Astro-Tech 127mm Triplet Refractor
Comparison of .8X and f/6.3 Focal Reducers
May 23, 2008
Skies were overcast with thin clouds so I decided to take some test images of M3 using the following two focal reducers I had on hand for other telescopes:
Reducers attached to modified Canon XSi (450D) in stock focuser:
Photos below show the position of focuser at focus with no reducer and with the focal reducers used.
No Reducer:
Televue 0.8X Photo Reducer / Flattener:
Celestron f/6.3 Reducer / Corrector:
Test Images M3 Globular Cluster: One minute exposures were stacked in Deep Sky Stacker with a level stretch and resize being the only processing. No Flat or Dark calibration was done and all images are full frame.
Images below were taken with the f/7.5 Astro-Tech 127mm Triplet that has a focal length of 952mm as follows:
1. AT 127mm - No Reducer................................................................................................f/7.5
2. AT 127mm with Televue 0.8X Photo Reducer / Corrector for 400-600mm focal lengths...f/6.0
3. AT 127mm with Celestron f/6.3 Reducer / Corrector for Schmidt-Cassegrains..................f/4.7 (providing f/5.2 with spacer I use)
1. No Reducer:
2. Televue 0.8X Photo Reducer / Corrector:
3. Celestron f/6.3 Reducer / Corrector:
Test Results:
The Televue 0.8X reducer causes stars to become elongated at around 70% into the Field of View. This is not unexpected since this version of the focal reducer is not meant for the longer focal length of the Astro-Tech 127mm. I would expect the Televue 800-1000mm version of this reducer to perform better but don't have one to test.
I was surprised by how well the Celestron f/6.3 Reducer / Corrector did since it is not designed for refractor telescopes. There is some noticeable star distortion in the corners of the Field of View that would be easy to improve with image processing software. I plan to do longer exposure deep sky imaging with this 5 inch triplet using the Celestron corrector at f/4.7 to test this configuration further.
After taking images at both prime and with the Celestron f/6.3 reducer/corrector, I made measurements and found that the scope is operating at f/5.2 with the f/6.3 reducer:
Previous Astro-Tech 127mm pages:
Photos of the Astro-Tech 127mm Triplet Refractor along with specifications can be seen HERE.
On April 28, 2008 took first deep sky images with the Astro Tech 127mm Triplet using a modified Canon 450D camera and images of M51, M13 and the Virgo Core of Galaxies can be seen HERE.
I am now using both the Astro-Tech 127mm and the Orion ED 80 for imaging on the Atlas EQ-G mount using an ADM side-by-side system and a photo of the setup can be seen HERE.
Lunar images using the Astro-Tech 127mm at prime and also with a Televue 2X Big Barlow along with star testing results and video can be seen HERE.