4 replies to this topic

[Blue Sky Idiot]

Hello all.

 

I have a Orion Optics GX250 - 10", F4.8, 1200FL, Newtonion telescope tube C/W original 1.25" focuser and basic finder scope.

 

See here for the exact model:

http://www.users.glo...d/newtonian.htm

 

I just have the scope itself, no mount.

 

I bought it in ~2000(?) to make a Dobsonion out of it but time, family, kids etc got in the way and it's been stored in my attic for the last 20+ years (well sealed up).

 

Anyway, time is being kinder to me and i've been able to pull it out of storage.

I've stripped it down and the mirrors are intact and still look ok (I suppose). I've given them a quick rinse with distilled water, nothing more.

The focuser, tube, etc are still in fully working order, as is the finder scope etc.

 

I had intended to build the dobsonion as a project, but on revisiting the subject, it seems that a Dob mount is fine for the tubes these days so in order to get it up and running, i knocked up a dob mount in the shed from some old sheets i had lying about. It's a proof of concept really, but works. I'll do a proper one, with more care and attention in the winter.

 

As for the tube, it seems things have moved on somewhat in the last 25 years!

My current thoughts for updating it (based on reading this site recently)  are:

 

Easy/Basic stuff:

 

1) Bobs knobs or similar for the columation screws, etc. The hex heads are useless.

2) Flocking and glare control: Apply flock opposite the focuser and near the primary (i understand big telescopes don't benefit from full flocking), Check for light leakages, black out non sliding surfaces at the eyepeice and internal bolts, etc.

3) Ventilate the mirror holder and add a fan: The mirror backplate is solid and fully seals the tube at the back(!). I can drill large (better for airflow?) holes in the plate carefully to allow for airflow and cooling. Perhaps add a fan as well.

4) Build a dew guard. The tube is short and the secondary is close to the end, a dew guard will help shutter stray light.

 

Difficult/Advanced stuff:

 

5) Update the 1.25" focuser for a 2" unit. Not sure about this. It's a fast scope and might only really benefit a 30mm+ large eyepeice for wide FOVs?

6) Update the spider: The spider is a thick, 2 vane affair. i'm not bothered about difraction spikes and i like the solidity of the brisge but don't really know if i would gain anything from changing it to a 4 vane thing. Maybe a different difraction image rather than actually better?

7) Build that dob proper....

 

Any thoughts on the above?

Anything else i should be looking at?

I'll post seperately about the Dob mount as thats a bit, er, unusual...

I'm on a tight budget so would like to keep things sensible. I'm not fussed about fit and finish as long as it works.

 

Should i consider getting the mirrors recoated? They look ok. No holes, flakes, etc.

Maybe if i had to recoat the mirrors, i'd consider getting something else and sell this one for parts. dunno.

 

Comments appreciated.

 

 

 

 

 

 

 

[happylimpet]

1 very good idea

2 i bet you notice surprisingly little difference. do it, but dont prioritise it.

3 very good idea. get a fan blowing on the back of the mirror and allow flow up the tube. will improve planetary considerably.

4 yup, especially to keep dew off secondary. on that note, consider a secondary heater, as per the kendrick ones (split heaters, maybe)

5 yeah sometime. mid priority.

6 low priority unless you cant collimate well with the existing one.

7 high, depending how nicely the current one moves.

[walt r]

I'll say, 'use it for a little while to see what you like and dislike.

Do learn to collimate and then evaluate how good the optics are. If good then do the mods.

If not good reevaluate the time and money to mod it.

Then revise your list.

[Blue Sky Idiot]

1 very good idea

2 i bet you notice surprisingly little difference. do it, but dont prioritise it.

3 very good idea. get a fan blowing on the back of the mirror and allow flow up the tube. will improve planetary considerably.

4 yup, especially to keep dew off secondary. on that note, consider a secondary heater, as per the kendrick ones (split heaters, maybe)

5 yeah sometime. mid priority.

6 low priority unless you cant collimate well with the existing one.

7 high, depending how nicely the current one moves.

Thanks for the reply.

 

Did some research on point#3.

 

So, as i have a solid back plate, i'm going to drill a  singular 50mm hole in the middle and place CPU fan on/in it to suck (not blow) air down the Newtonian. This disturbs the mirror air better than blowing on the back.  10-20 CFM seems to be a reasonable flow rate.

 

I have a CPU fan scavenged somewhere but am looking for a more appropriate 5V USB version i can power from a powerbank.

[star acres]

If it ain't broke, don't fix it. And I have a bias against flocking. You will probably like the contrast result but the flocking will fade badly in a few years. Then you'll have a mess to remove and wish you never flocked. If you wrapped your telescope up and stored it 25 years, it's probably just fine.