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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial
Thread started 25 Jun 2017 (Sunday) 15:34
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Beginner questions.

 
Bassat
"I am still in my underwear."
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6,288 posts
Joined Oct 2015
Bourbon, Indiana - USA
Jun 25, 2017 15:34 |  #1

I am an experienced amateur shooter, but know nothing of astrophotography. My grandson wants a telescope. I have considered photography with it. I may at some point buy a motor thingy to track objects with, but I will never stack multiple photos or that kind of stuff. Some serious moon shots and maybe the gas giants if I can pull them in.

What kind of 'scope' do I need/want. I am looking at a 120mm refractor and 6" reflector. The 6" gets me more glass on target, and costs less, but requires collimating from time to time. I can just just 'point and look' with the refractor. I could stretch the budget to an 8" reflector, if the gain over 6" is significant. Photography is not required, but would be nice at some future point. I just want to buy enough 'scope' so the kid finds it interesting.

If the photography is too involved, I won't mess with it. In that case, should I just get the kid a 90mm refractor?


Tom

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TCampbell
Senior Member
Joined Apr 2012
Jun 25, 2017 16:25 |  #2

Since you're just starting out, I'd skip astrophotography and just get an 8" dobsonian reflector. This will be a completely manual scope and it won't be suited to astrophotography but it will be easy to use and affordable.

Keep reading if you want more details on options...

There are two classes of refractors... the less expensive being "achromatic" and the more expensive being "apochromatic". The difference (besides price tag) is that the achromatic refractors will tend to show some color-fringing around the edges of the field. The lens behaves a bit like a prism and separates the color channels and while you don't quite see "rainbow" colors, you do see the color just beginning to separate. The "apochromatic" scopes use better glass and/or more corrective elements to substantially eliminate this issue and the results is a much sharper image. But whereas you can pick up an achromatic refractor for a few hundred dollars, most apochromatic refractors are likely over $1000 and up (they're not cheap).

So this brings us to the reflectors.

Physically larger diameter scopes are able to resolve more detail than smaller diameter scopes. So an 8" reflector will show more detail than a 6". A 90mm (a 3.5") will show even less detail.

If you're not going to do imaging with the scope, then I typically suggest getting a newtonian type reflector in a dobsonian mount. e.g. something like an Orion SkyQuest XT8 ... that's an 8" diameter newtonian reflector but is otherwise a no-frills scope (no computer, no motors, etc.). Basically all your money is going to the optics. This scope would need to be collimated from time to time, but collimation really isn't a big deal. You learn the basics of it, do a few time (you can pick up an accessory such as a cheshire eyepiece or a laser collimator to help) and it's fairly quick and easy to perform. For a bit more, you can get the "intelliscope" version of the same scope which they call at XT8i. That scope has no motors, but it prompts you through a simple two-star alignment process and from there you can tell it what you'd like to look at and it will tell you which way to move the scope to point it to that object. There are also fully motorized versions... for more money of course.

Most Newtonian type reflectors do not allow you to bring an image to focus with a DSLR camera. The sensor is too far to bring the image to focus and as you run the focuser tube all the way in you'll notice the image starting to improve... but will run out of focus travel before the image is sharp. A mirrorless web-cam type imaging camera will work better because the sensor doesn't have to be 2" farther back to make room for the camera's reflex mirror. There is a variant design of the newtonian reflector called a "newtonian astrograph" and these scopes have the focal length optimized for DSLR cameras.

The moon is a very easy object to image because it's big & bright. It actually uses a very fast exposure (not entirely unlike taking a daytime exposure -- it is lit by the full light of the sun after all).

Planets, however, are a bit trickier mostly because of their relatively small size. The atmosphere slightly distorts things and they'll wobble around while you're trying to image them and this causes the focus to come and go. Generally a single photo of a planet (grabbing just one frame) isn't very sharp. Planetary imagers typically grab a minute or so worth of video frames and then they use stacking software to improve the quality.

Big Schmidt-Cassegrain Telescopes (SCTs) are good at this because they tend to have high focal lengths. For example... an 8" Newtonian reflector might be an f/5 telescope which would mean the scope has an 8" diameter (about 200mm) and the focal length would be 5x that (f/5) or about 1000mm. Compare that to the typical 8" SCT which also has a 200mm diameter, but these tend to be f/10 scopes which means the same diameter scope gets you a 2000mm focal length. Of course the major downside to a higher focal length telescope (when used for imaging) is that you have to take longer exposures because the higher focal length means less light is coming through.

