A challenge with using a video camera with a small chip is the small field of view (FOV). On my VRC-10 (Focal length 2032 mm) and Mallincam XT-418 at prime focus provides a 14.24 arcmin FOV which is roughly equivalent to a 6 mm eyepiece. This offers great magnification for small objects but the FOV is too small to capture many cosmic objects. To obtain a larger FOV I could change to a shorter focal length scope, or change to a camera with a larger chip, or use a focal reducer - often called a telecompressor. Another benefit of using a focal reducer is that it effectively reduces the focal length of the scope, thereby decreasing the F ratio and increasing the speed of the system.
I own several focal reducers, all sold by Mallincam;
- MFR-6 (short half of the MFR-5) - an ~ 0.80 reducer,
- MFR-3 - an ~ 0.64 reducer,
- Mallincam 2" reducer - an ~ 0.75 reducer,
- MFR-8 (long half of the MFR-5) - an ~ 0.68 reducer, and
- MFR-5 - an ~ 0.44 reducer.
These reducers can be used singularly or in many combinations. To understand how best to employ these focal reducers, I've conducted experiments where I've measured the FOV with the plate solving website astrometry.net. The software provides a solution of your image that includes the FOV size, as shown below.
The results from my experiments are shown below. This data has been very helpful in planning observing sessions so I can match my optical system to the size of objects being viewed. It also shows the required extension rings, tubes, and reducers and where the resulting focus point falls. This greatly speeds up my setups.
|Mallincam XT-418 Setups on VRC-10|
|Focal Reduction||1” Focuser Ring||Blue Fireball = 50 mm||Blue Fireball = 80 mm||Badder 2” - 1-1/4” adapter = 8 mm||Filter||Focus position||arcmin, horizontal||arcmin, vertical||diagonal dimension, arcmin||Calculated Reduction||Resulting F Ratio|
|prime Focus||x||x||x||Lumicon DS||25||11.4||8.54||14.24||1.000||8.00|
|MFR-6 (shrt hlf MFR-5)||x||x||x||Lumicon DS||26.5||13.1||9.85||16.39||0.869||6.95|
|MFR-8 (lng hlf MFR-5)||x||x||x||none||15.5||16||12||20.00||0.712||5.70|
|M 2” 0.75x||x||x||Lumicon DS||31.5||14.8||11.1||18.50||0.770||6.16|
|M 2” 0.75x + MFR-8||x||x||none||12.5||21.4||16.1||26.78||0.532||4.26|
|M 2” 0.75x + MFR-3||x||x||none||18.6||13.9||23.22||0.613||4.91|
I experienced no comma or vignetting with any of the above systems, however I suspect that if I couple the 2" 0.75 reducer and the MFR-5 that I likely will see some vignetting. My results are similar to those of others, although not exactly the same due to differences in the spacers used. I plan several more experiments to try different spacers to alter the distance between the focal reducer lens and the camera chip. NOTE: Additional experimental data on numerous focal reducers is available at Jim Thompson's highly informative Abby Road Observatory website (look under Test Reports).