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Author: TMFOrangeblood Big gold star, 5000 posts Old School Fool CAPS All Star Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 1805  
Subject: Telescope talk Date: 2/6/2002 9:19 PM
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jiml8 said: My wife teaches astronomy at an area university, and we have the school's 9 inch reflector set up in our house.

With the new blood in here from Selena's post, this seems a great time to get some advice on a couple of telescopes I'm considering. Below is the text of an email exchange I had with someone who just bought an Orion SkyQuest™ XT8 Dobsonian Reflector:
http://www.telescope.com/cgi-bin/OrionTel.storefront/3c61000700870bd2272180f272880630/Product/View/A302

...but it hasn't been delivered to him yet. If you're interested in this kind of stuff, please read on and feel free to reply with any help. Thanks!


=========
Him - I recently purchased such a scope from Orion Telescopes and Binoculars (www.oriontel.com) based on the recommendation of Ed Ting of www.scopereviews.com and the Sky and Telescope review (http://www.skypub.com/resources/testreports/telescopes/0001sixdobs.html) that rated it best in its category.

Me - I, too, am considering the very telescope you purchased. However, I think I'm leaning toward spending $40 more and buying the Orion SkyView Deluxe 8" EQ Newtonian:
http://www.telescope.com/cgi-bin/OrionTel.storefront/3c5dfe41052b1d92271d80f2728806b4/Product/View/A319

For the extra $40, I would get an 8" scope with an equatorial mount... and I could throw in another $35 and get the AccuTrack Electronic Drive for the mount... which would allow me to do some limited astrophotography, I think. However, I have the following concerns:

1. The SkyView is f/4, compared to your SkyQuest's f/5.9. I know that gives the SkyView a wider field of view (and better for deep sky objects)... but I read in a book that the faster f-ratios aren't as good for planet-viewing as the slower f-ratios, and I can't quite understand why. Also, the book said faster f-ratios (especially under f/6) are far more likely to suffer from comas and will likely need re-collimating every session. Do you have a grasp on any of this, especially the advantages/disadvantages of f/4 vs. f/6?

2. While I am a fairly experienced photographer... I know nothing about astrophotography. For all I know, I'll get the SkyView and find out I really can't take decent pictures after all. I'm only looking to be able to take photos occasionally, and it needn't be anything fancy. But I'd like to know if the SkyView can handle that, even though the web page indicates it can handle some simple photography. (I know the SkyQuest cannot handle astrophotography because of the Dob mount.)

3. I don't understand the shipping charges on the page I linked above. It shows 2nd day shipping for the "reflector" at $100, plus $45 for the tube assembly, plus $20 for an oversize charge! Do we have slower shipping option? I see your scope was supposed to cost $120 2nd day.... plus a $30 oversize charge? Yikes! Are there no Orion dealers around the country? I don't think I'd be able to bring myself to pay $165 for shipping.


Him - I have been reading about scopes for a couple of months now, but I am still a beginner, so please take what I have written below with a large grain of salt. Also, I know nothing about photography and even less about astrophotography.

I myself was considering the equatorial version of the 8" scope, but the impression that I have of the mount is that is not very good for larger loads. My concern was that it would be shaky - which is a real problem visually and worse yet for astrophotography.

Planetary viewing and photography require high magnifications to allow for an image size large enough to display details. A given eyepiece will yield higher magnifications with higher focal ratio scopes. High magnification in a low f/ratio scope may require a Barlow lens, which causes a dimming of the image. As the f/ratio decreases, the curvature of the mirror increases, which causes increased coma. This increased coma is evident in a larger portion of the field of view. Also, low f/ratio scopes are less tolerant of miscollimation and may be more difficult to collimate. After a scope has been set up properly, there is really no visual difference between different f/ratios, except for the coma. Photographically, the camera would probably affect the f/ratio of the total camera/scope system.

The Sky View mount may be unsuitable for long exposure photography, but may be adequate for short exposure shots of the planets. Also, while the Dob cannot track for long exposure shots, you may be able to take very short exposure shots (a couple seconds).

I myself do not understand the shipping charges, but if you are not in a real hurry for your scope don't worry about the 2nd day option. My scope was shipped to Miami via FedEx ground and arrived five business days after the shipping date. The total shipping cost, inclusive of the oversize charge, was approximately $50.00. This standard shipping option was available for selection during the ordering process.

I have not yet received my scope as it has to be shipped from Miami to me. I will be sure to let you know how it works out.


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