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2" Oculars on my LX 90?

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#1 Hal Pollner

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Posted 27 October 2005 - 11:35 AM




Gentlemen:

It would be nice if I could use some of the 2" eyepieces I use with my 10" LX200GPS on my new LX 90.

What would be required in the way of adapters?

What about clearance with a 2" diagonal?

Thanks...
HAL

#2 jrcrilly

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Posted 27 October 2005 - 12:36 PM

What would be required in the way of adapters?


Any 2" visual back with SCT threads would do the trick.

http://www.scopestuff.com/ss_adt2.htm

I think Tele Vue may make a shorter one which would reduce the clearance problem somewhat.

#3 Starman1

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Posted 27 October 2005 - 04:59 PM

The LX90 is kind of tight. I'd recommend, if you can afford it, the TeleVue Everbright Shorty with Shorty adapter. This I KNOW will clear the base at all altitudes.
You'll never see the full 1.3 degree field this instrument is capable of until you change to 2".

#4 John_Gillies

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Posted 27 October 2005 - 05:32 PM

Hi Hal,

The Denkmeier 2" Star Diagonal ordered with the shorty SCT adapter and shorty nosepiece will give you the clearance you need to get through the forks. I use one for my 8" LX10 only with the Power x Switch. These particular adapters were designed to be used for the 8" SCT's. The diagonals are made by William Optics. See them at www.denkmeier.com

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#5 Hal Pollner

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Posted 27 October 2005 - 05:43 PM


Thanks, Men!

I'm waiting on my William Optics 2" Dielectric Diagonal (99% reflectivity) on back order for my 10" LX200GPS/UHTC.

I'll check into those diagonals you mentioned for my LX90.

HAL

#6 Hal Pollner

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Posted 28 October 2005 - 10:46 AM


HOLD IT...

What's all this about adapters, shorties, and nosepieces?

I thought there would be 2" Diagonals available which would thread right onto the 50mm (2") male threads emerging from the visual back of my LX90 without any intervening hardware.

HAL



#7 jrcrilly

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Posted 28 October 2005 - 10:55 AM

HOLD IT...

What's all this about adapters, shorties, and nosepieces?

I thought there would be 2" Diagonals available which would thread right onto the 50mm (2") male threads emerging from the visual back of my LX90 without any intervening hardware.

HAL


Sure, any SCT diagonal will fit the back of the telescope. I thought you wanted to use the diagonal you use with your 10". If you are going to buy a 2" SCT diagonal you should know that Astronomics is offering a closed-out Celestron unit for $40.

#8 Starman1

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Posted 28 October 2005 - 11:16 AM

HOLD IT...

What's all this about adapters, shorties, and nosepieces?

I thought there would be 2" Diagonals available which would thread right onto the 50mm (2") male threads emerging from the visual back of my LX90 without any intervening hardware.

HAL

The clear openings of these thread-on diagonals varies from brand to brand. Usually, no matter what brand, the thread-on diagonals have a much smaller clear aperture than the tube-type (refractor style), which is usually attached to the tube with a short adapter tube threaded on to the SCT threads. To get the full illumination of the largest field stop in a 2" eyepiece, always emply a tube-type 2" diagonal with an adapter tube, NOT a thread-on SCT diagonal.
The tube type diagonal can be rotated to the side more easily and can also be used in a refractor.

#9 Hal Pollner

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Posted 28 October 2005 - 11:45 AM



HEY DON:

The W.O. Crayford focuser on my 10" LX200GPS has that rotating feature tou mentioned.

About the clear apertures through the eyepiece field stops:

The largest field stop of any of my 2" oculars is 1-3/8" in my 32mm Meade SWA. Therefore, I would not need a light column any greater in diameter than 1-3/8" to satisfy the illumnation of ALL my 2" oculars!

Am I right?

Thanks again... (God, I love these discussions!)
HAL

#10 Starman1

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Posted 28 October 2005 - 12:47 PM

No, you wouldn't need illumination to exceed 70% at the edge of the 1-3/8" (35mm) field stop to have vignetting be non-visible. Ergo, the fully illuminated field can be smaller than that.

If you look through the back of an SCT with all the back end stuff removed, and you move your eye to the side so the straight-in point of view is parallel to the edge of the baffle, you'll notice that the image of the primary mirror gets progressively more cut off until you see only about 50% (or a little more) of the primary. That says that the baffle lengths and diameters on SCTs are chosen to fully illuminate fields smaller in diameter than the full aperture of the baffles, and they produce light drop off at the edge of fields large enough to engage the full exit field of the baffle.

There is a rule in newtonians, and it works in SCT's too, that the fully illuminated field should be at least 1/2" (12.7mm) and that light drop-off isn't noticeable when that parameter is chosen. In an SCT, that means you could move your eye either way from an on-axis position by 1/4" and still see the entire primary.

As you can tell, the farther back your eye is, the less field you can fully illuminate with no change in baffle size. Ergo, 2" diagonals, with their longer internal light paths, cause more illumination problems for, say, the 8" SCT than does a 1-1/4" visual back. At the same time, though, there's less vignetting. And there are short 2" diagonals with less internal length for the light to traverse. It's why I think all 8"+ SCTs should come stock with 2" diagonals, but then they'd make less money on upgrades ;).

With today's 2" eyepieces, some with quite large field stops (46mm), the 1/2" rule no longer works. It should be larger, say 3/4". But few of us have 2" eyepieces with field stops that big, so we can get by with the fully illuminated field of the old standby rule.

So if you use a calculator to pick a diagonal size for a newt, or you wonder how the baffle diameter and length were chosen, just remember the size of the field for full illumination (100%) should be at least 1/2" so the 70% level isn't reached until the edge of your largest field stop eyepiece.[Yes, I know this is dependent on focal length, but we aren't talking about f/3 or f/16 systems, here]

Photographically, that's not large enough. It's why I contend the 8-11" SCTs weren't really optimally designed for photography, but were designed so photography could be done as a secondary use.

The good news for the SCT makers is that the chip sizes in digital cameras aren't as large (usually) as the 35mm film frame, so the illumination figures for SCTs are much more appropriate for photography. Now. Until CCD chips get cheap enough that large size chips are in common use.

Oof! I rambled. Well, I hope that made a couple things clearer. At some point, a light bulb goes on and you recognize how many compromises have to be made in telescope construction. I'm just glad I don't have that job.

#11 davidpitre

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Posted 29 October 2005 - 10:14 AM

I use a W.O. SCT diagonal on my LX90 and highly recommend it. It will not clear the base with a focal reducer attached.
I love the idea of the Denkmeier 2" Star Diagonal, but it's not in my budget.
David

#12 Hal Pollner

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Posted 29 October 2005 - 10:28 AM



HEY DAVID:

Will the W.O. 2" thread-on diagonal clear the fork base on an LX90 WITHOUT a focal reducer attached?

What is the diameter of the light path through the screw-on fitting? If it's less than 1-1/2", then it will be restricting the 1/1-2" light column from the rear cell.

Do you have the high-reflectivity dielectric diagonal? (99% reflective)

Thanks...
HAL


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