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Viewing terrestrial with a 120 refractor

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#1 lekingaz

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Posted 26 March 2013 - 09:10 PM

Viewing terrestrial objects in daylight with my ed 120 refractor leaves me disappointed with the viewing. I'm using a Pentax XW 10 mm and observing a chimney about 300 yds distant , it just doesn't seem to resolve well enough. The viewing seems soft as well as low contrast . I feel like I'm looking through a low quality scope and eyepiece . It was in the afternoon with some heating but for the most part the viewing was across grass and trees, no roads or rooftops .

Lunar viewing in the last couple of evenings gave me a similar feeling, the seeing was ok i think, but i really had to work hard to get decent resolution of craters and most of the viewing was at the limbs as the moon was getting kinda full . My only other eyepiece is a 28 mm so it's not fair to compare with the 10 mm

I'm really not knocked out so to speak with the Lunar views and definitely not with daylight viewing . Am i expecting too much from this equipment or is this normal ? I don't have that much experience with astronomy scopes but the reason i bought a refractor was because it is reputed to give the best WOW factor over other varieties of scopes

Opinions welcomed

#2 GlennLeDrew

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Posted 27 March 2013 - 03:39 AM

Have you considered the exit pupil diameter? The smaller, the more sensitive is the view to atmospheric seeing (turbulence.) Once the exit pupil gets down to the 1mm range, the steadiness of the atmosphere becomes of prime importance.

#3 Tony Flanders

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Posted 27 March 2013 - 04:03 AM

Viewing terrestrial objects in daylight with my ed 120 refractor leaves me disappointed with the viewing. I'm using a Pentax XW 10 mm and observing a chimney about 300 yds distant , it just doesn't seem to resolve well enough.


Yes, that's fairly normal. Seeing is pretty bad when you're looking horizontally during the day, especially if it's sunny. That's one of the reasons that terrestrial observers rarely use big telescopes.

Your experience on the Moon may be due to pure bad seeing. On a good night, with the Moon high in the sky, you should be able to get a sharp view of the Moon at very high power. Mind you, the full Moon isn't very contrasty, any way you look at it.

#4 Mark9473

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Posted 27 March 2013 - 04:30 AM

One thing to check for the daylight views is if there is inside the telescope something that is reflective. Often happens with extension tubes for example, they will be painted black but if you look towards the light at a shallow angle they are seen to reflect a lot of light. That really kills the daytime contrast in my experience.

#5 droid

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Posted 27 March 2013 - 04:42 AM

What focal length? manufacturer? achromatic? was it bought used or new?

With this information we can make better guesses at the
problem.

#6 Mark9473

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Posted 27 March 2013 - 04:54 AM

He said ED.

#7 Jon Isaacs

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Posted 27 March 2013 - 05:13 AM

Yes, that's fairly normal. Seeing is pretty bad when you're looking horizontally during the day, especially if it's sunny. That's one of the reasons that terrestrial observers rarely use big telescopes.



:waytogo:

300 yards is a long ways away for a terrestrial object, it's like looking through a small mirage but magnified 90 x (120ED = 900mm focal length). There are situations where the view can be sharp and clear at that distance but it's not to be expected, viewing across a body of water can provide stable seeing because the water is uniform in comparison to the ground which will have houses, trees, roads, rocks, radiating heat and turbulence.

If you look at high end birding scopes, Zeiss, Swarovski, Zeiss, these might cost close to $4000 for an 85mm scope with a zoom eyepiece, generally they top out at 60x magnification... That ought to be a hint...

Jon

#8 obin robinson

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Posted 27 March 2013 - 07:19 AM

300 yards is a long ways away for a terrestrial object, it's like looking through a small mirage but magnified 90 x (120ED = 900mm focal length).


Exactly so. I've participated in countless long-distance marksmanship events. Even with high end spotting scopes the target at 300-1000 yards can look like you are watching it through water. If you change the focal plane you can often actually see three or more different heat mirages going in completely different directions from one another.

obin ;)

#9 azure1961p

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Posted 27 March 2013 - 05:59 PM

On a sunny day you needn't be concerned about looking over rooftops degrading the view - the grass will do it in spades. The asphalt is worse but the fact is a sunny day makes for horrible horrible seeing. If you want relief then observe over water but make sure you are virtually on the water as gazing across a meadow that borders the sea will destroy the view.

Typically under these horrible conditions something like 15x-25x is about max. Conversely well chosen spots observing over water can support 80x-100x - particularly on cloudy days.

Cloudy days despite a little gloom is the best wether for spotting due to the ground not heating up. After a rain when the ground is all doggy is also great .

Sunny days over a lawn? Awful.

Pete






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