PetPirate
member
Reged: 02/23/09
Posts: 11
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Hello,
I have a meade etx-125 and currently use the meade #64 T-adaptor with Canon EOS T-ring to take prime focus astrophotos with my Canon 450D.
I would like to do prime focus astrophotography of the upcoming solar eclipse, and would like a wider field of view (2x sun diameters would be nice). I believe I need a focal reducer for this, but am confused about how to incorporate one into my setup.
I *think* I need:
1. An adapter to convert the meade etx-125 rear cell threads to a standard schmidt-cassegrain thread (?)
2. An f/6.3 focal reducer which can screw into he above SCT thread
3. then what??? can I now use my existing T-adaptor, or (I assume) I will need one with a shorter tube due to the decreased focal length? Which one do I need?
4. If I buy a shorter T-adaptor, can I use my existing Canon T-ring, or do I need to buy another one?
Any sources for buying these internationally at a sensible price would also be much appreciated... I'm not sure why focal reducers (an ostensibly simple lens) seem to be so expensive. 
Any way to forego step 1? Can I just use the eyepice port instead somehow (benefit would be that I could use alt/az mounting near zenith without things crashing into each other)?
Any and all help much appreciated.
Thanks,
John
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2114
Loc: 55.215N 6.554W
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Quote:
I would like to do prime focus astrophotography of the upcoming solar eclipse, and would like a wider field of view (2x sun diameters would be nice). I believe I need a focal reducer for this,
Honestly I think your problem is that the ETX 125 (1900mm focal length) is too long. A focal reducer will reduce the focal length but the illuminated field of view is not affected, or even reduced slightly; you will have vignetting caused by the baffle tube, which you can't remove because it's needed to prevent light passing straight through the corrector plate to the camera/eyepiece port, bypassing the mirrors altogether.
I'd suggest a short focus semi-apo refractor of 60-80mm aperture would be the ideal scope for your eclipse trip.
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Arthur Dent
Galactic Hitch-Hiker
Reged: 10/23/08
Posts: 1184
Loc: South Yorkshire, UK
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Yes John
You will need an ETX to SCT thread adapter (#1 on your list).
For some bizzare reason, Meade put an non-standard thread on the photo-port of the ETX (probably so that Meade could sell their own bits at extra cost).
Art
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If I like it, the wife says that we can't afford it!
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Meade ETX105 (a nice "Grab & Go" scope) & Celestron NexStar 6SE with Bob's Knobs.
Various EP's from 6mm to 26mm, Baader Hyperion 8-24mm Zoom, a 2x Barlow, 2" diagonal and 7Ah PowerTank.
MRF and Antares 8x50 RACI finder scope - both for the 6SE's OTA, whilst the ETX gets a plain RDF.
Canon EOS 400D DSLR (un-modded) and SPC900 webcam. Finally climbing the AP Learning Curve!
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2114
Loc: 55.215N 6.554W
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Quote:
For some bizzare reason, Meade put an non-standard thread on the photo-port of the ETX (probably so that Meade could sell their own bits at extra cost).
AFAIK Meade never marketed an ETX back port fitting focal reducer.
The "bizarre" reason might be that the smaller diameter was necessary in order to leave room for the focusing knob on the smaller OTAs - the back of the ETX90, in particular, does not have a lot of real estate.
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Arthur Dent
Galactic Hitch-Hiker
Reged: 10/23/08
Posts: 1184
Loc: South Yorkshire, UK
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To add to my above post,
The ETX-SCT adapter that I have is NOT a Meade accessory. As it came with the scope when I purchased it s/hand, I have no idea as to the make or where it came from.
Cheers brian. I have an ETX-105 and could see no logical reason why the thread was smaller.
Art
--------------------
If I like it, the wife says that we can't afford it!
=======================================================
Meade ETX105 (a nice "Grab & Go" scope) & Celestron NexStar 6SE with Bob's Knobs.
Various EP's from 6mm to 26mm, Baader Hyperion 8-24mm Zoom, a 2x Barlow, 2" diagonal and 7Ah PowerTank.
MRF and Antares 8x50 RACI finder scope - both for the 6SE's OTA, whilst the ETX gets a plain RDF.
Canon EOS 400D DSLR (un-modded) and SPC900 webcam. Finally climbing the AP Learning Curve!
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PetPirate
member
Reged: 02/23/09
Posts: 11
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Thanks everyone -- so, If I understand correctly, the focal reducer won't provide any additional field of view?
