Telescope Reviews: Re: CPC1100 Focal Length: Range?

Terry,

First, your 3" comment is interesting because the difference between my calculated and the published FL (200mm) is about 8", or 4" of primary movement. That tells me I'm within the adjustment range. Something else that occurs to me is that Celestron publishes a maximum FL, rather than an average, as suggested by the quote from their eyepiece booklet.

If there is interest, I can get specific: references for this include "Math for Amateur Astronomers," by James A. Fox (Astronomical League Publication, pages 19-23), and "The Celestron Micro Guide Eyepiece (#94171)," a 12-page booklet that comes with their 17mm Illuminated Retical Eyepiece (pages 3 and 4). Note that the Astro League book shows Spica's declination (I said Arcturus above, in error) at 11.0 degrees, while Starry Night shows it as -11.205, but the cosine is the same for both positive and negative angles between +/-90 degrees. (And now I'm wondering if the 54 seconds was for Arcturus or Spica?! I will redo these tests tonight, just to be sure.)

The Celestron booklet's equations are specific to the scale of their eyepiece, taking advantage of the 6mm scale with 60 divisions, so I'll paraphrase theirs.

SD = (Tsec)*(Cos D)/4

SD = Scale Division in arcseconds
Tsec = transit time in seconds
Cos D = the cosine of the chosen star's decination angle.

After adjusting the rotation angle of the scale in the eyepiece for convenience, I positioned the scope so that the star just approached the left edge of the scale. With the tracking off, I start the timer as the star crosses the zero mark; I stop it as it crosses the 60 mark. Three trials gave me an average transit time of 54 seconds.

SD = (54)*(Cos 11.2)/4
= 13.243 arcseconds

Page 4 of the booklet says

FL = (82506)/[(Tsec)*(Cos D)]
= 82506/52.97
= 1557.6

This is off by (1764 - 1557.6) = 206.4mm = 8 1/8 inches, but the construct of a SCT suggests this represents a 4" movement of the primary, since there is a reflection in the pathway.

All this started innocently enough. I wanted to know how big a Lunar dome or crater was...

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