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Terrestrial scope

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#1 aatt

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Posted 05 November 2013 - 11:46 AM

I have a friend who wants a dual purpose scope-one for terrestrial viewing and astronomy. They are looking at a Celestron NexStar 6SE Telescope. They would need an image erector correct? Is this sort of design a decent performer for both purposes? I am thinking not and that a refractor might be the way to go. I don't want to steer them wrong, hence this post. Thoughts?

#2 Jon Isaacs

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Posted 05 November 2013 - 12:00 PM

I have a friend who wants a dual purpose scope-one for terrestrial viewing and astronomy. They are looking at a Celestron NexStar 6SE Telescope. They would need an image erector correct? Is this sort of design a decent performer for both purposes? I am thinking not and that a refractor might be the way to go. I don't want to steer them wrong, hence this post. Thoughts?


A standard star diagonal erects the image but reverses it left to right. Birds are sitting upright as you would hope but appear to be looking to the left when they are actually looking to the right. I am OK with this, some are not.

The bigger issues are the long focal length and the relatively narrow field of view of the C-6. Most often terrestrial viewing is done at relatively low magnifications and a widefield of view is nice both for itself and for locating you target. The relatively slow focusing of an SCT can be a problem as well because moving from an nearby object to a distant one requires significant refocusing.

A C-6 would work for terrestial viewing but would not be an optimal choice. If the views were primarily long distance it would be a reasonable choice but if bird watching is part of the menu, a small refractor is a better choice.

Jon

#3 Joe Aguiar

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Posted 06 November 2013 - 10:09 AM

ya i would agree you will need a 45 degree diagonal, as john said it also doesn't bother me either.

A 6SE i think is also too big for daytime viewing, i think a 80mmF/5 refractor is best for bird watching/ daytime viewing and a 80mm scope is also decent for planets too.

#4 snommisbor

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Posted 06 November 2013 - 10:17 AM

The problem you will find with a scope like that is once you get above a certain magnification the heat inversions and the atmosphere just makes it very mucky no matter how good a scope or high end glass you have. A refractor would be better or even a spotting scope.

#5 Eddgie

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Posted 06 November 2013 - 01:11 PM

The bigger issues are the long focal length and the relatively narrow field of view of the C-6


Yes, this can be an issue. Depends on what the use will be, but for general use, it will be a little tight on true field for terrestial.

The problem you will find with a scope like that is once you get above a certain magnification


Can be a problem with any size scope during the day.

Easy to deal with.. Just stay below that magnification. 6SE cab be used with 40mm Plossl and get 40x, and I use this power during the day quite often, so the "Can't use high magnificaiton to me is not a compelling reason to drop the 6SE from consideration.

Still, you need to discuss further with your friends.

See if you can get them to decide on a priority. If you think they may want to look at the moon, or some brighter DSOs, and they belive that the primary use will be for astronomy, then the C6 may be the right choice for them.

If they are going to do a lot of nature watching and want lower power, wider fields, then astronomy would be the secondary use, and maybe a smaller refractor would be a better choice.

But this much for me is 100% for sure. Get then an astronommical telscope of some type. Most daytime spotters are simply not a good choice for astronomy, but most astronomical telescopes can indeed be used for very producting daytime observing.

Also, the optical quality of most astronomical telescopes is better.

And you can use either 90 degree or 45 degree, and while I agree that 90 works fine, 45 degree diagonals are avialible on the used market for such low prices that they can have one of each and use the one specific to the purpose. My guess is that if they have enough money to buy a 6SE, they can flip for a second diagonal if they want one.

So, just my recommendation.. Grill them a bit more and see if you can discern what the "primary" use will be, and if it is going to be astronomical, then lean towards the C6. If it is going to be terresital, then perhaps a nice 80ED.

#6 vahe

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Posted 06 November 2013 - 02:06 PM

For terrestrial viewing unobstructed scopes do a lot better job compared to any obstructed system regardless of the obstruction size, big or small.

For terrestrial viewing obstructed scopes do a so-so job provided the subject matter is at infinity distance once you come a bit closer than infinity then the out of focus areas are marred by overlapping donuts, generally considered as the worst possible “bokeh” whether visual of photographic.

A small refractor is ideal for terrestrial viewing.

Vahe

#7 Jon Isaacs

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Posted 06 November 2013 - 03:29 PM

6SE cab be used with 40mm Plossl and get 40x, and I use this power during the day quite often, so the "Can't use high magnificaiton to me is not a compelling reason to drop the 6SE from consideration.



That would be 37.5x with a 1 degree Field of view. Most terrestrial viewing is done at magnifications below 40x and again, a 1 degree TFoV is pretty tiny. One could get a focal reducer and increase that somewhat but the secondary shadow will likely become an issue during the day.

One issue that has not been discussed is the suitability of the SE mount for terrestrial observing. A GOTO mount is poorly suited for terrestrial viewing where you want to be able to quickly point the scope at a bird or an animal that may only be there for a short moment. I am not sure if the SE mounts allow full manual operation... I know some Nexstar mounts must motor driven in the azimuth axis. In any event, for daytime viewing, a solid, manual alt-az mount is preferable..


This photo, it never would have happened with GOTO SCT.. the bird was only there for a few seconds...

Jon

Posted Image
Jon

#8 Binojunky

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Posted 06 November 2013 - 04:58 PM

Thats nice Jon, what scope and camera :photo: DA.

#9 Jon Isaacs

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Posted 06 November 2013 - 08:21 PM

Thats nice Jon, what scope and camera :photo: DA.


The scope was a TeleVue Pronto, the camera, the digiscoper's ((afocal imaging) favorite, the Nikon Coolpix 4500, the eyepiece a Celestron 32mm Plossl.

Jon

#10 mattz

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Posted 06 November 2013 - 11:41 PM

When you say they are interested in terrestrial, do you mean birding?
Birders generally hike with their scopes. There is not much in the way of astronomy scopes that you really want to hike with. Jon has probably the one solution, a very light sub-80mm refractor.
The only kind of birding one could do with a C6 is something like a sea watch, where you drive to a fixed location and simply need all the light gathering and magnification possible.
The mount is another issue. A 3 lb. spotting scope just does not need much of tripod. A lightweight CF tripod works just fine for a 6 lb. set up that can be carried.
That said, an astro scope will be a better scope and will be brighter.
I have an 80 mm Swarovski spotting scope for birds and an 80 mm Stellarvue. The Stellarvue is much brighter, but I am not up for carrying a 20 lb. setup all day like I am a 6 lb. setup.
Of course, if your friends want to do fixed location terrestrial viewing, then never mind....

#11 Joe Aguiar

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Posted 07 November 2013 - 08:29 AM

i also recommended a 80mm refrector

#12 aatt

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Posted 07 November 2013 - 08:30 PM

I think they are doing the fixed locality viewing thing while camping and then when night falls they are going to give it a go at the sky.

#13 Jon Isaacs

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Posted 08 November 2013 - 04:56 AM

I think they are doing the fixed locality viewing thing while camping and then when night falls they are going to give it a go at the sky.


The scope itself is not ideal but probably workable. In my mind, the mount is the real issue. It's just not friendly for terrestrial viewing.

Jon






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