Last updated: 18 April 2005
Subject: re: Hello. I need some Advice for Telescopes. Sent: Thursday, April 14, 2005 20:48:34 From: Richard Seymour (firstname.lastname@example.org) You asked for the differences between a 105mm reflector and a 102mm refractor telescope. Look at the front corrector plate of your ETX105. See the secondary mirror? (also known as "the central obstruction") In 99% of reflector designs, there's that secondary mirror blocking a measurable percentage of the light hitting the front aperture. In the ETX 105, it's a quite reasonable (i.e. small) 9.4% But "size" is not the only factor in the choice of telescopes. For any given price-point, you can usually get larger or better optics in a reflector than in a refractor. That's because a refractor needs more than one lens out front to be at all good. That's because light splits into its component colors as it passes between air and the glass.. and it takes multiple lens elements to counteract that tendency. Multiple elements means more cost. Good optics means that those multiple elements must be all carefully ground and well-figured, often with many surfaces with sophisticated coatings to fight reflections between the various surfaces. Whereas a reflector telescope just needs to bounce the light. Since it's not going -into- the glass, reflectors don't have to have the complicated multiple elements that a refractor has. (if the refractor is poorly corrected for color separation, you'll see bright targets having purple or red halos... as Jupiter sometimes does in inexpensive refractors.) So size isn't everything... (lack of) lens design can easily render an inexpensive refractor almost useless for many targets. have fun --dick (who has an ETX90, since the 105 didn't exist when i bought it)
Subject: Hello. I need some Advice for Telescopes. Sent: Monday, April 11, 2005 17:59:11 From: Amy Spencer (email@example.com) I'm a complete amature when it comes to astronomy...umm, I did purchase a Sky and Telescope magazine to browse through though. I have one question however, regarding Refractors VS Reflectors. It's generally stated that Reflectors are better because they have larger apertures. What would happen if both the Refractor Telescope and the Reflector Telescope had the same aperture? EG. Approx 4 Inches. Would the viewing quality (Image Clarity/Size) be the same? Going further, I'd like to know if you could run a quick comparison for me on the Meade ETX 105 AT http://www.meade.com/etx/etx_specs.html VS the Nexstar 102 GT http://www.astronomyonline.com.au/cache/item-1825public.html?cache=no I look forward to hearing from you. Thank you for your time. Yours sincerely, Amy Spencer P.S. I couldn't find anyone else to E-Mail to request advice about this. You're my only hope. *Wince*Mike here: Generally aperture is aperture, regardless of the design of the telescope. However, there are other factors that come into play. But you will get more aperture for the same cost with a reflector style telescope. Glass is expensive when it is done right in refractors. As to that comparison, I have no experience with that NexStar model but you can read a lot about the ETX-105 on my ETX Site; check out the Helpful Information: User Observations page.
Thank you kindly for your prompt response. I have a newly purchased ETX 105 EC, Father has been helping me to put it together-it's quite a struggle. *Laughs* We're both very afraid that we might break something! Ohh, I just wanted to know if that Nexstar 102 GT Refractor Telescope would 'out perform' my ETX 105 EC by just looking at the Statistical data. Both are 4" but...the Refractor is actually cheaper by about $300.00 AUD. If you actually answered my question via the below E-Mail, then forgive me. I'm a complete amature and must have missed it! Sincere regards, Amy SpencerMike here: The price sort of tells you something about the capabilities of the NexStar refractor.
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