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kx9i
super member
Reged: 03/04/08
Posts: 112
Loc: Phoenix, AZ USA
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I was "calibrating" my Celestron 17mm retical eyepiece, which has a 0-60 scale right across the center. The extensive instructions say that in order to calibrate this scale to my particular telescope, I should pick a star and time it while it drifts across the length of this scale. Then plug that time (54 seconds) into an equation to get the exact number of arcseconds per division. Trig doesn't scare me, so off I went, using Arcturus. (Declination -11 degrees.)
The first equation is for those who happen to know the exact FL of their scope, while the second equation is for those who do not, or who own a SCT, which has a variable FL that depends on the actual location of the primary, which moves when you turn the focus knob. They go further, explaining that SCT manufacturers publish FL averages, not the exact value. Okay; I buy that.
My CPC1100 (with the reducer/corrector) is f/6.3, with a published FL = 1764.
When I use the proper equation I can solve for FL (to check my work) but I get 1500+ which is no where near 1764. I checked and double checked, but I think I did it right.
Can this 1500+ FL result be within the "range" of a CPC1100 at f/6.3?
Thanks in advance.
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letimotif
no complaints
Reged: 05/20/07
Posts: 911
Loc: Far Flung Isles of Langerhans
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Hi Alex,
I'd be interested in this, too. I know that the FRFF, like most optical adjusters, varies its effect depending upon the distance from the two other optical components, the EP and the OTA optics.
If you have your FRFF positioned slightly outside the optimum distance this can affect your actual focal ratio. I was talking with Ron at Moonlite the other day, and they calculate the sweet spot as 3" from the EP to the FRFF. I suspect when we get somebody here who actually knows what he's talking about, we'll find this distance issue is at the heart of your conundrum.
Sorry I don't have a specific answer for you, but I appreciate your posing the question because I think a lot of folks will learn something from this thread.
Regards,
Terry
--------------------
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Edited by letimotif (05/18/08 11:33 AM)
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kx9i
super member
Reged: 03/04/08
Posts: 112
Loc: Phoenix, AZ USA
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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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kx9i
super member
Reged: 03/04/08
Posts: 112
Loc: Phoenix, AZ USA
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Well, I tested Arcturus and got exactly 60 seconds, twice, so I'll assume the above numbers are correct for Spica.
Here's the bad news: Arcturus results showed the Scale Division to be 14.16 arcseconds, about a 7% increase from the Spica result of 13.2 arcseconds. I like to think I'm a pretty careful guy when it comes to accurate measurements, but this is not too easily explained. The only source of error I can think of is the transit time, and that's a pretty easy thing to check, especially when you average several trials using a stop watch...
Oh well, it was an interesting experience, and I'll try using SD = 13.5 arcseconds when I want to actually use the scale.
Clear skies all,
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