Shortening a Newtonian..future proofing.

[kalasinman]

I am preparing to do this because my DSLR will focus with 4mm to spare, but I want to mount a 12mm thick OAG. As this is not a process I want to repeat, I'm thinking about just how much I should shorten. If I'm imaging, I'll want to add a filter wheel in the future. OAG =12mm, filter wheel around 20mm which puts me at shortening up 32mm. I wonder what else, if anything , I might anticipate adding to the train? I've seen devices used to fine tune sensor tilt, but am I right in assuming that would be used with a CCD which would not require as much inward travel as a DSLR?

[Lonestar70]

With a DSLR camera I would think a filter wheel would be a non-required item, but you may feel that an 0ff-axis guider would be necessary, albeit there may be other things preventing this such as the body size of the DSLR getting in the way.

In this case a finder guider would be the better choice.

A short 1 1/4" or 2" nosepiece for your camera tee ring would hold a single filter... be it light pollution or other type... this would also allow you to rotate your camera for image alignment.

If you eventually decide that a Mono CCD camera with a filterwheel is the next upgrade then the existing backfocus will probably be sufficient anyway... most CCD cameras have a back focus length of around 17- 20mm typically...as against the 46.5mm of your D5100.

Moving the mirror by such a large amount will mean you would also need a larger secondary mirror or you would lose a lot of your captured light.

I would suggest you did no more than perhaps 12mm to 15mm of mirror movement max.

Hope this helps.

Clear skies and good luck.

Best regards.

Sandy.

[kalasinman]

With a DSLR camera I would think a filter wheel would be a non-required item, but you may feel that an 0ff-axis guider would be necessary, albeit there may be other things preventing this such as the body size of the DSLR getting in the way.

In this case a finder guider would be the better choice.

A short 1 1/4" or 2" nosepiece for your camera tee ring would hold a single filter... be it light pollution or other type... this would also allow you to rotate your camera for image alignment.

If you eventually decide that a Mono CCD camera with a filterwheel is the next upgrade then the existing backfocus will probably be sufficient anyway... most CCD cameras have a back focus length of around 17- 20mm typically...as against the 46.5mm of your D5100.

Moving the mirror by such a large amount will mean you would also need a larger secondary mirror or you would lose a lot of your captured light.

I would suggest you did no more than perhaps 12mm to 15mm of mirror movement max.

Hope this helps.

Clear skies and good luck.

Best regards.

Sandy.

Yes, I currently have an OAG I need to mount. It's 12mm. --Thanks