Building a truss dob...

[Alexandros]

Hello everyone,

I'm considering seriously to start building my own dob.

So after reading a lot of stuff and doing some searching on the web, there are a couple of things i need to ask.

1. Why is it so difficult to find an 18" or 20" aperture primary these days?
And, after finding a couple of interesting cases on e-bay, why do they have to be sooooo expensive?

2. The first question was just to satisfy my curiosity. The second is more practical. Having difficulties in finding a mirror of 18" or 20" aperture, and even if i could find such a mirror i possibly could not afford it, i decided to be realistic. So, i stumbled upon the following 16" inch primary.

http://www.jimsmobile.com/buy_mirrors.htm

Now, does anyone have any experience with these mirrors?
The fact that it's gonna be an obsession-like dob, with trusses, does it mean that the 16" BK7 glass of the primary is gonna be ever-changing during the night and never give me a steady and clear image, making the search for a pyrex-like mirror a one-way road?

Hope i was clear. Being Greek, my English are not perfect.
Thanks in advance! :)

Alex

[billdan]

Hi Alexandros

You can buy a GSO 16in f4.8 from Andrews Communications in Sydney, may save you the cost of freight from USA.

GSO have a good reputation for mirrors, I believe Meade use them in their scopes.

Regards
Bill
Edit: cant help you on the difference between BK7 and Pyrex though.

[Shiraz]

Pyrex vs BK7 may not be a major issue - there is certainly a difference, but see
http://www.skyandtelescope.com/howto/visualobserving/3305656.html?page=2&c=y

The 16 inch GSO mirrors can be pretty good (not premium quality but good commercial quality) - see for example
http://www.astro-foren.de/showthread.php?9274-Die-neuen-GSO-16-Z%F6ller-im-Vergleich
others have reported less favourable outcomes on these forums, but I cannot find the posts

I have a 12 inch with a BK7 mirror - the focus wanders around a little bit if the temperature changes a lot, but that would happen anyway due to tube/strut expansion/contraction, regardless of the mirror material. The figure of the mirror does not seem to vary too much with normal temperature change rates.

[Alexandros]

Many many thanks guys!

Bill,

Thanks for pointing out "Andrews communications", i visited their official webpage and i' ll certainly get in touch with them. :)

Ray,

I was aware of the specific article and when i read it the first time it was like "phew... that's good to know".
It's good to have an opinion of someone with a 12" BK7 glass. Is your tube solid or truss? And also, do you have a fan behind the primary?
Many thanks! :)

[Shiraz]

the tube is solid. I use the scope for planetary imaging, so even minor faults in the figure would be fairly obvious - I have had some problems with astigmatism from edge supports, but have never felt that thermal distortion of the mirror shape was a major issue. Getting the thing to track the ambient temperature (and minimize internal air currents) is the biggest issue and I use a combination of aircon + Peltier cooling before use and a fan during use. A thin mirror would help in that regard and Pyrex would also be better at shedding heat - but the current system works OK. If you haven't already checked it out, the following is worth a look:
http://www.cruxis.com/scope/mirrorcooling.htm

[Alexandros]

No, i haven't used a fan before or any relevant software, up till now. I' ll definitely check it out.
My current equipment is the classic 10" (pyrex) Skywatcher dob, which i use in all my observations, and it doesn't necessarily need a cooling system.
Thanks again for the info! :)

[Shiraz]

OK, you have reference point. according to the software, your Pyrex mirror without a fan will have roughly the same thermal response as a GSO16 with efficient fan cooling - if you can live with what you have, you can probably live with a bigger commercial quality BK7 mirror.
There are two ways to look at mirrors. the first is to decide that you are going to put in a lot of time and expense making a scope, so skimping on a mirror is false economy. the second is to say that you will be able to afford a larger mirror if you go for lower specs, so you will be able to see more, but with slightly lower image quality than from a perfect mirror of the same size, in the best seeing conditions. How much of a perfectionist are you?

[GrahamL]

obssesion list a 20" primary .. in stock ? .. for $3300

fyi gso used to ship direct some years back in US dollars

[Satchmo]

There are more than 10 manufacturers of 18" +mirrors in the USA alone.