Contructing a pair of 6 inch f/10 dobs

[malclocke (Malc)]

I recently picked up a couple of identical 6" mirrors, progeny unknown, and decided to try my pretty shoddy hand at telescope construction.

Having been told they were f5, I later discovered they are f10, which means my original idea of having a nice portable dob is out the window, but never mind. I'm treating it as a learning exercise more than anything.

The first thing everyone mentions is 'binoscope' but I'm not going there right now!

I'm following the plans at http://stellafane.org/ fairly faithfully. I'll post pics as I progress (slowly).

Here are a few of the first mirror cell. I'm finding so far that I can make all my mistakes on the first attempt at a part, and then get it right for the second. So one of the scopes is going to be terrible, the other even worse.

[hickny (Peter)]

I know how you feel. I built my scope with the help of a friend and learnt from my mistakes.
The images of your work to date show great workmanship. The "stellafane" site has lots of good ideas.
I'll ckeck back on the progress. Good luck.

[anj026]

This article about a 6" f9 reflector might be interesting for you.

http://www.garyseronik.com/?q=node/141

[malclocke (Malc)]

Originally Posted by anj026

This article about a 6" f9 reflector might be interesting for you.

http://www.garyseronik.com/?q=node/141

Thanks Andy, that makes an interesting read. My design is fairly similar, minus the supreme optics and low profile focuser. I've minimized the secondary size based on what I was able to purchase (31mm). Still undecided whether to baffle or not, I like the sawdust finish idea though.

The stellafane designs use Gary's curved vane secondary holder, which I'm currently in the process of building.

[Wavytone]

Nice ! Lunar & planetary observer ?

Looking at your photos, it appears your mirror could slip out of the mirror cell during transport or if the scope is up-ended, or did you use silicone glue to attach the back of the mirror to the supports, but allow it to expend/flex ?

Also... its important that the 3 side clips around the primary don't apply pressure on it - this pressure will distort the mirror. It should be an easy smooth fit to slide in.

BTW the sawdust finish inside works - I have seen a scope like that. Not pretty but very effective at maximising contrast.

[malclocke (Malc)]

Originally Posted by Wavytone

Nice ! Lunar & planetary observer ?

Looking at your photos, it appears your mirror could slip out of the mirror cell during transport or if the scope is up-ended, or did you use silicone glue to attach the back of the mirror to the supports, but allow it to expend/flex ?

Also... its important that the 3 side clips around the primary don't apply pressure on it - this pressure will distort the mirror. It should be an easy smooth fit to slide in.

BTW the sawdust finish inside works - I have seen a scope like that. Not pretty but very effective at maximising contrast.

One of the scopes is destined for my daughter, aged 5 but probably 6 by the time it's finished . She is going to decorate the tube, which will be ... interesting! It will probably be used mostly for lunar and planetary. The second scope I will probably donate somewhere, not sure where yet.

Yes, the mirror is held onto the base and side clips with silicone glue. It is basically floating on these glue pads and makes no other contact with the cell. An obvious caveat of this is to maintain 'this way up' at all times!

[Wavytone]

"This way up" isn't always a good idea.

If the OTA is stored pointing up the result is a horrible film of grime collecting on the primary which will need to be recoated rather sooner than later - even if you think you have a dust-free garage and a good cover over it, it won't last long. For storage its best if the tube is upside down (primary pointing down) with the secondary mirror wrapped in an airtight bag sealed with a rubber band.

A "next best" solution is to use a shower cap over the primary mirror, though if you have an old-school tube you'll have to make a hole it it to reach the mirror.

Recoating the primary is relatively expensive. Recoating or replacing the secondary is cheap.

[malclocke (Malc)]

A little more progress. Secondary holders are built, focuser number 1 fitted, and first (day)light bagged!

Performed the first focus test out on the street to ascertain where to mount the secondary. Unfortunately don't have anything available from home as a practical infinity point to focus on, but found a tree about 1km away and got the mirror position for as close to middle travel on the focuser for my available eyepieces.

Will be fitting the cell in its (hopefully) final position, then disassemble the first tube for painting and on to the mount Progress on tube number 2 is on hold as I still haven't found a second focuser, but all the other parts are being made in tandem.

I'm very hopeful from my 'distant tree' test that these are going to be capable planetary scopes. At f/10 the magnification from even my longest fl eyepiece is pretty high. Intrigued to see how well these hold up for planetary compared to my GSO 10" f/5.