Expert: Paul Wagner - 3/23/2004

Question
Paul,

Thanks very much for the VERY prompt and informative answer. If I may. I'd like to follow up, as per your kind offer of more assistance. I haven't quite grasped the relationship of aperture to optics. I think I understand that larger apertures gather more light, but how does the magnification of the image relate to it? Are optics the key? Why does it take a photo to make the image look larger/closer? Somehow I get the idea that it's the aperture + optics + time of exposure that makes a good-sized image. Am I even close? Any ideas on a primer on this subject? Your advice would be more than sufficient, but I hate wasting your time with stuff you know, and yet have to explain to a novice. Again, I'm not a dummy, so if you do have the time, that's fine. Maybe it's simpler than I think? Thanks.

Tom

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Followup To
Question -
I'm looking to take up the astronomy hobby I had at age 13 - after a hiatus of 48 years. Retiring soon, then relocating to a better site, I'd like to come back in a fairly big way. I'm not a dummy but I've never owned a good telescope, so I'd appreciate your advice on the best way (including making my own if I can) to begin fairly quickly, including any manuals or books on telescopy and astrophotography. I'm hoping to make a jump-start, and pass up the beginner telescopes unless you consider it wise to extend my learning curve. Astrophotography will be all new to me, but I'll want to get a CCD also (on a budget if possible, including used equipment). I've heard that the Newtonian is best for CCD work - do you agree? I have heard of one guy that's adapted a Dobsonian for this purpose. I'd appreciate any thoughts or advice you deem applicable to my situation. Thank you for your time.
Answer -
Dear Tom:

Great question.  First of all, go to a few star parties and see what other people are using.  I have a huge dobsonian (17inches) that is the envy of most star parties.  But I don't take photographs, and I don't use a CCD.  Those are each major new undertakings, and I don't have the time to tackle all the intricacies.  I think it takes at least one full year of astrophotography before you can take a photo you really want to show another astrophotograper....and CCD adds another $1000 to any equation, right off the bat.  

For my money, I would start with a nice 10-16" newtonian or Dobsonian.  If you can't have your own observatory, go for the dob--the Newtonians with equatorial mountings are just too damn heavy for someone our age to pick up and carry.  My dob is about 75 pounds total--but it is pretty carefully designed to keep the weight at a complete minimum.

I would first check out Orion telescopes (Telescope.com), and Discovery Telescopes.  Both offer some pretty good deals on some nice sized Dobsonian scopes.  ORion's go up to 10 inches, and they sell optics for a larger scope, I think.  Discovery (http://www.discovery-telescopes.com/TD.html) has truss designs up to 24 inches....but I think that it is tough to move anything above about 16-18 inches.  

And if you want to make your own, here are a couple of great sites that wil give you lots of ideas and inspiration:

http://members.efn.org/~mbartels/

http://www.sfsidewalkastronomers.org/sfsidewalk/cdobplans.htm

Does that help?  Give it some thought, and drop me a note with more questions, or just to let me know how you are getting along...

Paul Wagner  

Answer
Dear Tom:

It's not as simple as it sounds.  On the one hand, to see dim objects, you need aperture and/or time exposure.  That's what collects the most light, and allows you to see the nebulae.  And since the light gathering power is directly related to the surface of the mirror, an 8 inch mirror gets almost twice as much light as a 6 inch one does.  That's why I go big first--because I love seeing those dim things.  

Photography can make up for a lack of aperture by doing time exposures...but they are very difficult to do well. Those last Hubble shots were combined over HOURS of camera time to get the images.

To see fine detail, you need very good optics, and it doesn't hurt to have a larger aperture as well.  You usually can't use more than about 50x the aperture in inches on any scope...the image deteriorates too much beyond that power.  At the same time, it is pretty hard to use any amateur scope at above about 300X--too much vibration, unsteady air, etc.  So by the time you get to 6 or 8 inches, you have maxed out your magnification for most conditions. Then it is time to go for aperture.

Optics do make a difference...really good optics will show finer detail on the planets...but I have yet to see a scope that shows a great deal more than mine does.

Paul Wagner