Diagonals and image quality

[rathalian]

I'm using a 10" Dob and the thought of using a diagonal for the modified viewing angle and also being able to see things right side up (i.e. no reversing looking directions) is appealing.

Has anyone used this kind of thing before?

[dannat (Daniel)]

I presume you are screwing the diag into the focuser? This will add to the focal. Length & you may not be able to reach focus, you may need to move the mirror slightly up the tube..sounds interesting, though if you have it tilted up won, t you be moving your yes further, ie from face down to face UO so does the correct orientation matter?

[rathalian]

I don't have a diagonal - was wondering what peoples experiences were.

[Blue Skies (Jacquie)]

They probably don't have any, because of what dannat says - you wont get it to focus because you wont be able to rack the focuser in far enough. You would have to move the mirror up the tube to get it to work. I'll use my favourite analogy, and get you to imaging the focal length is a piece of string a certain set length; you can bend that string anywhere you like but you can not lengthen or shorten it. If you put a diagonal in a dob it means you would have to lengthen the string to get it to focus, but the laws of physics wont let you. You have to move something so you focus point is at the top of the diagonal. You could shift the focuser down, but it would be easier to move the primary mirror up.

The only time I've seen dobs with diagonals is in binoscopes (two newts set up side by side to act as a binocular) and in those cases the above problem I've described has been allowed for in the design.

[cookie8 (Vincent)]

Owned 2 Dobs 8" and 12". Never used a diagonal with them & I don't think its needed.

[Allan_L (Allan)]

Originally Posted by rathalian

I'm using a 10" Dob and the thought of using a diagonal for the modified viewing angle and also being able to see things right side up (i.e. no reversing looking directions) is appealing.

Has anyone used this kind of thing before?

Hi Rathalian,
I had the same thought with my first Newtonian.
Unlike you I did not stop to ask.
Yeah, it did not work, probably for the reasons stated.

[Barrykgerdes]

In most cases the use of a diagonal on a newtonian will prevent focus being reached without moving the main or secondary mirror to take care of the extra length. However if you must use a diagonal you could try this:

If the nose of the diagonal is threaded it may accept the lens from a barlow (mine does). Then use a longer focal length eye piece to compensate. I have used this idea many times to increase the focal distance.

Barry

[rathalian]

Thanks guys - I'll focus on getting used to looking upside down and will search for a decent chair to make the viewing experience easier on the back.

Any ideas re the chair (that won't break the bank)?

[DavidU (Dave)]

I thought I would try a diagonal in my 12" Dob for you tonight.
I used a 2" dialectric diagonal and a 10mm plossl. It did not work. I estimate I would have to move the mirror up the tube by 30 -35mm, much the same as modifying a Newt for imaging.
Hope this helps.