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Question

I've used a combination of an Explore Scientific 2" dia. 2x tel-extender plus an 80 mm. extension tube at the end of 112 mm. of draw tube of my ES 127 mm., f/7.5 refractor to capture images with a Canon T1i.  I use this combination satisfactorily for imaging almost all deep sky objects, but planets need to be larger on the camera's sensor.  For that reason, I bought a 1-1/4" dia. ES 3x tel-extender.  Unfortunately, that 3x tel-extender required 251 mm. of extension tubes beyond the end of a 103 mm. of draw tube in order to bring the object to focus.  Adding all components together, including the 3x tel-extender itself, I had 421 mm. (16.57 inches) of stuff hanging off the rear of the scope's main tube, NOT including the camera body and its 1-1/4" dia. nose-piece!  Does this arrangement sound reasonable?  If I had to do a fast slew to an object with all that weight hanging onto the rear of the scope, I'm afraid there'd be whiplash damage :o !  How do you folks handle Barlows and tel-extenders of 3x and greater power?

Bob Z.   

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Bob,

I typically image with fast short focal length refractors. This is why I don't even try to shoot planets. I think that your solution is to use a scope with a longer focal length. A focal length of 4-5 meters is usually pretty good for planetary imaging. A 9.25 or 11" f/10 SCT with a 2X Barlow would get there easily.

Having 16 inches of adaptors and cameras off the back of your scope seems very difficult to manage, as you have suggested.

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Rick,

I know that a longer focal length scope would help, but I'm too old and poor to invest in more equipment of that type and scale.  Tele Vue offers 4x and 5x Powermates; I can't conceive of what those brutes would do and require for extensions to reach focus?!  I'll give eyepiece projection a try.  Thanks.

Bob Z. 

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Bob,

Yes, the 3x TeleExtender will increase your Focal Length to a very significant 2857.50mm (give-or-take); and with that FL you will have much greater Planetary Imaging Reach.  In fact it is conceivable that you could combine the 2x and 3x TeleExtenders for a whopping total of 5715mm.  That puts you at or beyond the reach usually afforded by the 16" Celestron SCTs.

But...  At only 127mm of Objective Aperture, your Images will be substantially dimmer, as that 3x of TeleExtender also impacts your Focal Ratio to f/22.5.  AND, you will have substantially less Detail, as that is a factor of Focal Ratio as well.  Beyond that, you can just imagine how difficult it will be to get the Planet centered in your 3x Smaller Frame of View.

So, not everything comes up Roses when you add a TeleExtender / Barlow...

Now, back to the question of Extension Tubes to accommodate your 3x ES TeleExtender:

No, it doesn't sound correct that you would require such substantial Backfocus beyond the TeleExtender.  Most Optical Designs are set for Focal Distances of 55mm or 85mm or 110mm (the latter two to accommodate OAGs and Filter Wheels; the former to accommodate a straight DSLR T-Adapter. 

I'd suggest that before you spend hard-earned money for a 1/4th meter of Extension Tubes, that you simply do the Paper Focus test:

  1. Set your Scope up on a Moonlit Night, using a regular Eyepiece to center on the Edge of the Moon (make sure Tracking is ON).
  2. Remove the Eyepiece and attach your TeleExtender to the End of the Focuser (without upsetting your Tracking of the Moon); extend the Focuser 1/2 of its travel.
  3. Hold a sheet of Paper in the Light-Path behind the TeleExtender;  Move the Paper (not the Focuser) Inward and Outward until you see a Well-Focused Image of the Moon on the Paper.
  4. This is your Focal Plane with that TeleExtender.
  5. Measure the Distance from the back of the TeleExtender to the Paper; that Distance less 55mm for the T-Adapter and Camera is the length of Extension Tube that you require.

Let us know how this turns out.

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Bob,

In your OP, you make No Mention of BYE's Planetary Imaging Mode.  If all of your Imaging has been of the DSO variety, perhaps you've never used Planetary Mode.

You NEED to Try It.  Actually, you NEED to USE it.  Planetary Imaging isn't all that much like DSO Imaging, as you need Fast 1:1 Video Frames of a Small Area of the Sensor Cropped around the Planetary Target.  That is exactly what BYE Planetary Imaging give you; but the DSLR Movie Mode cannot.

BYE uses 5x LiveView Zoom functionality to Capture 1:1 Pixel Ratio AVI Video that you then feed directly into RegiStax6 or AutoStakkert2.

Oh, and as a by-product, it produces the Optical Effect of about a 4-5x TeleExtender / Barlow.

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s3igell,

Thank you for your generous responses.  I will certainly perform those tracing paper tests/measurements and report the results to you and all.  Last evening, before knowing of your answers, I did try the 3x tel-extender again but without my camera body.  I used a 26 mm. eyepiece instead and every extension tube I have (but no diagonal).  Not only did focus require the almost full length of the draw tube, but I had to extend each extension tube to the point where all the thumbscrews were just barely holding on to the succeeding (following?) tube!  At that point, it didn't make sense to take a measurement because the assembly obviously was unstable and ridiculous-looking....and, yes, the image of the almost-full Moon was dim and was lacking contrast in the terminus area.  It is difficult to face the fact that the 3x tel-extender was a waste of money.

Now, I must admit embarrassment over having forgotten the 5x BYE Planetary Imaging function.  I hadn't imaged planets for so long that it simply didn't occur to me.  :huh:  So, it's back to the basics for me.  Thanks again.

Bob Z.   

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Bob,

Perhaps you could return the 3X extender. Also, putting adding the diagonal to the "ridiculous" light path could give you the additional length so that the extensions could be more fully seated.

And yes, you should always use the 5X zoom mode in BYE for planetary imaging.

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Rick,

I agree that the diagonal would probably provide the extra extension I need in addition to the extension tubes and a more stable arrangement.  However, imagine the look of that configuration!  I found another extension tube, of sorts:  an eyepiece projection adapter without an eyepiece inside it.  I'll try both configurations and then send photos.

Bob Z.

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s3igell and Rick,

I did finally manage to assemble a combination of extension tubes that resulted in a reasonably stable arrangement.  I have a link to the photo that I promised:  https://www.dropbox.com/s/e7kw0hjmceas3al/IMG_3725_1.JPG?dl=0

Another photo shows the arrangement with labels for each component:  https://www.dropbox.com/s/zavwofhq4vr7916/IMG_3731.JPG?dl=0

Per Murphy's Law, as soon as I had the tubes and camera assembled and the Moon focused on the camera's LCD screen, clouds rushed in!  I never got around to using my diagonal, however.  The "straight-through" arrangement measured a total of 438.8 mm., including 108 mm. of drawtube extension, from the rear of the Explore Scientific ED127's main tube to the focal plane of the Canon T1i .  I hope to try imaging the Moon with this setup and also with eyepiece projection.  These images will be compared to images captured at prime focus and also with a 2" 2x tel-extender.  Results to follow ASAP.

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You're a Brave Man for mounting your T1i at the end of that Rube Goldberg arrangement and then hanging to the back of your Scope out in the Dark...  GOOD LUCK!!

I can just see it Swaying and Oscillating in the Breeze or after each Shutter Movement.

I still CANNOT believe that ES designed their TeleExtender / Barlow with that sort of BackFocus!!

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s3igell.

A Rob Goldberg arrangement labels it better considering my name, and if my setup isn't well balanced, I'll have a pendulum effect, wind or no wind.  Camera shutter shake could be eliminated by holding a piece of dead-black material over the objective lens while the shutter is opened.  Not very accurate or scientific but worth trying just as an experiment.  I look forward to the next cloudless night.

Bob Z.

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