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Observing the Moon with a spotting scope (1 Viewer)

BoldenEagle

Well-known member
Finland
Couple of days ago I took my Kowa TSN 883 spotting scope to see the Moon as it was very high on the night sky. Seeing was very good so I installed APM 3.5mm XWA 1.25" astro eyepiece to have about 145x magnification and the view was very crisp, lots of detail to be seen. APM 3.5 XWA has very limited eye relief so I took my glasses off and I was very surprised how sharp I could see; normally my bad astigmatism prevents me to see any small details without glasses. With my glasses on, I could still see sharper but the difference was small. With more "normal"spotting scope (25-60x) magnifications viewing with my glassen off is out of the question as everything is totally blurry. I suppose that with a very small exit pupil, with an 88mm aperture about 0.6mm, I'm using such small area in the center of my eye's lens that the astigmatism has not very strong effect yet. Or is there some other explanation for this?

Regards, Juhani
 
It's not surprising that you got sharp views at 0,6 mm exit pupil.
Come to think of it, there are probably not that many birding scopes that offer sharp views at magnifications corresponding to 0,6 mm exit pupil.
My ATM65 HD and 883 used to, but nowadays my own eyes are the limiting factor and floaters begin to get annoying below 1 mm. :(

John
 
I had to look up 'floaters' as I only knew the term in conjunction with police slang. Wikipedia's article on them explains why they may be more of an annoyance when one is looking up - as in observing the moon.
 
Come to think of it, there are probably not that many birding scopes that offer sharp views at magnifications corresponding to 0,6 mm exit pupil.
My ATM65 HD and 883 used to, but nowadays my own eyes are the limiting factor and floaters begin to get annoying below 1 mm. :(
My 883 sample is not perfect but pretty good still. If I remember correct I measured 1.41" resolution at 96x magnification. Maybe using 145x would make it even little better as my own vision is just 20/20, haven't tried though. In a good seeing the 145x is subjectively perfectly sharp for the details of the Moon but if directly compared with optically more perfect sample of Zeiss Victory FL85, there is small but detectable difference favouring the Zeiss.

Attached is a smarthphone image through the 883 and 3.5mm eyepiece at about 145x magnification.

Regards, Juhani
 

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My 883 sample is not perfect but pretty good still. If I remember correct I measured 1.41" resolution at 96x magnification. Maybe using 145x would make it even little better as my own vision is just 20/20, haven't tried though. In a good seeing the 145x is subjectively perfectly sharp for the details of the Moon but if directly compared with optically more perfect sample of Zeiss Victory FL85, there is small but detectable difference favouring the Zeiss.

Attached is a smarthphone image through the 883 and 3.5mm eyepiece at about 145x magnification.

Regards, Juhani
Coincidentally, that is the exact value I measured on my 883 using a 3,5 mm Nagler.
Although I had hoped for the Dawes' limit of 1,32", that was probably not on the cards as the element steps on the 1951 USAF chart at around 10% are not going to be surmounted by a small change in target distance.
My ATM65 HD was diffraction limited at 1,78".
Surprised, btw, that you found such a good sample of an FL85. When Jan Meijerink tested several back around 2007, most of them were worse than the 883s.

Best,
John
 
Surprised, btw, that you found such a good sample of an FL85.

I don't know exactly how it was before but the scope has been fixed in the Zeiss repair, because it fell with the tripod and it's objective lens cracked and so was replaced. Maybe the scope went through more precise adjustment than out of the factory new ones, who knows? Nevertheless the star test reveals just mainly some undercorrection of spherical aberration but astigmatism, coma and prism defects are virtually absent, as far as I can tell. My 883 shows very little astigmatism, just a bit of coma, some spherical aberration and the vertical prism line(s) is present. I have earlier posted star test pictures of both these scopes below Kowa and Zeiss sections. I haven't done resolution measurements with the Zeiss but I think it could be at least close to be a cherry one.

Regards, Juhani
 
My 883 sample is not perfect but pretty good still. If I remember correct I measured 1.41" resolution at 96x magnification. Maybe using 145x would make it even little better as my own vision is just 20/20, haven't tried though. In a good seeing the 145x is subjectively perfectly sharp for the details of the Moon but if directly compared with optically more perfect sample of Zeiss Victory FL85, there is small but detectable difference favouring the Zeiss.

Attached is a smarthphone image through the 883 and 3.5mm eyepiece at about 145x magnification.

Regards, Juhani
Nice photo, Interesting object in the center of the crater in the SW quadrant casting a shadow.
 

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