MallinCam FOV tests
My test equipment:
8-inch Celestron NextStar SE SCT F10
2” WO diagonal
Color Hyper MallinCam
MFR-3
10mm, 10mm, 5mm extensions
Philips PET 1002 10.2-inch DVD player
Canon Digital Rebel – screen photographs at 1/6-second with image stabilized lens
My target:
Pleiades, M45 portion of cascade of magnitude 7 stars
Control:
25mm Orion Sirius Plossl
Software:
Starry Nights Pro
The goal of this project was to come up with a reasonable approximation of the true field of view shown with the MallinCom Color Hyper video camera when shooting through an 8-inch F10 SCT using a diagonal and with various focal reduction devices. The target was M45 on the night of November 23, 2007 when a nearly full moon was just 10 degrees away, significantly washing out the sky. Aside from that, conditions were poor, but tolerable. It was a decent night to do this kind of test, but not much else.
I used the MallinCam MFR-3 focal reducer alone, then added a 10mm extension, then a second 10mm extension and then a 5mm extension. In each case I took either a 6 second – or 2 second – integration and photographed the screen of the portable DVD player I use as a monitor. These screen shots were with a Canon Digital Rebel and a 75-300mm zoom lens at 1/6 a second using image stabilization.
My results give a smaller field than what others have reported or estimated and I can attribute that to two factors. First, I'm using an eyepiece equivalent for the smaller dimension of the horizontal field. Second, I use a diagonal and place the camera in it. I find this an easier arrangement alla round, but it also may be reducing the field of view. I simplyd on;t know enough about wha's involved to say, but I suspect this is the case. The MallinCam field is not circular, but rectangular. I did my best to estimate the vertical of the fov from the software, then simply did the math figuring the screen was set at 3:4 ratio.
This is a crude approach, but yields images useful for this purpose. I determined dimensions and eyepiece equivalents with the Starry Nights software which allows you to measure the separation between two stars. I estimated the total field of view by using the feature in the same software to provide a foc for eyepieces of various sizes. In each case I assumed an eyepiece had a 50 degree apparent field of view.
Just to roughly test the software against reality I also compared the fov it showed for a 25mm Sirius eyepiece with what I could see visually. I found in this case – as I have many times in the past – that this rough approach to determining eyepiece field gives a workable solution. (I’m pretty sure all the software is doing is calculating the power, then dividing the apparent field of view by the power to come up with an approximate true field of view.)
Here are my results. Keep in mind that images are just screen shots under poor conditions and no attempt was made to have them all exactly the same size. However, the fov was evaluated simply by measuring distances between stars in the various images. Also - in some images you don't quite see the bottom of the machine. Trust me - I was tired and cold, but I'm convinced all these results, while approximations, are in practical terms accurate.
1. MallinCam alone – This yields a true field of view roughly equivalent – in its smallest dimension - to a 5 mm eyepiece with a 50 degree apparent fov. I would estimate the fov on the screen as 7’20” x 9’ 35”
2. MallinCam with MFR-3 – This yields a true field of view roughly equivalent – in its smallest dimension - to a 6.4 mm eyepiece with a 50 degree apparent fov. I would estimate the fov on the screen as 9’30” x 12’ 40”
3. MallinCam with MFR-3 and 10 mm extension – This yields a true field of view roughly equivalent – in its smallest dimension - to a 7.5 mm eyepiece with a 50 degree apparent fov. I would estimate the fov on the screen as 11’15” x 15’
4. MallinCam with MFR-3 and two 10 mm extensiona – This yields a true field of view roughly equivalent – in its smallest dimension - to a 8.9 mm eyepiece with a 50 degree apparent fov. I would estimate the fov on the screen as 13’ x 17’ 20”
5. MallinCam with MFR-3 and two 10 mm extensiona plus 5mm extension – This yields a true field of view roughly equivalent – in its smallest dimension - to a 10.6 mm eyepiece with a 50 degree apparent fov. I would estimate the fov on the screen as 16’ x 21’ 20”