Mirror for Dobsonian

[drewblank]

I plan on constructing a dobsonian over the summer and was wondering which mirror would be best to use. Two options I have right now are: a 114mm Astromart Classifieds - Telescope Making - 2 small mirrors and cells or a 127mm Antares OBJECTIVE MIRROR D=5 F=650mm, 1/3 wave, green glass [5F5] : Hands On Optics :: Telescopes, Accessories, CCD and Digital Cameras. The 127mm is also spherical instead of parabolic, would that make a big difference? Thanks!

[Peter Drew]

Couldn't resist replying to this one . I've no direct experience of either of the options you mention, but on paper, the 127mm spherical mirror at F13 could be the best bet. At F13 the difference between a sphere and a paraboloid is almost negligible and as long as it is a good sphere it should perform well.

[Moonshane]

is there a reason you wish to use such a small mirror?

I'd suggest at least 6-8" as a first dob. this will reward you with far better views and almost certainly not cost the earth as parts are pretty standard.

[Cornelius Varley]

Couldn't resist replying to this one . I've no direct experience of either of the options you mention, but on paper, the 127mm spherical mirror at F13 could be the best bet. At F13 the difference between a sphere and a paraboloid is almost negligible and as long as it is a good sphere it should perform well.

I didn't see any mention of the mirror being F13. The 5" mirror appears to be similar to the Skywatcher 130/650 which is F5. There also no mention of the mirror being spherical.

Peter

[Ad Astra]

Spherical mirrors are never a bargain at any aperture or focal length.

They do not focus properly and will always have 'spherical aberation' which makes focus difficult and robs you of resolution and contrast.

IMHO, if you are going to build a dob from scratch, I cannot see the advantage of anything less than a 6" mirror. You deserve some reward for all your hard work and effort!

Dan

[John]

I believe the TAL 6" F/8 newtonians have spherical mirrors don't they ?.

They seem to have a pretty good reputation for optical performance.

[drewblank]

Well the main reasons for those mirrors was cost, I was trying to get the best I could get for the least. Both of those weren't over 50 bucks. But it looks like if I really want to construct something worth while I'll have to shell out for a nicer 6" or 8" mirror.

[Peter Drew]

Oops!, misread the posted specs. However, good spherical mirrors of long focal lengths do give excellent astronomical images. In the 70's we made several 41/2" 48" focus Newtonians which outperformed all but the very best 4" refractors. I agree that today, the OP would do better to consider at least a 6" paraboloid.

[Ad Astra]

Well the main reasons for those mirrors was cost, I was trying to get the best I could get for the least. Both of those weren't over 50 bucks. But it looks like if I really want to construct something worth while I'll have to shell out for a nicer 6" or 8" mirror.

After you have been here for a month and have 50 posts, you can have access to the Buy-Sell forum, maybe something there would be available.

If you are in the States, Astromart has used stuff - costs like $12 to join, but you can look for free and the deals can be pretty good.

Dan