Dobs and binoviewer

[aerobrake (Michael)]

Hi everyone,

I've completed the top half of my 18in mirror dob and have a question before I start the mirror box and fine tune the length of the truss tubes.

I almost exclusively use a denk binoviewer with my other scopes and just wanted to ask you guys is there any reason I cant build my dob to suit just the bv's? The issue is focus and length of the truss tubes will be such that if I build the dob for exclusive bv use a single ep will not come to focus.

I can use a power switch and have the dob usable with both but its extra money that I dont know is worth spending.

What do you think????? Pros/cons???

Cheers
Mike

[Saturnine (Jeff)]

Hi Mike

There's no reason that you can't build for binos', a great idea, if you design your truss tube lengths so that the binoviewer comes to focus at near to the focusser in travel limit, then you still should be able to come to focus with single eyepieces simply by using an extension tube, albeit a long one, with the drawtube racked out. Used in cyclops mode the eyepiece shouldn't be much further out than the binoviewer eyepieces. You may need about an 100mm ext tube though.

Regards

Jeff

[aerobrake (Michael)]

Hi Jeff,

Yeah I did think about using an extension tube but wasnt sure how long it would have to be, thanks for your input. At least I now have an idea,

I think Ill go with the dob being bv specific, the thought of cyclops viewing scares me

Cheers
Mike

[Satchmo]

You should give yourself the option of both. Cyclops viewing has the edge for viewing fainter galaxies as there are some losses in using a beamsplitter and the brain doesn't recombine the light split between two eyes to equal the brightness of a single eye. You'll also get a much larger field of view with a single 2" low power eyepiece. You can get an 80mm extension for about $35 but bear in mind your diagonal size might be inadequate at this greater diagonal to focus spacing, and there might be some vignetting by the base of the focusser tube.

Mark

[aerobrake (Michael)]

Thanks Mark,

still up in the air which way to go. You are correct in that I will loose field with with the bv's. I got some software I can use to compare the difference so I'll have a look at that. Thanks for reminding me about fov, slipped my mind.

Cheers
Mike