Star diagonal on a Dob?

[koputai]

Hi Guys,
Very noob question here:
Is it possible, useful, or even desirable, to use a star diagonal on a Dobsonian Newt?

Thanks,
Jason.

[Astroman]

Simple answer... NO.

You will be placing your head pretty close to the OTA whilst viewing and the OTA's do get really cold on some nights, you may end up freezing your ear to the Tube, which wont be much fun. Also putting the eyepiece further away from the designed focal point so you may not even reach focus with it attached. The dobsonian mount was designed to put the eyepiece at a usable height so you didn't need to bend over backwards to look through the scope.

[OneOfOne]

With most Newts you will find they won't have enough "in focus". You will find the same problems with a bino viewer too, although using a barlow can sometimes push the focus out enough...but I wouldn't bet my life on it though unless I tried it before.

[casstony]

A two inch diagonal with a GSO barlow element screwed into the nosepiece might work. I can't imagine a use for this though - you'd probably attract a few curious people at a star party.

[Zuts]

Hi,

I am a bit of a newb but....

Ideally no-one would use a diagonal if they didnt have to.

Why?
(1) They cost money
(2) They dim the image. Not by much, but it is still occurs as there is one extra reflection for the light so some will get absorbed.

Why do people use them?
(1) Better eye placement, so they dont break their backs.
(2) Some scopes wont come to focus without one.

If your eye placement is OK and it should be on a dob then there is absolutely no reason to stick a diagonal in the optical path.

Paul

[koputai]

Thanks for the replies guys.



I actually find that using an 8" Dob is not that comfortable, with lots of bending over, squatting down. On a smallish Dob like this you're sort of in an in-between position.

Cheers,
Jason.

[PhilW]

There is a place for this. Check out the binoscopes in these threads:
http://www.iceinspace.com.au/forum/showthread.php?t=24892 (great construction notes for a 12" bino)
http://www.iceinspace.com.au/forum/showthread.php?t=25742 (includes pictures of the 10" at the Snake Valley star party).

These scopes use diagonals as tertiary mirrors to fold the light path, so that two dobsonian optical trains can be used in effect as a set of binoculars. However this is a niche application, & you wouldn't use a diagonal on a conventional monocular dobsonian for the reasons stated earlier in this thread.

Phil

[casstony]

You need an observing chair Jason. You can build your own or buy one from Bintel. I do all my observing seated. http://members.tripod.com/denverastro/seat.html