The "mount" is actually fairly important for astronomy. Cheap refractors tend to have mounts that might look solid enough in the showroom... but just wait until you use them. Remember you are magnifying objects more than you typically would with a camera. So it doesn't take much vibration to make the image shake so horribly that focusing is a real challenge and the scope isn't fun to use. (this, btw, is another reason why I tend to suggest dobsonian mounted reflectors... the mount is much more stable for the price tag.)

You can't just put a motor on an alt/az mounted scope because this requires a computer to interpolate the axes of motion as the Earth spins. But you CAN put a motor on an equatorially mounted scope because the axis are aligned to the Earth's poles. This means the major axis truly does move the mount in true east/west direction and the declination axis does move the mount in true north/south direction.

A fairly low-cost go to mount would be the Celestron Advanced VX mount. This isn't a particularly high-end mount when it comes to imaging, but it's great for starting out and it's fairly easy to use. A Celestron Advanced VX mount with a 6" SCT would run around $1300.

The eyepieces that come with the scope tend to be good for starting out (I wouldn't race out and buy lots of eyepieces and I would never recommend buying an eyepiece "kit"). Generally two eyepieces are sufficient. If you do get another eyepiece, as a general guideline, no eyepiece should have a focal length which is *shorter* than the focal ratio of the scope... otherwise you're unlikely to be happy with the view (it'll probably look to fuzzy).

I usually suggest also building or investing in a comfy height-adjustable seat so you can be comfortable enough sitting at the eyepiece for minutes on end studying objects ... rather than bending over at the eyepiece and giving yourself back pain.




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Bassat
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"I am still in my underwear."
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6,288 posts
Joined Oct 2015
Bourbon, Indiana - USA
Post has been last edited 3 months ago by Bassat. 2 edits done in total.
Jun 25, 2017 16:38 as a reply to TCampbell's post |  #3

Wow. Thanks for such a thorough response. I think all of this means that I will pass on the photography aspect. I can do the moon well enough for me with a 100-400L on crop body. I hadn't consider a Dobsian, but the XT8 is only $379 at telescopes.com. That is a lot of light gathering.

Is the equatorial mounted reflector significantly different from the Dobsian? Also, I am essentially blind in my right eye. Will that affect my use of the 'spotter' scopes?

Edit:
I did look at the XT10. It is at the upper limit of price and size for me. Is it worth the $600 over the $379 8"?

EDIT:EDIT:
I live out in the middle of nowhere Indiana. Not much light pollution out here.


Tom

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MalVeauX
"Looks rough and well used"
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Joined Feb 2013
Florida
Post has been last edited 3 months ago by MalVeauX. 3 edits done in total.
Jun 25, 2017 17:56 |  #4

Heya,

If you're not going to get a tracker, forget about trying to do any real photography with it.

But for visual, aperture is king, so get the biggest aperture you can afford, be it 8" or 10" or 12" or more. A dobsonian reflector is the fastest way to get the most aperture for the least cost. Aperture is where the detail comes in on those planets and craters on the moon. A big refractor is a wonderful thing, but a cheap big refractor that is fast will have a lot of CA on bright targets, so I wouldn't go with a refractor for what you're looking to look at (moon, planets). A SCT or reflector would be the fastest way to get aperture and no CA on big bright targets.

You can get a 8" SCT for about $400 used. This is a very, very good value. Quite compact scope for what it is. CloudyNights has them on the for sale forum fairly often at that price.
You can get a 8" or 10" reflector in the $300~500 range. They're heavy and big, but they're the best ratio of aperture per dollar you're going to get.

Mounts are important so that the scope will settle fast and not wobble violently as you breathe.

Dobsonian reflectors have their own mount (the dob portion). You can save money there that way.

+++++++++++

Example, SCT:

https://www.cloudynigh​ts.com ...se-ota-fastar-compatible/ (external link)

8 inches of aperture, compact SCT design, great for planets & moon.

Add to that an eyepiece or two that's great for planets & moon at low and medium powers, and a diagonal. Add a good alt-az mount that can handle it.

http://agenaastro.com ...dual-ed-eyepiece-8mm.html (external link)
http://agenaastro.com ...ual-ed-eyepiece-25mm.html (external link)
http://agenaastro.com ...escope-star-diagonal.html (external link)
http://agenaastro.com ...w-deluxe-altaz-mount.html (external link)

+++++++++++++

Otherwise, for a good deal, look for packages like the Zhumell Z8 (8" Dob Reflector) with everything needed, collimator, eyepieces, etc. It goes on sale often.

https://www.telescopes​plus.com ...onian-reflector-telescope (external link)

Very best,


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