Built in field stop in C11 Celestron w/1.25 visual

Does the use of the standard 1.25" visual back restrict the visual image in any real way with a C11 Celestron? I have read various discussions but haven't run into the definitive treatise yet (except for Eye Openers webpage, but its still very fuzzy to me). It seems it may be an issue with long focal length eyepieces, and it seems obvious it is not with shorter eyepices, but at what focal length eyepiece does it start to become an issue? Is there a chart or diagram somewhere? I have virtually no interest in imaging, and I want to make sure I am getting all the C11 is willing to deliver. Seems like a pretty small hole back there compared to the size of the tube! I like Moonlights solution for both restricted light path and a truly awful focus feel of the C11 whereby they just bolt a spacer block onto the 3.5" threads on the back of the C11 tube, and then have a 2" Crayford hung onto that. I am planning on doing that to improve the focus experience when the budget allows.

Thanks in advance for any and all responses.

Oddog

Hi Tom

I bought a CPC 1100 (slightly) used and the original owner supplied some nice addons. Am not expert on C-11. Here are some speculations maybe someone will kindly correct if they are wrong.

I measure about 54mm diameter in the rear of the internal baffle. Just peering up thru the baffle from the rear, it looks like the baffle is about 12 inches long, and appears to widen out slightly toward the front of the baffle.

A refractor 2" diagonal is about 50.4 mm diameter on the 'outside', and a few mm less on the 'inside' light path. Since the converging light path has to travel several inches thru the diagonal to the eyepiece, then whatever is vignetted by the front aperture of the diagonal would be 'smaller than that' by the time the light cone reaches the eyepiece field lens.

Am guessing that a 2" diagonal is about as wide-open as is practical on a C-11. One would only get a few mm diameter increase by finding some esoteric diagonal/focuser with inside diameter larger than 2"? The widening diameter at the front of that long baffle-- the widening doesn't look extreme enough to me, that you would have a 54mm diameter un-vignetted light cone at the rear of the baffle. Am guessing the light cone gets vignetted somewhere farther up that long baffle. Am guessing the practical light cone is somewhat smaller than 2" at the rear of the baffle.

My CPC 1100 came with an addon Astro-Physics 2" visual back which screws onto to 3.25" C-11 back threads. This AP part is a fine impressive hunk of machining for such a 'surprising low' price. Its great!

http://www.buytelescopes.com/product.aspx?p_id=5668&store_id=2&did=4

Already had a 2" refractor diagonal, which works just fine in the AP visual back. The AP visual back is probably about as 'open' as one will get.

Some have alleged that the C-11 max true field of view is about 1 degree. An Orion 38mm Q70 doesn't seem to vignette in the CPC 1100, about 70 degrees AFOV, about 0.95 degree TFOV and 74X mag. With a pricier/heavier eyepiece-- ferinstance a Celestron 2" Axiom LX 31mm would have about 0.91 degree TFOV at 90X.

OTOH, in 1.25" eyepieces, a Pan24 gets about 0.58 degree TFOV at 117X. An 18mm Meade 5000 UW has about 0.51 degree TFOV at 156X. A less-expensive Celestron Axiom LX 15mm (nice for the money IMO), has about 0.44 degree TFOV at 187X.

So I doubt if will anytime soon buy any 'premium' big heavy UWA 2" eyepieces with focal length > 24mm. The $100 Q70 works pretty well, but since about 1 degree TFOV is the max, it seems easier to mainly use 1.25" 'good quality' medium-focal length EP's. The Pan24 or Meade 18mm UW TFOV's are not drastically narrower than 'the max' available from the C-11. I don't often use the Q70. The Meade 18mm UW seems the 'most-used' EP so far in the CPC 1100.

Folks say that a 2" dielectric diagonal is better than 1.25" dielectric diagonal, even with 1.25" eyepieces, because the central area of a 2" diagonal can be more accurate than the 1.25" diagonal. Given so much money spent on a C-11, it doesn't seem irresponsible to spend a little on the AP visual back and a 2" diagonal, even if mostly using 1.25" eyepieces?

