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An Astro-Monocular

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#1 desertlens

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Posted 09 April 2013 - 04:41 PM

I've been experimenting with the Stellarvue FM60 finder for over a year now and it has surprised me more than once. Originally acquired with a RACI prism it was intended as an observing instrument rather than a finder. I've had some excellent views with it including the Veil Nebula with a 25mm Ortho and an OIII filter. It proved to be much better optically than I had any reason to expect from an ƒ3.75 achromat.

I've long toyed with the idea of a telecope I could use hand-held. I enjoy my binoculars but am cursed with a narrow face which can make them tedious at times. I also wanted some eyepiece flexibility and the ability to use filters at these magnifications. Here's what I've come up with:

Posted Image

I recently replaced the prism with the 2" helical focuser offered by Stellarvue. Frankly, I'm totally surprised (again) with what I'm seeing. If anything, optical qualities have improved without the RACI prism. The image is inverted but I find it no more distracting for astronomy than with a Newtonian. The focuser is well made: versatile, smooth, with no play and a movement I would describe as "fine" rather than "coarse". Also, both the focuser and the supplied 1.25" adapter have compression rings, a nice touch with heavy eyepieces.

I estimate that my real limit for a hand-held instrument is about 12x. My eyepiece selection reflects that. 25mm=9x and 18mm=12.5x. I will, of course, push that to 18x with a 12.5mm Ortho on those nights when I'm feeling "stable". I had high confidence that the Orthos would do well but was struck with the performance of the 18mm ES82... huge field and very sharp until just before the field stop. The 24mm ES68 is very nearly as good (actually a bit better than a 19mm Panoptic). The Orthos are used with the high hat adapter to get a head start on out-travel and for a bit of face clearance.

I doubt that I have enough magnification to do much with Jim Barnett's 60mm challenge but I'm looking forward to continued use of this little gem.

#2 JKoelman

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Posted 11 April 2013 - 12:28 PM

A reincarnation of the pirates scope... :grin:

I was eyeing these finder scopes (80 mm version) as ultra-light widefield travel scopes.

If anything, optical qualities have improved without the RACI prism.

Have you done flashlight tests to check the unobstructed aperture with and without RACI prism?

#3 desertlens

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Posted 11 April 2013 - 03:00 PM

Well, this Roger is jolly. No flashlight test, but I did an a/b comparison with and without the RACI prism and see no radical difference in brightness. My impression was that illumination was a bit better without. There is a baffle but I see no evidence of any obstruction with any EP including the 2" 18mm, 82º. Stellarvue even suggested a 22mm Nagler.

#4 JKoelman

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Posted 12 April 2013 - 08:30 AM

Hmmm... the tube OD is smaller than my IPD. I could buy two of these...

But I wouldn't like the ergonomics of a straight thru design. Petty they don't come as shorter tubes so that 2" Amici's can be inserted...

#5 desertlens

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Posted 12 April 2013 - 10:43 AM

True, there are certainly better binocular options. I thought I'd try a cyclops unit and it's working well. The scope is small and the straight through design works... rather like holding a long lens on my camera.

#6 Midnight Dan

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Posted 12 April 2013 - 12:53 PM

Now all you need is a holster! :grin:

-Dan

#7 desertlens

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Posted 12 April 2013 - 03:35 PM

A holster! Yeah, that's the ticket. :cool:

#8 StarStuff1

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Posted 12 April 2013 - 04:05 PM

I have heard that the SV 60mm scope can take much more power than 18X.

As an experiment a couple of years ago I modified a no name Chinese 8x50 straight through finder scope. Part of the rear tube was cut off and a 2-1 1/4-in adapter slid right in. With decent eyepieces I was able to get decent images up to around 50x. Castor was not a clean split but rather two fuzzy balls barely touching.

#9 desertlens

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Posted 12 April 2013 - 05:14 PM

I have heard that the SV 60mm scope can take much more power than 18X.



That's true. I've had it significantly higher (32x) and it held up well. I'm primarily thinking hand-held (possibly a monopod) so 18x will likely be my upper limit with 12.5x more common.

#10 JKoelman

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Posted 12 April 2013 - 10:49 PM

Craig -- your posts suggest you made the transition from 90° RACI to straight-thru to get better hand-held ergonomics/stability? However, I would expect the 90° to be easier to hand-hold for stargazing, particularly when sitting in a chair with the elbow of the arm holding the scope resting on the armrest. Have you tried this?

