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Old 12-07-2010, 10:03 PM
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mldee (Mike)
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Question: Canon 1000d Sensor to mount distance

I want to take some SCT FR6.3 shots with my 1000d. Does anybody know the distance from front of bayonet mount to sensor, so I can calculate spacing for various brands of FR's?

Thanks,
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Old 12-07-2010, 10:31 PM
adman (Adam)
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Mike - I am pretty sure that all Canon EOS cameras are 44mm - can someone else confirm that?

Adam
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Old 13-07-2010, 04:46 PM
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mldee (Mike)
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Thanks Adam, seems about right.

I'm doing a little project to try and verify Focal Reducers' published focal length figures, just using handheld FR, sunlight and a piece of white paper to adjust for focus point with tape measure.

Did my Meade 6.3, 3.3, Antares 6.3 and 5, and GSO 5. All 2" or T size and got some interesting numbers today.

Some jibe quite well with published figures, some are totally different. After I play with them + spacing in my C8, I'll publish them here for others' possible edification.
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Old 16-07-2010, 11:31 AM
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Reducer spacings v's effective focal ratio

I've prepared a spreadsheet which may be of interest. It calculates the final effective focal length and f ratio for various reducer fl's and spacings.
It also takes into account the changes in EFL due to the primary mirror shift in an SCT (based on Chris Lords original data, but updated)
You need to measure the final image plate scale (CCDInspector) to verify the optical train and actual spacings.
Ken
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File Type: zip reducers_V2.zip (6.0 KB, 26 views)
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Old 16-07-2010, 12:24 PM
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mldee (Mike)
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Thanks for that info Ken, I'll add the spreadsheet to my little text file I've been compiling, showing all the trivia relating to various focal reducers, it's quite handy. I'll link the most recent copy here for your review.

I've tried the Antares 6.3 a couple of nights ago with my 1000d and it gave very good results at my suspected distance, 85mm, very little star distortion too. Interesting that as I wanted to keep my Meade 209 (remote) electric focuser attached, I put the FR and spacer array with the camera, at the back of the focuser. Little or no vignetting on the C8.

I'm presently looking for a used Celestron 6.3, as its longer FL would allow me to put it in front of the focuser directly on the OTA, which is my preferred position. With the QHY8 Pro I'd have more latitude due to shorter internal sensor distance.

I want to verify similarly for the other FR's I've collected, but been too busy.

Will be interesting to see how they jibe with your spreadsheet calcs. It was strange that some of the FL's I saw were so different to the published ones. Only a real star test will tell.

I like the range of FL's and speeds that a C8 or such will give with FR's or a Hyperstar for the really wide stuff. Suits my type of imaging activities, which are basically happy snaps.

Cheers,
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File Type: txt 100713 FR sensor distance list-4.txt (5.1 KB, 18 views)
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Old 16-07-2010, 12:31 PM
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mldee (Mike)
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BTW Ken, (forgot to ask in my previous post), your 3.3 numbers don't seem to reflect a focal reduction to 3.3, but up in the 7's. Am I misunderstanding something?
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Old 16-07-2010, 01:39 PM
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Merlin66 (Ken)
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Mike,
The spreadsheet is reflecting the x0.63 reducer focal length in cell A26 (240mm) - change this value to 88 for the x0.33 reducer and you'll see the results.
I should make the entry of user focal lengths a bit more obvious!
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Old 16-07-2010, 02:04 PM
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Doh! Bit silly of me not to realise that..Spoon feeding is so uncool
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Old 16-07-2010, 02:09 PM
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Merlin66 (Ken)
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That's OK!
The original layout had fixed values for the reducer focal lengths, but I quickly found out that not all "standard" Meade/ Celestron reducers are the same. Also there are Orion, Antares, Optec, SkyWatcher and Lumicon reducers etc which are very different!!
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Old 16-07-2010, 03:40 PM
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True, BTW, as one last final question, do you see anything wrong with my "old schoolboys trick" of using the FR like a kid's magnifying glass, with the sun's image to measure the FL?

All of them gave a very clear image on the white cardboard of the cloudy sky/pinpoint sun when at hand-held focus, I then just measured the distance from Paper to rear of lens (glass) and assumed that to be the ~focal length. Is that OK?
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Old 16-07-2010, 04:02 PM
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Nothing wrong with that method.
You'll get a very close approximation. The final answer will come from measuring the plate scale of the image.
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