It seems strange to me -- as the camera's field of view seems to be less than when looking through a larger, say, 40mm eyepiece. is there any way to widen the field of view without resorting to eyepiece projection?
The Canon 450D is also an APS-C sensor camera (1.6x crop factor) -- if the vignetting occurs outside of the sensor's field of view, then it might still have some benefit in squeezing more into the frame?
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duro
journeyman
Reged: 04/10/08
Posts: 6
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If I remember correctly the FOV for your setup using a Canon 450D is about 40 arc minutes times 26 arc minutes. You can do the calculation yourself using a FOV calculator in the net and data for the chip used in the camera. You are right that a 40 mm EP will give you a larger FOV than this. A reduser will enlarge your FOV, but there might be other problems occuring as brianb pointed out. You will have to test it out to know for sure.
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pfile
member
Reged: 06/14/09
Posts: 17
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i have the same setup (etx-125 + canon 50d). i've been considering getting the adapters and a focal reducer... did anyone above actually try this?
also can the etx-125 mount be driven by something like orion's starshoot guider? or is this all good money after bad?
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AlexDJ30
sage
Reged: 09/02/08
Posts: 276
Loc: Monterrey, Mexico
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Many people try it pfile..the problem is the weight! the ETX is not very happy with lots of weight on it...his wheels are spur or plastic gears..so there is a lot of tracking errors when you out a good amount of weight on it.
If you plan to imagine the sun and planets its much better to use a webcam than the 50D...less heavy!..there are many astroshops that selles focal reducers for CCD cams (like DSI, Nextimagen, LPI) that goes on the noce piece of the camera like a filter and they reduce the focal, maybe this is a better way than 50D + big focal reducer on the back.
Yes its a little dissapoiting that the ETX is not suited for DSAO photography (I tried myself...and yes i went away from it, i buy a GEM..), the ETX its nice Solar system photography with webcams..but from that...DSO's are out of the question.
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Equipment:
- ETX 125 AT
- C6 ASGT
- WO 66mm SD
- 2.5X ED Barlow, 8-24mm Zoom Eyepice
- Canon EOS Rebel XSi
- Celestron Nextimagen
- Orion Starshoot CCD cam
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pfile
member
Reged: 06/14/09
Posts: 17
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hmm. i was more interested in DSOs, so that's too bad. i also have an ancient C8 but it probably needs to be collimated and it does not have an electronic mount, so i have no hope of autoguiding without upgrading the mount somehow.
generally speaking can one do serious DSO work with an SCT+focal reducer, or do people tend to use fast refractors for DSO?
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2114
Loc: 55.215N 6.554W
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Quote:
generally speaking can one do serious DSO work with an SCT+focal reducer, or do people tend to use fast refractors for DSO?
Oh yes, even without the focal reducer - the longish focal length means a good mount & very likely autoguiding or adaptive optics are required - but there are lots of people producing very very good DSO images using SCTs, including searching for supernovae & asteroids.
Suggest you cut your DSO imaging teeth on a short focus refractor first, though, it's a lot easier to learn that way & there is a lot to learn!
Edited by brianb11213 (06/17/09 04:26 AM)
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pfile
member
Reged: 06/14/09
Posts: 17
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i have been using a 50d with the 70-200L f/2.8, sometimes with a 1.4x tele-extender. but the field of view with this lens is still way too wide to get decent angular size out of anything except stuff like the rosette nebula, M42 and maybe the north america nebula. so i thought it would be interesting to try using the ETX-125 for the sake of narrower field of view.
as you pointed out, the problem is that without precise guiding, the f/15 ETX-125 is just too slow. i figured if i could get it down to f/6 or so then the ~90s exposures i've been taking with the canon lens and an orion skyview pro were probably possible with the ETX's clock drive.
i guess in my quest to keep it simple at the outset, i wanted to avoid seriously long exposures, and it seemed like the focal reducer would let me do this. of course the field of view would increase but hopefully it would still be narrower than 280mm.
Edited by pfile (06/17/09 03:27 PM)
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pfile
member
Reged: 06/14/09
Posts: 17
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well just to follow up on this thread - i decided to try attaching my 50d to the etx-125 without any kind of focal reducer or what have you, and i can't seem to focus the image. i know i've done this in the past with a 20d and an etx-90, so i'm left kind of puzzled by this. i assume i should be using the short tube with the t-adapter. is that correct?
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