Getting focus on Celestron C8-N

[SteveBz]

Hi Guys,

I recently received a question about getting focus on my C8-N with a DSLR, which I thought I would post here because the answer might be useful for someone else.  As I'm travelling at the moment, I can't post any photos except ones that I already have, so when I get back, I'll add some better ones.

When I first attached my DSLR to my scope via the 1.25" nose piece adaptor, it was apparent that focus was a problem.  I couldn't focus inwards far enough.  I looked around and there was a lot of advice here on SGL. 

In summary, at 1.25" there were only two ways of getting focus:

1. Use the cell from a 2x Barlow lens and screw it into the nose piece.  This will give you focus, but at the expense of greater magnification and longer exposure times.  Thanks to Gina on this site for this idea (reference to follow)
2. The other way is to unscrew the 1.25 eyepiece holder from the adapter.  Look closely, the adaptor is two pieces.  Lift the adaptor out of the focus and unscrew it (photos to follow).  (This advice does not apply to the Celestron 114 PowerSeeker, which I also have.  Here the adaptor is one piece.).  Then take your nose piece adaptor and unscrew the nosepiece from the T-adaptor.  The focuser attachment part of the 1.25" eyepiece adaptor will screw directly onto the T-adaptor.  Again, you can get focus, but the price is you will no longer have an internal thread from the nosepiece adaptor for filters and so on.  I worked this way for a long time, but eventually I moved to 2".  Here there is a better answer, although obviously everything is a bit more expensive.

At 2" there are also 2 ways.  The Barlow solution still works for the price of a 2" barlow, but the main way is to shorten the optical path.  For this I used two components:

• You need a low profile adaptor.  I bought a silly 2" ring off eBay for £5 and hacked it about (photo to follow), here you will save about 20mm off the optical path, but it still wasn't enough;  This photo is not perfect, but you can see the low profile adaptor with a lens cap at the top, in place of the standard adaptor.

• I then bought some thumbnail 5mm knobs at, I think, 20 and 25mm, you should measure to be sure, and raised my main mirror inwards.  Here I took anther 10mm off the optical path and I now have focus, with a grand 2mm to spare!  I'll take a better photo later, but this shows the sort of Bob's knob's style screws that I used.

 

With the two inch solution you can still use your nose-piece adaptor and there is space for filters too (of the more expensive 2" variety). 

So now this is what I'm doing and I'm gradually building up a collection of 2" filters, Barlows and eyepieces for the new 2" setup.  The advantages are many.  I can have filters (especially light pollution filters), there is less vignetting, the 2" setup is much more stable and less wobbly making focus easier to find.

More detail and photos to follow.

Regards

Steve.

[jschusterman]

Thanks for this Steve! So, based on what I've read here, you CAN achieve focus simply by attaching the camera via t-ring directly to the 2" attachment part of the focuser, yes? You didn't have to ALSO use a 2" barlow, move the primary mirror forward, or use a low profile focuser? Is that how you did it when you said "I worked this way for a long time"? Thanks again!

 

- Josh

[SteveBz]

It's the 1.25" focuser attachment. The 2" is rigid and one-piece.