#11 desertlens

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Posted 13 April 2013 - 11:29 AM

Johannes,
Yes, I tried working with the RACI but found it difficult to get oriented to the target when my eye-line was at 90º to the scope axis. I had to keep looking up and away from the scope. With the straight-through approach, I'm looking along the scope axis and have a free eye for orientation. It just seems more intuitive for me. Also, stability was no better for me with the 90º approach.

#12 GlennLeDrew

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Posted 13 April 2013 - 12:26 PM

Craig,
My home-made 60mm aperture, 90 degree bino, even at 21X, is not so difficult to aim. It's definitely worth the ease on my neck and back to have the less easily aimed 90 degree configuration.

#13 desertlens

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Posted 13 April 2013 - 01:35 PM

Glenn,
I'll probably work both ways but for now the inline approach seems more intuitive for me when working hand-held... just the way my brain works. I also think the prism wasn't helping optically and it limited me to 1.25" EPs. No big deal since I often use orthos but I'm very pleased with the 18mm ES82 in this scope. I'm working on a way to mount on a monopod which should also help with stability while preserving much freedom of movement. Thanks for your input. BTW, I've always enjoyed the Rickover quotation.

#14 JKoelman

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Posted 13 April 2013 - 10:39 PM

I also think the prism wasn't helping optically and it limited me to 1.25" EPs.

I suspect the prisms to be undersized for the steep f/3.75 light cone.

This thing could be a wonderful ultra-compact widefield travel scope (and times 2 a great 90° travel bino), if putting my WO 2" Amici in the straight thru version would allow focus at infinity. But I am always hesitant to place a handsaw against an OTA...

#15 plyscope

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Posted 13 April 2013 - 11:17 PM

Craig what is the OD of the tube at it's largest? What would be the minimum possible IPD for a double tube bino setup?

I'm thinking if the lens is 60mm then the diameter of the dewshield is probably close to 70mm therefore the IPD would be 70. But then I'm only guessing.

#16 Midnight Dan

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Posted 14 April 2013 - 09:11 AM

I have heard that the SV 60mm scope can take much more power than 18X.


I think the limitation is the how steady you can hand-hold the scope, not how much power the scope can take.

On the other hand, some people scoff at hand-held limitations! :grin:
http://farm3.static....1e46dab.jpg?v=0

-Dan

#17 desertlens

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Posted 14 April 2013 - 11:00 AM

Andy, the dewshield is 72mm in diameter, so your estimate is pretty close.

#18 desertlens

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Posted 14 April 2013 - 11:11 AM

On the other hand, some people scoff at hand-held limitations!



... and some people have a good deal more arm strength than I do. :grin:

This little scope will take more magnification and I'm working on a monopod solution but I think I probably stop around 18-20x. If I want more than that, I have a small mounted refractor. Dancing stars have a rather psychedelic effect on me and I can only take so much of it. :p

#19 desertlens

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Posted 14 April 2013 - 11:18 AM

I suspect the prisms to be undersized for the steep f/3.75 light cone



That's what I'm thinking. We have to remember this is a finder and wasn't really designed for my tinkering... but it works and that's remarkable.

#20 plyscope

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Posted 14 April 2013 - 03:59 PM

Thanks Craig,

I think 72mm rules it out as a binocular concept for most people, me included. Oh well back to the drawing board. :)

#21 core

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Posted 14 April 2013 - 11:28 PM

I had my sister's Borg 45ED for a couple of months, absolutely fantastic little combo; very conservative specs at 325mm f/7.2 , upper limit of magnification was limited only by exit pupil. Plus you could 'upgrade' the objective to a 50mm or 60mm with the rest of the OTA remaining the same. The only down side was the cost though ... not sure I'd spring for one myself.

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#22 core

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Posted 14 April 2013 - 11:30 PM

Will take 2" as well. A pair of 'em (better yet 60mm and up) would make a fantastic little bino scope; there are a couple of dual mounting plates out there iirc.

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#23 desertlens

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Posted 15 April 2013 - 01:30 AM

Peter,
A pretty little thing. Is that the helical focuser up front? I think it was Steve Martin that coined the phrase: "Let's get small". :cool:






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