Mallincam MFR-3 vs MFR-5 question
Posted 26 October 2012 - 12:59 PM
Posted 26 October 2012 - 03:36 PM
If you look through the posts on the Mallincam tech yahoo forum you will find a discussion thread that discusses this whole focal reduction issue. However, you can also roughly approximate your effective focal length by downloading CCDcalc from http://www.newastro..../camera_app.php and then comparing the images on your screen of a well known object like M57 for instance with the FOV depicted in CCDcalc for the Mallincam Chip size. In addition, i recommend you check out www.astrobin.com and upload a JPG file of a few different focal reduction combinations and plate solve them for the effective FOV, again pluggin in the plate solved FOV from astrobin into CCDcalc will give you the effective FL of your scope and reduction factor. For my purposes with my F10 C8 or C11 I find with the MFR3 its about F8 and with the MFR5 its F4.5 and with the MFR5+10mm spacer its like F3.9 or thereabouts. I have not used the F6.3 celestron reducer with the MFR5 as some posters on the Yahoo forum suggest this gives way too much vignetting.
Posted 26 October 2012 - 06:27 PM
Posted 27 October 2012 - 07:04 AM
Posted 27 October 2012 - 07:44 AM
My MFR-5 with no additional spacers yields a reduction factor of almost exactly 0.5X, as measured on an f/5 refractor. On an SCT, or any scope having adjustable spacing between mirrors for focusing, the exact degree of reduction will likely differ from this, depending on the difference in position of the camera from the back end of the scope with and without reducer. (Not to mention the change in system f/ratio as the focusable mirror is moved.)
OK, thanks. So clearly I'm not an optical guy. It looks like a MFR-3 with 2 10mm spacers in front is roughly equal to a MFR-5 in focal reduction. Probably focal reduction isn't the whole picture. I'm trying to decide if it's worthwhile to buy a MFR-5. Is there advantage for my situation (SCT & Dall Kirkham OTA's) for MFR-5 vs MFR-3 ?
Posted 27 October 2012 - 11:23 AM
From the comments from MFR owners it appears that the -5 gives you better optical correction (tighter stars) when used at reduction factors below .5x. With the ability to add spacers both between the elements and between the whole optical stack and the sensor the -5 seems to give you more flexibility. Or to put it another way - it's easier to get more reduction with the -5 than with the -3, albeit for a price.
The -5 is a very optically "neutral" focal reducer, it seems to work well on a whole variety scope designs when used with the smaller sensor size (1/2") of the traditional Mallincams and other video cameras.
There was a lot of unclear information about the -5 when it was first introduced. It was labeled a .33x or f3.3 focal reducer. Clearly in it's stock form it does not offer that aggressive focal reduction. However with the optional spacers it certainly does give you the ability to get down into the .33x range with a good quality field of view. When you are looking at the -5 just don't think of it as a $250 focal reducer, rather it's more like a $300 focal reducer with the required spacers.
If I was looking for a focal reducer for just one scope and I didn't need too aggressive a reduction factor I'd look at the -3. If I was planning to use the reducer on a variety of different scope types and/or I was looking for a reduction factor of .4x or better I'd probably spend the extra money on the -5.
Posted 27 October 2012 - 11:30 AM
Thanks for the info. Can you tell me what the configuration is to get to .33x? Or better yet a chart that calls out the various configurations?
Posted 28 October 2012 - 03:30 AM
Your question comes down to wording. Increasing the reduction factor means making the reduction more aggressive, which decreases the focal length and f/ratio.
The MFR-5, due to it being a dual-lens reducer, can achieve more aggressive reduction before vignetting occurs. This is because the rear element provides additional reduction if the front lens is already as far forward into the light cone as it can be.
Note, however, that with any reducer you can go only so far with the aggressiveness before you enter the realm of reduced system aperture. Once the front element has been moved as far up the widening light cone as its own clear aperture allows, any further ingress--via increased separation between reducer and focus--will effectively reduce the aperture of the telescope. Yes, the image scale will continue to shring, but so will the aperture, resulting in a fairly constant f/ratio. And edge darkening will become quite awful, too.
The extent of permissible reduction before aperture reduction occurs depends on the characteristics of the reducer (principally, its own f/ratio) and the scope's f/ratio. The faster the reducer, and the slower the scope, the more aggressive the reduction can be made. Other things like field curvature and image aberrations permitting, of course.
While I haven't tested this, I suspect that the MFR-5 may cause aperture reduction--and certainly quite obvious vignetting--before a reduction factor of 0.33 is reached. For this level of reduction I use a Meade f/3.3 reducer on my C8; no field curvature, aberrations and vignetting of note here.