ASTROPHOTOGRAPHY TIP Last updated: 23 November 2004

Subject:	review - focussing adaptor
Sent:	Saturday, September 4, 2004 08:58:55
From:	Michael Morris (michaelmorris4@hotmail.com)

I have recently ventured into the realm of astrophotography with my trusty ETX90EC. I know its mount makes it unsuited to long exposure photography but it's sometimes great to take pictures of the Moon or Sun when I'm observing them.

This is when the problems start; focussing the camera. The combination of the glass screen on the camera (Canon AE-1) and the ETX's small aperture mean that the image on the camera's focussing screen is often too dark to focus accurately. It's also a real drag spending ages trying to focus the camera and then to have to refocus if you turn the flip mirror back to view through the eyepiece. The ideal solution would be to view an object through the eyepiece then be able to quickly turn the flip mirror to take a photograph at prime focus and then move the flip mirror back to continue observing through the eyepiece. There's only one problem with this, the camera plane and any eyepiece are nowhere near parfocal.

The answer was a simple adaptor that extends the barrel of my 26 mm eyepiece by a little over 2 inches. A parfocalising ring attached to the lower section of the adaptor's barrel allowed me to fine tune the system to my eyepiece-camera combination. I attached the camera to the prime focus of my ETX using a BC&F ETX Combi-Adaptor (www.astro-engineering.com) and a Canon FD T-mount. I fine tuned the system with the parfocalising ring in daylight on a distant object to get the eyepiece and the camera perfectly parfocal.

Parfocal Focussing Adaptor
(note parfocal ring near base).

Adaptor in situ

Now all I have to do is focus the telescope at the eyepiece, then turn the flip mirror, centre the image in the camera viewfinder then press the cable release. All pictures are beautifully in focus. This was fantastically useful for observing and photographing the Transit of Venus earlier this year.

The eyepiece/camera adaptor will also allow the use of screw-in filters on the eyepiece. This is useful for observing the moon near full with a neutral density filter or for using a green filter to highlight faculae on the surface of the sun (with a Scopetronix screw-in glass solar filter over the front lens). The inside of the adaptor barrel in baffled and painted matt black to eliminate internal reflections.

This adaptor was made for me by the excellent Chris Livingstone Telescopes in Worcester, UK. (www.livingstonetelescopes.co.uk). I'm sure if you contact him he will make one for you too.

To complete the set up, I am hoping to create a simple rectangular reticule for the 26mm eyepiece that allows me to frame the image in the camera at the eyepiece, reducing alignment errors in the camera viewfinder and making composing and taking photographs even easier.

Subject:	focusing adaptor
Sent:	Sunday, November 21, 2004 09:21:53
From:	Michael Morris (michaelmorris4@hotmail.com)
"I have just tried Chris Livingstone's excellent astrophotography
focussing adaptor in an ETX 125 and it work fine.  Long gone are the
hours trying to get a sharp image on my camera.  Now all I do is just
focus and centre with the eyepiece,turn the flip mirror and shoot."

thanks

Michael Morris

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