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Help! Unable to get sharp focus with DSLR at prime focus / eyepiece projection


Jove
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Hi all,

The other night I tried attaching my Nikon D80 to the back of a Skymax 127 telescope. The thread on the back of the scope matches that in the camera T-ring so that's all I have: camera-t ring-scope. There's no extender or anything in between the scope and the camera.

Now, when photographing terrestrial objects I can get a good focus with this set up and have taken some nice wildlife pictures. The other night, though, I was trying to photograph Saturn and try as I might couldn't get a sharp image to appear through the viewfinder, and all the pictures I took were blurry. Is there anything I can to to rectify this? The focus on the scope is quite sensitive, so perhaps I am just not able to adjust it finely enough to get a crisp focus?

I also have a 40mm eyepiece that has a thread to allow eyepiece projection. I attached the camera to this eyepiece but had similar problems - I couldn't get a good focus. I have no instructions on how to to EP projection so simply extended the tube to its maximum length - can anyone suggest what I may be doing wrong here?

Thanks for your help !

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When focusing the D80 on my scope crisp focus is VERY hard to achieve -it's taken me at least 5 minutes of slowly adjusting the focus wheel before now to get anywhere near good focus.

Does the focus through the camera's viewfinder change much? Is it like you're heading in the right direction and the image is getting slowly sharper but then you cannot adjust the focus any further?

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Yes, that sounds about right. Or that the focus point is so finely balanced it is impossible to actually hit it! Someone suggested I might need a bit more space between the scope and the telescope to focus at infinity - are there little tubes you can get to provide this space?

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I am suprised that you could not get your mak to focus, maks have a large focus travel to cater for a large number of attachments. Focusing on Saturn would be quite tricky as it is a small object to be photographed with a DSLR. You could try taking photographs of the moon first to see if you can reach focus. Small amounts of focuser movement can send the image from one side of out of focus to the other very quickly. If you have not run out of focus travel adding an extension tube to the visual back will not help you achieve focus. When using the 40mm eyepiece did you obtain focus when looking through the eyepiece without the camera attached?

Peter

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Thanks for the suggestions. I'll try focussing on the moon and see how that goes. I'll be a bit more systematic too, rather than just fiddling around and hoping for the best, and will pay attention to whether or not I'm running out of focus travel.

When using the 40mm eyepiece did you obtain focus when looking through the eyepiece without the camera attached?

Peter

Yes, the 40mm eyepiece focuses beautifully visually. It's this eyepiece:

Revelation 40mm Projection Eyepiece. You unscrew the end with the rubber eye shield (or whatever that bit is called) and screw on the camera t-adapter in its place. You can vary the length of the tube (and hence the distance of the camera from the eyepiece lens) and use a little screw to lock it at the distance you want.

Thanks for your help :D

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Further to this I did some experimentation with the moon. I found I can get at least the middle of the frame in focus if extend the tube a little bit - I guess this indicated that I needed a bit more clearance between the eyepiece and the camera focus than you get with the projection tube at its shortest. The images I got were not the sharpest, but were OK. I don't think I can get the whole field in focus at once, just the central region - does that sound likely? Is that suggestive of a scope with a curved field (and the kind of thing Astrographs and scopes iike Celestron's 'Edge' SCTs go to great lengths to avoid?). If so, would I face the same problem with something like a DMK camera, or are the CCDs on those small enough to be entirely within the relatively flat bit of field?

As ever, thanks for any help or advice :D

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To work out how big the moon will be on the camera sensor, using the prime focus method, divide the focal length of the telescope in millimetres by 110. This will give give you the diameter of the moon on the sensor in millimetres.

For more information on the eyepiece projection method look at this site Ann Dittmer's Lunar Photography. Look at the "nuts and bolts" page for calculating the magnification of the eyepiece.

Peter

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