Problem focussing camera on ed80

[hatman]

Ok, I have a beautiful ED80 and have no problems attaching and imaging with an EOS on a direct straight through attach on the back of the scope (2").

No problems with focus and all perfect.

Where I get the problem is if I attach a 2" diagonal and then attach the camera to the diagonal (this allows easier operation when at zenith).

No problems with the hardware but I cannot get focus, no matter how far I move the focussing unit.

Can anyone suggest what they may think the issue is?

I would take it that if you can gain focus in an eypiece with this arrangment then surely the same should apply to a camera. After all their is no issue with an eyepiece in direct straight through viewing.

What am I doing wrong?

[jjjnettie (Jeanette)]

One normally wouldn't image through the diagonal, you won't be able to get enough infocus.
I know that it's very uncomfortable trying to get to the back of the camera when you're close to zenith but one just has to make the best of it.
One of those plastic backed picnic blankets is better to lie on than a regular tarp.
Invest in a Right Angle Finder for your camera. They're not too dear and you can increase the magnafication on the view 2x if you want to.
Or sell your refractor and buy a newt.
They both have their pro and cons re. comfort, but when I get sick of one I can always swap to the other.

[dpastern (Dave Pastern)]

hatman - I've just realised in the past week the same thing - focus is not achievable with my WO 2" diagonal. The odd thing is, I know I've achieved focus with it the first time I used the setup with my DSLR. If it worked then, then it should work now, or that's my logic.

My suggestion is to use an eyepiece to frame the subject roughly, etc, then remove diagonal and eyepiece and add the camera. Re-focus, check framing etc, take images.

Dave

[Ian Robinson]

Don't use the diagonal .... you shoudn't need to use the LCD screen on the back of camera at all .

For focusing and view finding via the camera I use a chinese clone of a Canon rightangle viewer which can be used at 1x and 3x . and has dioper adjustment , and which also rotates about so I aways see that I'm targeting without the need to be a contortionist.

Mine is similar to this one
http://cgi.ebay.com.au/2-5X-Right-An...d=p3286.c0.m14
I refuse to pay through the nose to buy the Canon branded equivalent.

I focus through the camera and without the angleview in place first on a handy brightish star about 30 degrees above the horizon, then attach the angle viewer , focus at 3x via the focusor that's part of the angle viewer to ensure sharp focus , then I am ready to aim at anything in the sky , I simply rotate the angleviewer to suit and for most comfortable use .... it's all very easy and straightforeward .

These (right angle viewers do not interfer with the light path to the camera like a stardiagonal will .

Plus the star diagonal will mean you need more infocus to reach focus as they add distance when fully racked in to that due to a straight through configeration.
Plus it will likely cause vigetting of the image or will even obstruct the image circle.

[jjjnettie (Jeanette)]

I have the Seagull brand finder.

[Ian Robinson]

Originally Posted by jjjnettie

I have the Seagull brand finder.

Not sure what brand mine is , I've forgotten , but it came from HK and I've very pleased with it.

Tan in HK I think , they have some great photogadgets at really good (cheap !!!) prices.

[hatman]

Hey thanks heaps for the tips. Still confused as to why you can get focus in an eyepiece out the back of the scope and focus in the eyepiece in the diagonal but can only focus the camera in line, not through the diagonal.

[Terry B]

Originally Posted by hatman

Hey thanks heaps for the tips. Still confused as to why you can get focus in an eyepiece out the back of the scope and focus in the eyepiece in the diagonal but can only focus the camera in line, not through the diagonal.

Think where the focal plane of the camera is-the same place as in a film camera. The focal point of an eyepiece is in the barrel of the eyepiece. You can't get the focal plane of the camera close enough with a diagonal.

[astro744]

Originally Posted by hatman

Hey thanks heaps for the tips. Still confused as to why you can get focus in an eyepiece out the back of the scope and focus in the eyepiece in the diagonal but can only focus the camera in line, not through the diagonal.

The focal plane from your main objective is at a fixed distance for focusing at infinity (600mm for the ED80) In that 600mm from the main lens is the telescope tube, focus unit and focus draw tube (the sliding bit). The draw tube has a limited range of travel. When you focus with an eyepiece and no diagonal your focuser is racked out more than when using an eyepiece with diagonal. Not all eyepieces focus without a diagonal and you may require an extension tube since your focal plane may sit too far back.

The cameras focal plane (film plane) is about 46mm back from the front of the T-ring and this is usually about 46mm further back than an eyepiece focal plane (changes with eyepiece design). Adding a diagonal in the light path chews up the available distance and you may get less than 46mm to play with. The travel of the focuser needs to be the distance through the diagonal plus the distance to the film plane; usually not achievable.