View Full Version : Prime Focus

2006-Nov-06, 01:57 AM
I hope its ok to ask a question in this category. I just tried to setup my digital SLR (cannon D60) with a Tmount and prime focus eyepiece adapter. I cant bring it in far enough to focus. I have a 130mm orion relector telescope. Any suggestions? Also I treid in the daytime and it is really close to reaching focus. I just need like another 1/4" or so(guessing).:doh:

2006-Nov-09, 09:16 PM
You could try using a barlow lens without the lens so your just using it as an extenstion tube. I have a similar problem with my 2" focusor when i use 1.25" eyepieces and this works for me.

2006-Nov-10, 07:28 PM
Well, I need to focus in farther than my focuser will go. Does that mean I need to move my primary mirror up farther somehow?

2006-Nov-12, 08:46 AM
you could try and turn your colimating screws on the rear cell on the outside of the rear of the tube in as far as they can go, but yet leaving enough space to keep it in colimation. That would move the mirror up some. I kinda have a similar problem with my orion 8inch refrector, but it has a 2inch focuser and it came with a prime focus adapter. With my Canon 20d it verily reaches focus with it turned all the way in, but it doesnt leave enough room to use filters.

The only other no cost idea I can think off is if the hole in the tube is big enough, you may be able to take the knobs off the focuser and mount it from the inside with the flanges on the inside. For example, take the focuser off, knobs off, etc. Then insert it from the inside of the tube and put the screws back on with the flanges on the inside. Of course this depends on the size of the cut out hole in the tube.

2006-Nov-14, 05:35 PM
Thank you for the advice. Ill see if I can mount it on the inside. Will using a barlow lense help at all? What would happen if I was to trim off a 1/2" of the optical tube so I could remount the mirrior up higher? I guess that could be an expensive mistake.

2006-Nov-15, 09:06 AM
Most Newtonian's never have enough out travel, the nature of the beast. I build my own and allow for using a 35mm or DSLR camera, this needs a slightly larger elliptical diagonal to fully illuminate the field as the mirror is nearer the diagonal.

If you can move the mirror cell even an inch higher up the tube, you will not destroy anything, don't cut the end of the tube off, you can always put the cell back to its original position if you need to.

The Meade SN series have their focus a long way outside the tube so one can fit a 35mm camera.

All my homebuilt Newtonian's I have the focuser & diagonal mounted of a single plate set in tracks so I can move the whole assembly closer to or further away from the main mirror. I do have two separate elliptical flats to ensure full illumination at the extreme ' in ' position.


Shevill- 42 South