Friday, June 15, 2018

Eyepiece Adapters

It is often a surprise to those without any interest in astronomy that maximum magnification is often not very useful.  This is why cheap, department store telescopes often advertise "up to 500 power!"  Most of the time, atmospheric turbulence or poor clarity (usually because of humidity trying to become clouds) means that as much or more detail can be seen at 120x as at 500x, sometimes more.  A common rule of thumb is that 50x-60x diameter of the objective diameter (big lens on refractors and mirror on reflectors) is the highest power you are ever likely to find useful or pleasing.

There are exceptions.  Extraordinarily made refractors, like the Televue and Astro-Physics refractors and the Questar Maksutov reflector (for those of you with money to burn) can often benefit from much higher magnifications.  Here I describe a comparison test I did many years ago of a Televue Ranger to a couple of the cheap refractors, where I found even 97x aperture on the Ranger to still be a very pleasing result on Saturn.  I understand Questars often tolerate 100x.

Planets and the Moon are sometimes beneficiaries of stupid-high magnification. (Yes that's the technical term among amateurs for >50x per inch.)  One night in California, we had an extraordinarily hot day, and because it barely cooled at night, there was no humidity and nearly no turbulence.  I found 700x on the Moon with my 8" reflector (which has a very fine Coulter mirror) was still sharp and beautiful.  But this was a rare night, one that I have never repeated.

You get some often startling results.  When I first built that 8" reflector with my father, we lived in Santa Monica.  What I lost in transparency from smog was compensated for by low turbulence.  (I think the inversion layer that gives Los Angeles its smog problem does something good for reducing turbulence.)

If you are hunting for deep sky objects, like galaxies and nebulae, which usually occupy lots of area, a wide field of view and therefore low magnification are of high value.   Eyepieces have what is called the apparent field of view (AFOV); this is how much of the true field of view your eye will see.  So the 25mm eyepiece that I use with that 8" reflector is  1414mm/25mm = 56x.  AFOV of the eyepiece is 45 degrees.  Actual field of view is therefore 45/56=.8 degrees,  There is plenty of black field around the Moon which is .5 degrees.

When I started buying eyepieces, the 45 degree apparent field of view was considered awesome.  Then a troublemaker named Al Nagler who came off the Apollo mission as it ended, and started Televue.  He used his expertise to produce eyepieces that had 82 degree AFOVs, and now such absurdly wide eyepieces come from many makers; show it can be done and everyone applies their expertise to the problem.  Competition makes everyone better off.  These eyepieces are priced the way you expect, and are hand grenade heavy.

That older homebuilt 8" f/7 reflector was a project my father and I did in 1970. It has a 1.25" diameter focuser of course, as nearly all American telescopes did back then.  (Most Japanese scopes until recently used .965" barrels.  In spite of making splendid telescopes, the ,965" eyepieces sold with most cheap refractors are much worse than objective.)  I have acquired a few 2" diameter eyepieces over the years. Two came with a 17.5" Dobsonian that I bought at auction for $600: one is an 18mm University Optics orthoscopic, the other a 50mm brass barrelled item that might have seen service during World War 2. The third is an 85mm Super-Plossl made by a company that offered it as evidence of what they could do (about 35 degrees of AFOV).  That 85mm eyepiece gives about a 2 degree field.   I have wanted to use the 85mm and 50mm eyepieces to create a wider field for deep sky hunting. So I bought this Solomark 2" to 1.25" adapter. It naturally adds a couple of inches to where you are going to be able to focus, but my focuser is one of the absurdly long focusers that University Optics used to sell (3 3/4" of travel), and these 2" eyepieces are long focal length. I am able to use all three 2" eyepieces now.

I also bought a Solomark 1.25" to ,965" adapter as well.  I am supposed to help a friend collimate his Japanese-made reflector and my laser collimator is 1.25" diameter.  This adapter will let me do this operation.

Both of these products were cheap enough that turning adapters myself made no sense: division of labor wins.  They are beautifully finished, slide into the focusers as they should, and accept the eyepiece barrels by just sliding in,  They also use brass compression rings which clamp the eyepiece when you tighten the set screw by squeezing a ring of brass around the barrel.  This is both a tighter grab and does not mark up the barrel.  I suspect it better centers the eyepiece also.

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