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Spotting scope star test

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#1 ChristianG

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Posted 22 January 2016 - 04:54 PM

...Has anybody done it? Mine (a Pentax PF-65EDA II) shows some severe misalignment ot its optics, the star test at 122X looks like the cover of Harold Suiter's book, see below, minus the chromatic aberration:

 

bk_StarTest.jpg

 

I will post a picture of the actual star test soon.

 

Anyway, the question is: should we expect good collimation from expensive spotting scopes, or are they to be used only at low magnifications? Any thoughts on this would be useful...

 

--Christian



#2 DesertRat

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Posted 22 January 2016 - 07:58 PM

Since you asked for any thoughts...

 

1) I would expect better from Pentax, even for a spotting scope.  Something is misaligned or tilted, either an element or prism.  It does seem 122X fairly high for the aperture, and its normal intended use - but still this much coma would hamper even 20X views.

 

2) Its not chromatic aberration but just diffraction (and a lot of coma) on that front cover.  But you probably know that already.  Even a pure reflective scope will show it for an in-focus star - but its not easy to see.

 

3) Think more opinions would available over in the refractor forum, as I believe some of them have this scope or something similar.

 

Glenn



#3 Patrik Iver

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Posted 23 January 2016 - 02:19 AM

My PF80EDa star tests pretty well at ~100x (5,2 mm XL - the shortest EP I own). Concentric and even rings.

 

I've seen surface markings on Mars with this spotting scope.  :)



#4 IDONTSEEIT

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Posted 23 January 2016 - 02:19 AM

How do things look, in focus? That's what matters. IMHO, I believe this is normal for most prismatic spotting scopes. I don't believe their intended use is at magnifications much above ~60X.

 

My Leica 77-APO, when defocused on a star @~60X, or what ever the max magnification with the stock zoom eyepiece is, doesn't look so great, and if I remember correctly it shows signs of some slightly pinched optics, and some other over/under correction type crap. But when it's "focused", everything looks great.



#5 mooreorless

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Posted 24 January 2016 - 09:30 AM

Everything I have read about spotting scopes, even the high end ones can have some problems with sample variations. You could just use lower magnification. If you just bought this and can send it back that is what I would do.  If you want any more info contact me I don't want to post a link on here.


Edited by mooreorless, 24 January 2016 - 09:39 AM.


#6 ChristianG

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Posted 25 January 2016 - 06:27 PM

Hi all.

 

I was a joking a bit about the the book cover, it's not that bad... but almost.

 

I did star tests with a Celestron Neximage 5 and a 2X Explore Scientic focal extender. The target was my artificial star at the end of a long hallway (80 feet approx.). This is what it looks like. The problem is visible with a 5 mm eyepiece (78X), and rather evident with a 3.2 mm eyepiece (122X). 78X should not be too hard on a 65 mm f/6 objective like the PF-65EDA II. I understand that spotting scopes are not designed for kigh magnification, but as others, I had higher expectations from Pentax. By the way, the problem is clearly due to objective lenses misaligned relative to each other, not from the 45 degree prism. I have included a comparison with my Lunt 70 mm f/6 telescope, and this one has a symmetrical diffraction pattern, and pinched optics...

 

I have asked the vendor to exchange it. If they send me another one that has the same problem, I mighty suppose that they are all like that, and leave it at that. But I would like the instrument to live up to its full 65 mm aperture potential! Thank you all for your input. Let's see what the vendor proposes...

 

--Christian

 

PF-65EDAII.jpg

 



#7 Patrik Iver

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Posted 26 January 2016 - 03:12 PM

As I wrote above, my 80 mm Pentax star tests decently, and therefore it would surprise me if the 65 mm version was significantly worse (unless we are talking about sample variation). They might have a different optical configuration, but I see no indication of a diffraction spike (like from a roof prism in the lightpath) in your star test either, so probably they are roughly similar in basic design?



#8 curiosidad

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Posted 09 February 2016 - 01:30 PM

Hello

Anyone  finding any significant reduction in clear aperture among spotting scopes that use internal focusing lenses, or maybe they   that use a moving prism for focusing ?

they  suffered a gradual loss of clear aperture as the prism is moved out of its optimum position toward close focus(prism for focusing). ?

thanks

regards

paul.



#9 Patrik Iver

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Posted 09 February 2016 - 05:32 PM

Hello

Anyone  finding any significant reduction in clear aperture among spotting scopes that use internal focusing lenses, or maybe they   that use a moving prism for focusing ?

they  suffered a gradual loss of clear aperture as the prism is moved out of its optimum position toward close focus(prism for focusing). ?

thanks

regards

paul.

 

According to some threads on Birdforum, the Pentax ED80 is among the worst offenders in this regard (moving prism), reportedly going down to ca 70 mm aperture at close focus.



#10 nixnix

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Posted 12 June 2016 - 11:29 PM

I think you are asking a lot w/122X out of a 65mm objective though I understand the star test. 

 

From what I read they should render 1-1.3mm per mm of objective though not that bright. The max allegedly is 1.5mm.

 

My Celestron Regal M2 100ED does 36X & 77X very nice and clean from modest Luminos 82* 15mm & 7mm.

 

I'm going to pick up another eyepiece to put me on one side or the other of 100X. 

 

I've no desire to try 188X. Spotters are not built to specs that incorporate high magnification.

 

Doesn't imply that some cannot do well at higher powers though. I would expect better from your Pentax and would send it back as well.


Edited by nixnix, 12 June 2016 - 11:35 PM.



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