Observing the Moon with a spotting scope (1 Viewer)

[BoldenEagle]

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

 

[Binastro]

With high magnification and small exit pupils astigmatism is less of a problem.

There is a discussion of this somewhere but I cannot remember where.

Regards,
B.

 

[Tringa45]

Televue sells cylindrical adapters for some of their eyepieces to compensate for the viewer's astigmatism.
See chart on the right: DIOPTRX™
It's not surprising that you got sharp views at 0,6 mm exit pupil.

John

 

[Tringa45]

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

 

[Ignatius]

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.

 

[BoldenEagle]

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

 

[Tringa45]

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

 

[BoldenEagle]

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

 

[dries1]

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.

 

[Yarkho]

I think your star test and your Moon views show that your spotter is a good optical instrument, performing as it should. Some spherical undercorrection is normal, probably you are dealing with spherochromatism, which is hardly avoidable at these f/ ratios.

Not to worry.

The Moon really gets good at magnifications above about 100x; your use of 3.5mm eyepiece at 0.6mm exit pupil means your image is already seeing-limited and at the max the optics can provide. I generally find that at <0.5mm exit pupil the image degrades fast.

 

[Binastro]

With a spotting scope, maybe, but not with an optically excellent astro scope.

My Pentax 100mm f/12 certainly was used visually on planets at 300x and tested at 400x.
The image did not break down at 0.25mm exit pupil.

Even large telescopes do well at 0.5mm exit pupil.
My 317mm Dall Kirkham worked well at 700x, but the image broke down at 1100x.
My friend uses 650x regularly on his Skywatcher 350mm Dobsonian.
Horace Dall's 8 inch Maksutov was beautiful at 400x.

A double star observer uses 850x regularly with a 10 inch Newtonian.

One of the best planetary observers uses 600x regularly in good Seeing with his 300mm Newtonian.

My 120mm refractor performs beautifully at 250x terrestrially at 4.7 miles at 3a.m.

William Herschel used 900x to discover the planet Uranus with his speculum self made 6.3 inch reflector.

The limit is usually the observer's eye or the Seeing.

Scopes up to 5 inch, if really good optically, do not break down at 100x per inch of aperture.
That is 0.25mm exit pupil.
In the real world in good Seeing.

Regards,
B.

 

[Yarkho]

Hooray for your eyes then. At 0,5mm exit pupil my floaters kill the image, even if without them it would probably be still fine.

Also in several years of observing there were only a few nights when the seeing permitted anything close to 350x. I generally find that a ~1D magnification is the most pleasing to my eyes (seeing permitting). Maybe I just don't have a big enough telescope.

 

[Binastro]

It is not just my eyes.

Rev Dawes of the Dawes limit used 420x for his drawings of Mars and Jupiter and its moons. With his fine 6.3 inch refractor.
He saw detail on Jupiter's moons, which I haven't.
He was, I think, very short sighted and could not recognise friends walking by.

My starting magnification for planets with the 12.5 inch Dall Kirkham was 265x, then 400x and on excellent nights 600x.

For fine detail, many planetary observers use 65x per inch.

However, Paul Doherty who could see Jupiter's moons without optical aid in his forties used 190x on his 12 inch Newtonian.
People with exceptional eyesight need less magnification.

But Dawes and others had very fine eyesight.

Some went to Greece where the Seeing was better and the planets higher

Lately my eyes aren't so good but until five years ago I had no trouble with 0.3mm exit pupils, and when younger much less than this.

Spotting scopes, however, don't allow the magnifications used on fine astro scopes.

The Czechs excelled at meteor observing, but this is mainly without optical aid.

Regards,
B.

 

[Yarkho]

Given the era Dawes lived in, his refractor was a long focal-length instrument, where achieving low magnifications was not so easy. I am unable to find the focal length of his 6.3" refractor, but considering that it was a proper achromat satisfying the Fraunhofer condition, we arrive at f/18 or longer to have acceptable levels of CA.

I am not disputing the fact that high magnifications are good for lunar & planetary work, or that they are indeed preferred for many people,

it's just with my eyesight and local seeing conditions there are no advantages in going above 1D magnification (objective diameter in mm). There are only exceptional circumstances when the conditions allow me to see any more detail on Jupiter than at say 150x - and at that magnification I can see details within the belts, GRS, etc. just fine.