Help! Lens to use for Parfocal focusing with Canon XSi and SCT?

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  • Member since
    September, 2013
Help! Lens to use for Parfocal focusing with Canon XSi and SCT?
Posted by jlmanatee on Wednesday, October 02, 2013 7:58 AM

Has anyone worked up a parfocal lens to use in focusing their DSLR with their f/10 SCT?  I have a Canon XSi and a Celestron CPC 800 SCT.  I am not using a focal reducer at this time.  I use LiveView to focus on a bright object.  Then I use the GOTO to swing around to a dimmer object.  But it may not be centered in the scope.  Heck, it might be almost off the field.  No matter how carefully I've synched the Telrad finder to the scope, some adjustment may need to take place.  So to get it in the field, I then slap on the 2" star diagonal and an eyepiece.  Of course, I have to refocus for the eyepiece, thus losing all my hard work of focusing for the camera.  I've read that creating a parfocal lens, set the correct distance, can make all this easier.  I tried a 25mm lens and my diagonal, but I only have about an inch of play to work with and I could not get the lens to focus without adjusting the focus knob (ruining my camera focus).  Do I need to use another size lens?  Do I need to extend my 25mm lens out further? Or is the 2" star diagonal causing it to be set out too far?  Would I have better luck with a 25mm 1.25" lens and diagonal?  

  • Member since
    November, 2009
Posted by Poppa Chris on Wednesday, October 02, 2013 3:01 PM

Two solutions come to mind.

1) An eyepiece extension ring - screws into the eyepiece like a filter and extends the barrel lengh allow for more back focus.  For your Canon DSLR you should be approx. 155mm from the plane of the rear cell.  Your Diagonal and eyepiece combo may not be giving you that much distance.  The extender will do that for you.  Add to it a parfocalizing ring to allow "fine-tuning" of the barrel length and you should be good to go.  Total cash outlay = about $25.00

2) Don't bother with the diagonal and eyepiece at all.  Download Backyard EOS software to use with your Canon DSLR and a laptop.  Then use it's "Frame and Focus" module to perform everything you are trying to do by swapping accessories.  Plus the focusing will be much sharper by using the FWHM algorithm to get your focus "just right".  Simple to use and allows for complete camera control.  Cost = $35.00

(I personally recommend the Backyard EOS option.  It has worked wonders for my own AP) 


---Poppa Chris---

Denham Springs, Louisiana USA

"Second star to the right - Then straight on until morning!" - Peter Pan

Celestron CPC1100GPS (XLT) - 279mm aperature, 2800mm Focal length. (f10) Celestron Ultima LX (70deg AFOV) Eyepieces 32mm thru 5mm, Canon EOS Rebel T2i DSLR, Backyard EOS imaging software, Orion Star Shoot Planetary Imager IV, Celestron Skymaster 15x70 binoculars


  • Member since
    May, 2009
Posted by Timthelder on Wednesday, October 02, 2013 11:06 PM

I also use a Canon XSi  (modified.) When I am done aligning my mount with calibration stars, (I use an illuminated reticle eyepiece to do this,) I will most usually use the last alignment-star to focus my camera, (unless it's too bright.)

I use Canon EOS utilities live view to do this.  When you remove your illuminated reticle eyepiece after aligning on the last calibration star...insert your camera and look through the viewfinder.  You should see an out-of-focus star. 

Focus it as best you can  using the SCT focus knob, when it's close to being in focus, then revert to using the 'live view' in Canon EOS utilities to tighten it up.  (remember...the last focus adjustment on the SCT  is in a counter-clockwise turn of the focus knob to eliminate mirror-flop.)

When you open 'live view' you should see the star.  Center it, magnify it using the little magnifying glass icon at bottom right of the live-view screen, then finish focusing the camera.

That's about as close as I can get you to being there...

Cheers, Tim.



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  • Member since
    July, 2002
  • From: Texas
Posted by chipdatajeffB on Thursday, October 03, 2013 11:28 AM

If you do this often, you might benefit from using a flip mirror or optical manifold (like the Taurus Tracker). Such a gadget holds Both an eyepiece And a camera in place at the same time.

With the flip mirror, a simple flip of a lever on the side of the box moves the light path from camera to eyepiece or vice-versa.

With the Taurus Tracker, the light path is normally fully open to the camera; to use the eyepiece, you loosen a set screw and then slide the eyepiece focuser barrel into the light path, where a small prism picks up the light and directs it to the eyepiece. The focuser is normally a fine helical thead, so you twist the eyepiece to focus it. To set one of these up the first time, you focus using the camera in live view as you've described already, then you slide the eyepiece focuser barrel into place, focus by twisting the helical threaded tube, then lock it into place with the setscrew. Thereafter, you simply slide the eyepiece into place, or out of the light path, depending on what you're doing at the time.

Here's a photo showing the Taurus Tracker on my C-11, which I use for spectrography:

The spectrograph and camera are at far left. The tracker is the gray gadget between the scope and the black box of the spectrograph. This model has two sliders: one for a centering and focusing eyepiece, and the other for a guiding eyepiece or camera. In the photo above, only the centering and focusing eyepiece is inserted.

Each of those two gray "barrels" is helically threaded, so you can make them parfocal, and each is held in place by a locking thumb screw. There are two positions for each: "in" the light path and "out" of the light path. They will lock in either position.

This is a Tracker III (three positions: camera, or straight through, and two eyepiece, or prism-fed). The Taurus Tracker II is similar but has only two positions (only one eyepiece position).


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Come visit me at Comanche Springs Astronomy Campus (we're on Google Maps) in Texas.

  • Member since
    July, 2001
  • From: PA, USA, Earth Moderator
Posted by DaveMitsky on Thursday, October 03, 2013 12:01 PM

See for more on flip mirrors.

There's a short video at 

Dave Mitsky


Sic itur ad astra!

Chance favors the prepared mind.

A man is a small thing, and the night is very large and full of wonders.

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