secondary upgrade, advice please

[Starkler (Geoff)]

The secondary mirror on my GS 10 inch has scratches on it.
Dont ask me how that happened as I dont know

I can buy a replacement for $50 from Andrews which is the cheap option , but I was considering perhaps going to a smaller secondary of 2.14" to reduce the CO and hopefully get better contrast viewing planets.

Standard secondary = 63mm + 3mm extra thickness for holder = 66mm obstruction . 66/250 = 26.4% central obstruction.

I have run the numbers and a 2.14" secondary will give a 21.7% CO, whilst giving a 1.5 degree field at 75% illumination. Fully illuminated field is 0.25 degrees.

My conclusion is that the GS is designed for maximum FOV use, but a smaller secondary would give me a happy medium between fov and smaller CO for planetary use.

To go to a different size secondary will mean buying a new spider and holder. I'm considering going for a three vane, again for diffraction spike reasons.

I would appreciate any comments and recommendations including what and who to buy from.

Thanks

[Dave47tuc (David)]

Hi Geoff,
Bad luck on last night Maybe next week

When i built my 8" Newt, I used a Astrosystems 39mm secondary. I brought from from Roger Davis, at http://www.batsc.com.au/

Also Bintel sell Novak gear which are very good.

If it was me I would go the Astrosystems way nothing but great reports on there stuff. Vist them at http://www.astrosystems.biz/

Hope this helps.

I'll be observing at home tonight

[mch62 (Mark)]

Hi starkler I used a Protostar ULS Quartz 2.14" Diagonal on my 12.5" Newt .
Rob Royce my mirror maker recommended them to me for the best in performance.
I also highly recommend them.

These are a very high grade diagonal with a price to match but Protostar also have a lower grade Diagonal as well.

There spiders are also excellent with very very thin vanes and show only the smallest of diffraction spikes. I use a 3 vane but they have 4 and curves spiders as well.
They also have heated diagonal holders which use the spider vanes as the conductors.

http://www.fpi-protostar.com/

You can increase the 100% full illumination by getting a descent low profile Crayford focuser.
Just make sure that any CCD chip you intend to use is going to be fully illuminated.
E.G. a ToUcam is a 1/4" chip but if you ever get a larger 1/2"chip you will want that covered as well.
Make sure that the 75% illumination is going to cover the lowest powered EP you have.

Mark

[gbeal]

Starkler,
sage advice from those above.
I built up my 10", and use a Sky Instruments secondary (and it was cheap, real cheap), as well as one of their primaries. There is a whole raft of options, but if you buy the best you won't be left wondering.
As an aside, my secondary is 2.1" and I have often thought of going smaller still, so at worst get the 2.1" model.
Protostar is marketd by a guy Brian in the US, and he also sells the flocking paper, a nice guy.
Gary

[Starkler (Geoff)]

After a reality check I decided its not worth spending a small fortune to upgrade a cheap scope for a small gain in planetary detail when the seeing where I live is pretty rotten 95% of the time.

Thanks fellas for your time and advice.

[Striker (Tony)]

Wise decission oh great one.....Geoff

Why not spend a small fortune and move house....

[Starkler (Geoff)]

If somebody would pay me a salary for observing from my dark sky observatory equipped home and fishing in a nice trout stream I'd be in it