F/3.3 issues

[Robbie]

G,day,
Im new to all this but learning fast, my question is on the use of a 10"meade sct coupled to a meade f/3.3 focal reducer.

when I use the focal reducer , stars start to smudge as they get closer to the edge, Its not too bad ,just enough to annoy me, Ive heared about taking "flat field" shots to correct, but no one has been able to explain them to me as yet.
The shot below is of the cluster M28 taken with a meade 10"sct @f/3.3, orion starshoot dsi2. unguided stack of 12, 40sec exp.
Its the first half decent shot Ive take since "mastering" alignment/drift alignment so try not to laugh too loud.
Many thanks in advance for any helpfull advice and many thanks to Chris James's atricals for getting me this far.

[Terry B]

Flat fielding wont help this problem. A flat will correct variations in light across the field caused by vignettiing or dust on the camera. What you have is an optical aberation from the focal reducer- probably "coma" and this needs an optical solution not a flat field.

[Robbie]

So the meade f/3.3 fr is dodgy??? its new and I havent heard of anyone elses showing the symptoms I am getting, What sort of "optical Solution" would you suggest?
regards.

[[1ponders]]

The Focal Reduce probably isn't dodgy Robbie. The "sweet spot" of the reducer is designed to illuminate a chip the size of the original DSI colour (3.7 x 4.9mm), but will handle slightly larger ships (SBIg ST7 etc). Unfortunately your Orion Starshooter has a chip about 20 times the surface area (15.6 x 23.4mm). To fully illuminate that size chip using and SCT you will have to look elswhere for a focal reducer. Actually what you are really after is a Focal Reducer/Field Flattener combination.

[mick pinner]

much better of with the 6.3 reducer, the 3.3 has issues with DSLR's and DSI's as far as l know.

[Merlin66]

The 3.3 FR is not dodgy. Any optical element which is required to compress the focal length is a design compromise.
These particular lenses are very sensitive to the distance from the CCD chip to the lens. I believe the Meade 3.3 is designed for a distance of about 20mm to the CCD to give f3.3. This is based on using the 15mm spacer + CCD.
If other distances are used the performance suffers i.e. coma etc

[centroid]

Robbie, I had a 10" LX200GPS, and now a 12" LX200R, and with an MX7C camera, I could use the f/3.3 focal reducer.

However, when I changed to the larger 'chip' SXVF-H9C, I could no longer use the F/3.3 reducer, as it produced the effect you describe. Its just not suitable for anything other than the smaller CCD's.

The f/6.3 is fine, but of course won't give as large a field of view, as did the f/3.3.

Forgive me if I'm "teaching granny to suck eggs" here, but with any focal reducer, its essential to get the spacing between the focal reducer and the CCD correct, in order to achieve the stated focal ratio.

There is a formula for calculating this. With your f/10 scope, and the f/3.3 focal reducer, a spacing of 57mm, will give f/3.29. This is of course the theoretical distance, and manufacturing tolerances, would likely require some small adjustment of the spacing.

I arrive in Perth this weekend, for a five week stay with my daughter, and if I can be of any help, just give me a 'shout' via the forum.

Regards

Dave

[Robbie]

ahhaa thanks guys thats a big help, I can get around the issue with a bit of cropping and yes that explains why when used at f6 the pictures show no smudging, Iwill take the ccd to reducer sweet spot into consideration as well but Im only using the adapters supplied by meade so Im not sure what adjustments I can make there,
Thanks again for the help.

[Merlin66]

Just found the Instruction sheet for the Meade 3.3 reducer.
Based on a f10 system and a Pictor CCD:
The 15mm spacer gives f5, and the 30mm spacer f3.3

[centroid]

You need something like AC 603 on this page:

http://www.iankingimaging.com/show_products.php?category=123&offset=10

A bit late now, or I could have brought them over for you :)

I'm sure one of the Aussie astro shops will stock something like it.

Dave

[Merlin66]

The standard Meade supply with the 3.3 reducer includes the 15 and 30 mm, spacers. The SCT to 1 1/4" is the standard visual back.
Should be in the box somewhere!!?????