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SkyWatcher Evoguide ED 50mm as a right angle finder

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#1 dhferguson

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Posted 08 October 2019 - 01:38 AM

Cheers,

 

I prefer a right angle finder for axis-symmetric telescopes such as SCTs and refractors.

 

I recently ordered an Evoguide 50mm ED mainly for AP guiding, on sale of course, from Astronomics, who provided the following caveat with regards to its use as a finder, to wit: "... Visually there will be some compromises.  Due to the 20mm of backfocus, or 60mm if you remove the 40mm extension, you would have to use it like a finderscope straight through.  There will not be enough room for a diagonal...."

 

Nonetheless, I thought I'd try using it as a right angle finder anyway, and here is my report. I decided to "donate" the Celestron 1.25" 90 deg  diagonal that came with my Celestron Edge 8, much preferring the use of a Baader Clicklock and 2" diagnonal/eyepieces instead with this latter instrument. Please note that, while this converts the Evoguide to a right angle finder, it is NOT a RACI finder. RACI diagonals generally use Amici roof prisms and have longer light paths though the diagonal than just a plain star diagonal gives. My inverted image finder thus yields the same field orientation as my Edge 8 and my Stellarvue 125mm refractor (I alternate using both). The shorter optical path length of the plain diagonal is important because, when I installed the Celestron diagonal on the Evoguide, there was just enough focal length to focus properly: any longer optical path would not have worked. Even so, I found it necessary to remove the extender on the Evoguide to provide more optical path length through the diagonal, and this was just enough.

 

I decided to also "donate" a 1.25" Besser 20mm eyepiece of 70 deg field of view to my new "finder." The result is very sharp (for a finder) images, and I am quite happy with the result. Below are listed the "specs" of my new finder. The only number I might have wished to be different is the rather short eye relief of 12mm. This turns out to be adequate for me but something like 18mm would have been nicer. However, the Besser eyepiece is best-in-class (inexpensive, that is) for optical quality, so I accept the compromise. A longer focal length Plossl would have worked too but ... the magnification would have been less for about the same field of view. BTW, the Besser barrel cannot be unscrewed to allow for easy installation of a human hair reticle. Instead, I will use a small dot from a magic marker directly on the aperture lens, probably best if it is not quite centered. 

 

Skywatcher Evoguide 50ED APO/Bresser70deg20mm"
  Objective Diam. (mm) 50
  Focal Length (") 9.4
  Focal Ratio 4.8
  Exit Pupil Diam. (mm) 4.2
  Finder FOV (deg) 5.83
  Magnification 12
  Weight (oz, w/ eyepiece) 38
  Eyepiece Eye Relief (mm) 12
  Eyepiece FOV (deg) 70
  Eyepiece* F.L. (mm) 20

 

 

 

I realize my setup is more expensive than the usual 50mm finder but now I also have an excellent guide scope too.

 

Happy observing always,

 

Don
  

 


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#2 wrnchhead

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Posted 08 October 2019 - 03:58 AM

How are the views? Well corrected? Ever since I saw that little scope I have been fascinated by it and I think it’s going to be one of those things I buy just because I want one. Also love the looks and finish of it.

#3 dhferguson

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Posted 29 October 2019 - 11:02 PM

Cheers,

 

 

Hello Eric. You asked about the optical quality of the Evoguide 50ED. I had a look through it with a 7 mm Televue Type 1 (x34) the other night to try to answer your question.

 

There is a was a very slight amount-barely detectable--of CA, which I confirmed by the pronounced asymmetry of color correction just inside and just outside focus. I believe this is an FPL51 as opposed to an FPL53/FCD100 ED objective, so very good but not outstanding color correction is to be expected, just don't expect visual perfection.

 

Unfortunately, I was limited to x34 using the star diagonal. My 5 mm and 3.5 mm eyepieces are XWs that require slightly more "in" focus than is available with the diagonal. I would have changed to the "straight through" mode except the seeing was pretty poor the night I looked. Anyway, the two close Epsilon Lyrae looked slightly elongated but I couldn't tell if they were split, which is a nice test for a 2". I'll get back to you with a better answer within a few weeks, OK?

 

At 34x and under crummy seeing, the diffraction disk was simply too small for a fair and objective star test.

 

I am not (yet) an astroimager but I noticed on the Astronomics website a blurb saying imaging should be OK within a 15 mm circle. Note that an APS-C detector (even the small pixel Nikon detectors) are larger than this. Evidently, this telescope will vignette an APS-C detector, and so the entire field of such a camera will not be available.

 

I am very happy with this telescope as a finder and it should also be great as a guide scope. Celestron owners, note that the tube is too wide in diameter to fit in that odd 50 mm finder holder that Celestron provides; the one with the rubber O-ring in front and the piston+2 screws adjustment mechanism in back.

 

Happy observing always,

 

Don


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#4 dhferguson

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Posted 06 November 2019 - 03:09 AM

Hello Eric,

 

I promised I'd get back to you here with definitive test results, and here they are. We've had a number of nights of good seeing out here in NorCal lately. I was able to use my 3.5mm Pentax XW eyepiece (excellent quality) with the straight-through visible back on the 50mm (AKA 2") Evoguide ED refractor.

 

The optics results were very good! I elongated both components Epsilon Lyrae but, given it was nearly at zenith, due to discomfort I could not fairly state whether or not they were split. Recall that the two components in each pair are roughly 2.3" separation, while the Rayleigh limit of a 2" telescope is 2.8" at the peak of human night vision, about 0.55um. This makes Epsilon Lyrae a tough test for a 2". 

 

Anyway, I next went to a lower and more comfortable target, a bright second magnitude star, and star tested the scope with the same eyepiece. The results were quite amazing: with a green filter (monochromatic, no CA), the little telescope showed only the very slightest spherical aberration, certainly Strehl > 0.97, whoopee! Unsurprisingly given the presence of just a slight amount of residual CA, the star test was a bit worse but, still, Strehl > 0.95 polychromatic visible. All-in-all, this is an optically fine little telescope!

 

As I'd stated, I intend mine to be used both as a finder and a guide scope. This little telescope is truly portable and also would do well on a lightweight camera tripod. That makes it packable in a carry on suitcase along with everything else you need for a trip, a truly portable telescope! Unlike finders made with binocular objectives, it will accommodate at least x70 for views of the Moon and planets. Still, it IS a 2" scope with 2" light collecting ability.

 

So here we go: mechanical--B (the very fine helical focuser and the inability to use a RACI diagonal can be lived with; the build is typical ES which is decent but not great), optics--A-minus.

 

Happy observing always,

 

Don


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#5 Jmdaniels

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Posted 06 November 2019 - 05:41 PM

Thought this may be useful. Using mine as s grab and go ruck sack scope. Getting up to 60x mag and  using fluid head to support. Great little scope



#6 wrnchhead

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Posted 06 November 2019 - 08:01 PM

This is great. Going to get one for sure! Maybe my ST80 is overkill for guiding and this would make a nice color matching weight loss addition to my rig.


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