned and I hope to never have to do it again but I know I will and it will be easier next time.
The other mod was to use the metal plate most people take off the back of the GS mirror cell as a baffle for the fan. Cut out a 70mm diameter hole, cut out three slots for the collimation screws and mount the fan directly to the plate. Cut down the locking screws that I don't use, so they would hold the plate on the outside of the mirror cell without interferring with the collimation. On it went easily and wait for a clear night.


How did they go? Well Saturday night cleared up beautifully and it was off to Linden. The fan caused no detectable vibrations and the mirror settled down very quickly a quick win. As for the secondary, I had a great time with Saturn and the usual targets, Omega Centuri was a stunner but when I got Jupiter up about midnight I nearly fell over. For the first time I could see festoons without even trying!! The equatorial belts have always been easy but the temperate belts were visible, the GRS was clear, I couldn't believe it.

What are the outcomes? If you have a GS scope run NEWT over it and check the size of your secondary. For me the results have well and truely been worth it.

ving
27-03-2006, 01:30 PM
thats great news rob!

but what is newt?

rmcpb
27-03-2006, 03:04 PM
A software package for designing newtonian scopes. You can download it free from http://home.att.net/%7Edale.keller/atm/newtonians/newtsoft/newtsoft.htm

Its a great tool and way to learn about the way scopes work, if nothing else.

I would hazard a guess that GS use the same size secondary in most of their scopes. A 2" would be about 17% in a 12" (about perfect), 20% in a 10" (OK) and 25% in an 8" (too large). Hopefully they don't use it in the 6"!! It would be a way for them to keep costs down when making the scopes by only having one size for all scopes.

The 1.5" one I now have is an 18.75% obstruction by diameter which is about perfect for the rule of thumb of between 16-20%.

ving
27-03-2006, 03:30 PM
so what hapens if you drop below 16%?

cjmarsh81
27-03-2006, 03:54 PM
I would guess that you would not see the full image. It would be like sticking one of those toothbrush mirrors in and expecting it to reflect the whole primary.

rmcpb
27-03-2006, 05:36 PM
If its too small the illumination on your large eyepieces will drop off noticably toward the outside. So not only seagulls but dim ones at that.

Tha advantage of the correct sized secondary is increased contrast so you get to split that close double easier, see more individual stars in Omega Centuri and the festoons on Jupiter.

Get NEWT if for nothing more than the Help section which explains a lot of this.

Starkler
27-03-2006, 06:08 PM
I have done the same calcs for my 10" gso dob.
The supplied secondary is 63mm but the holder thats the mirror sits in has a thickness that adds to central obstruction giving a total of about 26% CO.

One can go down a mirror size or more remembering two things:
1: Imperfections in secondary mirrors are more likely to be an issue at the outside edge, meaning a smaller secondary has to be of good quality because you will be using more of it.

2: You are sacrificing illumination at the edge of field at low power. If you dont own a 31nagler or similar low power wide field you can afford to go a bit smaller.

For the 10" , one can go down to a 2.14" inch secondary without loosing too much illuminated field.

matt
28-03-2006, 06:37 AM
Good stuff Rob

Was interested to read you've done exactly the same mod with the back metal plate that I'm about to do ... with vibration not being an issue.

Great news. Did you use anything to dampen vibration, ie place anything between the plate and fan to cushion the unit?

rmcpb
28-03-2006, 07:23 AM
Geoff,

Well there goes my theory about the same sized secondaries. I think you are right in that GS are allowing for less than premium secondaries to be used by oversizing them. The one I installed was a premium Discovery one and is great but I would advise noone to go below the diameter that is calculated by NEWT so they don't get noticable drop off in their larger eyepieces.

___________________________________ ________

Matt,

I used some velcro circles between the backplate and the primary cell to hold it in place while I tightened the studs and, of course, they act as a kind of vibration dampner but, how much, I don't know as I have the studs done up as tight as my fingers can do them. Maybe I just lucked in on a smooooooth fan.

matt
28-03-2006, 07:27 AM
good work!

Did you take pix as you worked through the 2 jobs???

Would be keen to see some of that cooling fan mod:D

rmcpb
28-03-2006, 07:44 AM
Matt,

The cooling fan mod looks just like the ones posted the other day as a source for cooling fans for the lazy I think it was called.

I will get the camera out tonight and posta picture though.

matt
28-03-2006, 07:58 AM
Goodonya Rob...

That's the spirit. Where would we be if we couldn't show pix of our own stuff. Be like showing someone pics of someone else's kids instead of our own and saying ... "well, mine are just like these ... you get the idea".

:lol: :lol:

ving
28-03-2006, 09:53 AM
hmm... looks like a smaller secondary will be on my wish list too :)

rmcpb
28-03-2006, 10:40 AM
Make it a good one Dave :)

Starkler
28-03-2006, 01:01 PM
Rob Its great that you are happy with the way your scope performs now but I have a question.
Did you ever check your old secondary mounting for pinching or astigmatism?

If you are seeing a huge difference in image quality I would suspect that something was wrong in your old setup other than a bigger C.O. alone.

I saw somewhere simulations of planetary images with various sized central obstructions and the difference between 20% and 25% was very subtle indeed.

In short, It's well worth checking the existing setup for issues and then a new GS secondary only costs about $50.

rmcpb
28-03-2006, 01:13 PM
Geoff,

I went through the procedure and it was OK. I think the differences may be many pronged:

This was the first outing with the fan fully baffled so the primary was probably closer to ambient than it was previously.
The smaller secondary would help in the contrast, this one is only 3/4 the size of the stock one.
The better quality of the secondary has to help.
And of course the great seeing on Saturday night helped but I have been out on nights of great seeing before and not had those results.


Too many things have been changed in one go to put the improvements down to only one thing. Another variable that is possible is that I just had a bung secondary and now I have a beauty.

Will never know the full story but I'm happy with the results.

Starkler
28-03-2006, 01:16 PM
Thats great Rob.
Just alerting others to the possibility that they may not get as dramatic an improvement as you did as its not a cheap exercise.

matt
31-03-2006, 06:33 PM
so what sort of obstruction would an 80x57mm represent to a 10" scope???

Is it in the right ballpark?

rmcpb
03-04-2006, 07:38 AM
Taking the minor axis of 57mm on a 250mm scope you get a 22.8% obstruction by diameter. This is in the ballpark of the generally accepted obstructions but toward the high end. Generally the rule of thumb is between 17-20% with obstructions up to 25% acceptable.

If you want to go for the low end you need a really good flat mirror that is good to the edge so you don't get distortion. That is why I spent the extra on a Discovery secondary for considerable more money than a GS one.

Merlin66
03-04-2006, 09:29 AM
I think you should be able to verify the performance of both the fan/ main mirror/ secondary by checking the optical train with a 150 line ronchi screen. It should show any distortions in the seeing/optics. Just hold the screen close to the focus while aligned with a 3rd mag star. Blue tac works wonders.