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How to Properly Center Short Focus Eyepieces

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#1 asaint

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Posted 23 November 2008 - 07:20 AM

Short Focus

#2 cosmicray

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Posted 23 November 2008 - 05:36 PM

Hi Ragold,

When I was at school in the 1960's my science lab had an observatory equipped with 6" reflector that was made by a Scottish company called Charles Frank. It had several eyepieces supplied with it and they were screw fit and if I remember correctly also helical focussing too.

When I got my first scope - a Japanese 60mm refractor I was dismayed at the smaller EPS and the fact that they were held into the focusser by a single set screw and also how sloppy the rack and pinion focusser was.

Now we have Crayfords and some improvement in holding EPs from my vintage 60mm scope but that Scottish scope had it right and your article reminded me of how good that school scope was all those years ago

Thanks for your article

ATB

Ray

#3 hoof

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Posted 23 November 2008 - 09:04 PM

Ah yes, centering high power eyepieces. This is why I *strongly* recommend collimating telescopes with the 1.25" adapter you intend to use, and with whatever locking mechanism you intend. If you align everything to center up through a cheshire or laser that's off-set by 0.5mm, then your off-set-by-0.5mm high power eyepiece will be perfect, right?

So rather than trying (in vain possibly) to center the focuser perfectly, then trying to center the eyepiece in that focuser, why not simply replicate the circumstances that the high-power eyepiece will be used in (offsets and all), and align to *that*?

Another aspect I use to check alignment is high power star test. If there is significant misalignment in a newt (enough to affect the view), it'll show up in the out-of-focus startest as non-concentric rings (the classic out-of-focus coma pattern). In addition, a rough check can be done ensuring the secondary "shadow" is centered both in focus and out of focus.

Bottom line, I believe in aligning to the axis of the eyepiece, and not wasting time trying to get the eyepiece centered on the focuser (and the focuser to the 'scope). If a 1.25" adapter adds 0.5mm of misalignment, and the set-screw or single-screw brass ring system adds a further 0.5mm, it'll do the same for your 1.25" cheshire sight tube or 1.25" laser. Thus, aligning with those with the same 1.25" adaptor you intend to use should solve your alignment problem.

I've always wondered why people bother with 2" laser collimaters. Unless your high-power eyepieces are 2", all you are doing is aligning the 2" mode of the focuser, and ignoring the effects of the 1.25" adaptor. Personally I like to align with the final configuration, this simplifies everything for me.

#4 Starman1

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Posted 27 November 2008 - 04:37 PM

Jonathan hits the nail on the head. If your collimation device is off-center in exactly the position occupied by the eyepiece when it is in use, centering the high-power eyepiece is not worth worrying about.

As to why to use 2" collimation tools (active or passive), the purpose is to get the optical axis coincident with the line of travel on the drawtube of the focuser. The 2" tool more simply aligns the setup because of the lack of an intermediary device with its possible misregistration.

Now if you add a 1.25 to 2.00 adapter, it would behoove you to move the eyepiece and adapter off center in the same direction to more closely track the position of the 2" collimation tool.

Ragold's hypothesis that a 3 screw compression ring would work best is, however, fraught with imprecision. To return the eyepiece to the same location each time would require tightening each screw exactly the same number of turns each and every time. In contrast, a single screw forces the eyepiece against one side of the adapter, where its barrel registers against the inside of the adapter. So long as you always push the eyepiece off center in the direction in which you pushed your collimation tools, the error is likely to be a substantially smaller amount than his worst-case scenario.

Now if the 2" focuser centered the adapter, and the adapter centered the eyepiece, then the optical axis would coincide with the center of the focuser drawtube. That would be great, except it is an extreme rarity in the telescope market.

Precision focusers and precision adapters have smaller amounts of play, and do not shove anything off center by amounts as large as 0.5mm. My Paracorr fits tightly in my focuser, just like my collimation tools, and I'd estimate the degree of off-center to be in the low thousandths of an inch.
Likewise, the 1.25" adapter is very snug.
Very poor tolerances are one of the reasons for an upgrade after-market.


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