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Focus and Photography

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I have connected a DSLR to my telescope and with a 20mm eyepiece I can manage to focus but if I use another eyepiece, say 12.5mm, then focus is not possible due I imagine to the eyepiece being to far out. Can anyone suggest a way around this or am I limited to the 20mm eyepiece due to the telescope I am using.

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It is quite common not to be able to achieve focus with a DSLR on a Newtonian reflector as they have limitations on how much focuser travel can be provided without the draw tube causing an obstruction into the light path.


It is easiest to set up in the day time, focus on something about 50m away then progressively aim for more distant targets, you need to wind the focuser in for further away targets.  Be careful not to point the scope at the sun as you will destroy your camera and may even melt part of your scope.

You may find that it is not possible to focus past a certain point as you have the focuser fully in.

If this is the case you either need to modify the setup to bring the camera closer to the mirror or use a Barlow lens that will effectively move the focus point further out.


Don't however confuse poor seeing with poor focus, if the image is swimming around this is the result of thermals either within your scope or in between your scope and the target you are imaging.

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Try using a barlow ahead of your eyepiece adapter with the 20mm to get to higher powers instead of using the 12.5mm eyepiece.  I'm going to guess that since the 20mm has longer eye relief than the 12.5mm, the image comes to focus farther away from the the eye lens of the eyepiece, which results in less need for in-focus, which you've apparently run out of with the 12.5mm.  You could also try using a longer eye relief eyepiece, but you have to be careful it will fit inside your eyepiece adapter tube.  Many long eye relief eyepieces are wide because of their large eye lenses.

BTW, the technique you're using is referred to as eyepiece projection photography.

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  • 2 weeks later...

I had a skywatcher 200p and couldnt achieve focus using my canon 450d.

I was told about a self centering adaptor from modern astronomy that solved this problem.

Give bern a ring there and he will tell you what you need.

Cost me around £80 to solve it.

Hope this helps.

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