Focal Reducer Test with C14 - Double Cluster

[Chuck]

With some earlier test images showing prime focus imaging, with my Meade 8" LX200 Classic OTA, was going to difficult w/o guiding, I wanted to try more testing using a focal reducer. I was able to get out this past Friday to do some quick testing using a recently purchased Celestron f/6.3 focal reducer and my old Lumicon f/3.3 focal reducer from the early 90's. Click each image to access acquisition details and higher resolution images.

Double Cluster with the Celestron f/6.3 FR



Only a small portion of the left and right sides were cropped out due to unacceptable coma. With only 5.5 minutes total imaging time, the results show the Celestron focal reducer to work quite well and should allow me to get some good images without guiding. If all goes well, then guiding will be something to try down the road.

Double Cluster with the Lumicon f/3.3 FR



This single image, with the Lumicon focal reducer, was enough to show it to be unacceptable for imaging due to the excessive coma and chromatic aberration.


----- Chuck

[Bruce D]

The 6.3 fr seems to produce a very nice image, beautiful capture Chuck

[Apollo XX]

Don't do that! I thought you meant you had a Celestron C14!

HAHA! The same thing happened to me for a second. I've never heard Double Cluster referred to by C14...sometimes the NGC's, but never the Caldwell.

Chuck, what I really like about the 6.3 pic is that it is very representative of what one sees in the eyepiece when looking at double cluster. Nice!

[Chuck]

Thanks Guys and sorry for the confusion using the Caldwell designation.

@ Bill: Looks like I had the Lumicon info wrong. I actually found the original instruction sheet for the reducer last night and it was called the Lumicon Rich Field Viewer. It was only described in the instructions for visual use and when used with 2" accessories, an f/10 scope would be increased to about f/5, not the f/3.3 I mentioned. Either way, it didn't perform well photographically with the test I did.

I've also heard good things about the Starizona reducer/corrector and may want to look into it down the road myself. For now, I'm hoping the Celestron reducer will work out well. There still some coma at the edges, after a little cropping, but not bad for a 24 year old scope that has never been collimated.

@ Mike: Yep, I agree about the f/6.3 image being like what you see in the eyepiece. I hope to try the double cluster again with my 200mm Canon prime lens as it should be a perfect match.

---- Chuck

[Pete]

Hi Chuck,

Focal reducers are probably the most misunderstood telescope tool. They are actually image size reducers. Typically they're used to fit a big image at the focal plane onto the tiny chips used 10 years ago.

Focal Length of focal reducer = FL
Distance of FR optic to focal plane = D
Compression ratio = CR = (FL - D)/FL This is the same as CR = 1-(D/FL)

You can typically make minor adjustments to compression ratio by changing the distance of the focal reducer to the chip. But the compression ratio of focal reducers that are also Field Flattners is not variable, according to some literature I've read. Also note that focal reducer spacing datum is typically from some point in the middle of the optic and not from the rear lens.

To get the size of the fully illuminated (non vignetted)circle, multiply the clear aperture times the compression ratio above.

It all starts with the size of your chip and you work backward from there.

Pete

[Pete]

Does anyone know what the baffle size is on a Meade 8"? I know the C8 is 37mm, which with the 0.63x reducer/flattener would cause vignetting with the 27mm diagonal of your Canon EOS 7D Mark II

The Meade 8" has a 1.5" (38mm) baffle tube and a 1.5" aperture at the Schmidt thread on the backplate. The Meade focal reducers also have 1.5" optics. Moving the focal reducer back a hair from the chip will allow you to fit the image to the chip without vignetting.

Be aware that while older Meade f/6.3 reducer/correctors had a relatively long recommended distance between chip and optic, but the distance on newer reducers is so minimal that there may be no room for a filter.

Pete

[Chuck]

But I was just working out how the reducers illuminate an APS-C sized chip in the EOS (and my Nikon D7)

Bill, I have tried taking a few flat frames with the Celestron f/6.3 reducer (at a back focus distance of 110mm used in this test of the double cluster) and only saw minimal vignetting at the corners with my Canon 7D Mark II. As I needed to slightly crop the left and right sides to get rid of the worst of the coma effected stars, this isn't a problem.

If you do wind up springing for the Starizona reducer/corrector, your crop sensor should be fully covered.

---- Chuck

[Pete]

Minor vignetting is easily addressed with flat frames.

Pete