Tom Duncan said:

Does the use of the standard 1.25" visual back restrict the visual image in any real way with a C11 Celestron? I have read various discussions but haven't run into the definitive treatise yet (except for Eye Openers webpage, but its still very fuzzy to me). It seems it may be an issue with long focal length eyepieces, and it seems obvious it is not with shorter eyepices, but at what focal length eyepiece does it start to become an issue? Is there a chart or diagram somewhere? I have virtually no interest in imaging, and I want to make sure I am getting all the C11 is willing to deliver. Seems like a pretty small hole back there compared to the size of the tube! I like Moonlights solution for both restricted light path and a truly awful focus feel of the C11 whereby they just bolt a spacer block onto the 3.5" threads on the back of the C11 tube, and then have a 2" Crayford hung onto that. I am planning on doing that to improve the focus experience when the budget allows.

Thanks in advance for any and all responses.

Oddog

My CPC 1100 came with an addon FeatherTouch SCT Microfocuser knob. This one:

http://starizona.com/acb/Feathertouch-SCT-MicroFocuser---Celestron-CPC-1100-P2518C654.aspx

That one is for CPC 1100 only. They sell a slightly different one for other C-11's--

http://starizona.com/acb/Feathertouch-SCT-MicroFocuser---Celestron-11-except-CPC-1100-P982C654.aspx

Dunno if the replacement Microfocuser is 'smoother' than the stock knob when twisting the 1:1 knob. Dunno if the Microfocuser is supposed to reduce mirror flop, or whether the only advantage is the 10:1 fine focus knob.

I do see some mirror flop, but it isn't bad for visual use on mine. Unless the Microfocuser somehow makes the thing work smoother, I'd be happy with the original stock focuser. Turning the 'big' 1:1 knob easily gets real close to focus on mine, though supposedly there is product variation and some are not so good. In fact, I rarely can get focus improvements using the 10:1 knob. There is some backlash in the mirror movement. To avoid backlash, have tried moving the 'big knob' till it is a little out of focus in one direction, then using the 'little knob' to take up the backlash and move to a perfect focus. But in my experience, the end result doesn't seem much better than just carefully adjusting the big knob. Using the little knob, it gives about the same focus result, but takes longer to achieve .

If I add another focuser, will probably use some kind of helical focuser on the front or back-end of the diagonal. Borg makes some nice helical focusers. I have one of the small rotating Borg helical focusers which works nicely, but haven't yet tried it in the CPC 1100.

http://www.sciencecenter.net/hutech/vis-acc/diag.htm

For 1.25" use only, the one I have is DP36.4 prism diagonal with Borg 4317 rotating helical focuser.

You can also find 1.25" helical focusers that sit down in a 2" diagonal. Orion sells one, though some folks say the thing isn't very precisely made.

Borg also sells big-aperture non-rotating helical focusers that could be attached to the front-end of a 2" diagonal. But those get into similar price ranges as crayford focusers. But if dropping the big-bucks on a focuser, perhaps a big Borg non-rotating focuser would add less distance on the back of the scope, compared to the Crayfords.

http://www.sciencecenter.net/hutech/borg/parts/html/7835.htm

They sell an SCT adapter for that 7835 focuser, but it looks like it uses the sct 'small threads' rather than the 'big threads', so might affect aperture.

http://www.sciencecenter.net/hutech/borg/parts/html/7428.htm

The Hutech folks are very nice answering email, and they have about a zillion parts and adapters.

Am gonna try making a simple rotating 2" to 1.25" drop-in helical focuser to experiment with on the CPC 1100. I don't use 2" eyepiece with it very much.

Tom Duncan said:

Does the use of the standard 1.25" visual back restrict the visual image in any real way with a C11 Celestron? .

Oddog

No. There will be a slight light loss in the
outer ~15% of the field of a 30mm Plossl eyepiece.
But you will never notice it visually.