The Celestron Micro Guide Eyepiece can be used to determine the effective focal length of a telescope. This could be helpful when using a binoviewer with a catadioptric telescope. With an SCT the focal length varies with the amount of back-focus required.
Thanks to terraClarke for bringing this eyepiece back to my attention. I had forgotten the reference for using the eyepiece to determine the focal length of a telescope. Looking at my written copy of the eyepiece manual (THE CELESTRON MICRO GUIDE EYEPIECE #94171), the formula for telescope focal length is:
- f = 82506 ÷ ( t* x cos δ)
t* = time in seconds for a star's passage across the entire eyepiece scale (0 to 60) with tracking off
δ = declination of the star in degrees
This could be used in determining the focal length of a telescope when operating with a binoviewer. One could place the Micro Guide eyepiece on one side of the binoviewer and cap the other eyepiece. I'd like to give this a try at next opportunity, using the Micro Guide eyepiece with my WO BV and Celestron-11. It appears that the manual for this eyepiece can be obtained from Astronomics. According to the manual, this procedure should be used with all accessories in place - such as diagonal, Barlow lens, adapters, etc.
Tonight I tried this method on my Celestron-11 and William optics binoviewer using Arcturus as my target.
- This star has a declination (according to SkySafari) of +19°11' 5" (19.18°).
- I measured 7 passages of the star from 0 to 60 on the scale and found an average of 27.62 seconds, with a standard deviation of 0.1169 seconds.
f = 82506 ÷ ( t* x cos δ) = 82506 ÷ (27.62 x cos19.18°) ) = 82506 ÷ (27.62 x 0.9445)
f = 3163 mm
- This gives a focal ratio of 3163 ÷ 279. [11-inches = 279 mm.]
I believe the optical path with my binoviewer is around 217 mm:
- SCT straight through eyepiece holder - 45mm
- Baader T2 prism diagonal - 67mm
- RAFCamera T2 to BV adapter 4.5mm
- WO Binoviewer - 100 mm
According CN forum member Eddgie for every mm in excess of 100mm back focus there is an addition of 3.1mm in focal length - see reply #8 in this thread. So in my case there is 117 mm of back focus beyond 100 mm. His rule predicts that would add 363 mm to the C-11's focal length. This results in a focal length of 2800 + 363 = 3163 mm. The fact that his formula predicts exactly what I measured gives me confidence the Eddgie knows what he's talking about when it comes to SCT optics.
So kudos to Eddgie!
I must mention that there might be a slight error in my measurements since the star wasn't exactly focused. This was necessary since I don't have an illuminator for the Micro Guide Eyepiece's reticule. So a slightly out of focus star expands to make the reticule markings visible. This eyepiece was given to me by Hardin Optical Company when I was working there. Since it was missing the illuminator, it couldn't be sold. So I was happy to have it. I could fashion a simple red LED light or buy a screw in illuminator for $30.
When making the measurements I found it useful to have both eyepieces uncapped. The Micro Guide Eyepiece at 12.5 mm (40° apparent FOV) gives 253X (0.16° actual FOV). The WO 20 mm eyepiece on the other side gives a wider FOV at 158X (~ 0.38°). So if I lose the star in the guide eyepiece, it can be re-centered with the WO eyepiece.
Edited by Rustler46, 15 June 2019 - 02:37 AM.