SCT, focal reducers and coma correctors

[Apostate]

Hello

I've just acquired an 10" Meade LX200 SCT and am wondering which focal reducer is best suited? 

Do they introduce or reduce coma and, regardless, is it possible to combine with them with a coma corrector such as the Baader MPCC to reduce coma further?  The MPCC would be attached to an APS-c sized DSLR. 

Many thanks

Simon

[sharkmelley]

Hi Simon,

Is this for imaging or for visual use or both?  Is your main priority to reduce F-ratio and increase field of view (which is what a focal reducer does) or do you want to achieve a flat field with no aberrated stars for imaging (which is what a field flattener does)?

Mark

[Apostate]

Hi

Yes, sorry - should have been more specific.. I will be imaging and would like a nice flat field.. 

Thanks

Simon

[sharkmelley]

Is your LX200 the ACF (Advanced Coma Free) version?

[Apostate]

It's an older EMC..

[sharkmelley]

I'm not familiar with the EMC but since it is older then I'm sure it will be the usual spherical mirror SCT design, just like my Celestron C11. For imaging, the main problem is coma followed by astigmatism and field curvature.  Forget the standard F6.3 corrector sold by Meade and Celestron - they don't flatten the field nor correct the aberrations (see spot digram for the standard Celestron "corrector" on Starizona's page: http://starizona.com/acb/Starizona-SCT-Corrector-P3230C0.aspx)
The standard Celestron (and Meade) correctors come with no instructions or recommended spacing to sensor and I wasted an awful lot of time trying different spacings camera to corrector spacings, none of which worked.  I've never had any luck with the Alan Gee corrector either - this corrector again comes with no instructions or recommended spacing to sensor.  Another frustrating lot of wasted time.

I haven't tried the Starizona 0.75x corrector from the link above but I have tried the Teleskop Service 0.8x SCT corrector: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p3291_Image-Corrector---0-8x-Focal-Reducer-for-Schmidt-Cassegrains.html
This was again frustrating until they recommended that I use a tilt adjuster - this recommendation is now on their website but I bought mine back in the early days.  With the tilt adjusment this corrector now does an excellent job.  You can see spot diagrams on their webpage.  The corrector to sensor spacing is critical and you must buy the right ring spacings to get this spot on (within 1mm).

If you want to risk other alternatives then take a look at the experiments done in this interesting SGL thread:
http://stargazerslounge.com/topic/128928-sct-reducercorrector-test/

I imagine the Starizona corrector works just as well.

However,  if your scope were the newer ACF version then the coma is already under control and you need a corrector just for the residual astigmatism and curvature. I'm afraid I don't know the answer to this one.

Mark

[Apostate]

Thanks, Mark

So I guess you get what you pay for with reducer/flatteners..

It sounds as though using the MPCC with the Celestron x.63 (for example) might work if I can get it into the optical train and keep the spacing between the reducer/flattener and the DSLR's chip in tolerance..  but doing so will be a pain in the backside.

I guess that the MPCC is going up for sale..  Shame, really - it is a decent bit of kit that does what it says on the tin - unlike, or so it sounds, the reducer/flatteners from M and C.

I'm a little perturbed regarding the tilt adjuster - I suppose it brings the chip back square to the light path after the weight of the camera and such like take up minute slack in the various components between the back of the scope and the lens mount and let it droop away from square?  Or is it correcting for something else?  I wonder if it is useful regardless of which reducer/flattener you use?

Simon

[sharkmelley]

The reason for the tilt adjuster with the TS Corrector I really don't understand and I don't see why it should be necessary.  All I know is that when I tried it, it worked a treat. 

Mark

[michael.h.f.wilkinson]

I have the 0.63x reducer/corrector from Celestron which claims to correct coma and flatten the field. This worked quite well on my 35mm SLR in the distant past, although it did cause vignetting in the corners. I got the correct spacing to the image plane by using either the bog-standard SCT-T2 adapter from Celestron, or their off-axis guider with T2-Contax adapter. That should allow you to work out the correct distance to the CCD. These reducers come up second hand  quite ffrequently, so buying one to try it and selling if a failure could be done without losing (much) money. The Baader MPCC is certainly not suitable, as it is designed for fast Newtonians. Field curvature and coma properties are different.

[Apostate]

Thank you Michael

I'll post the MPC up for sale and grab a reducer to experiment with..  From reading around the subject it seems that the Celestron is probably the most reliable since i have heard some flaky reports on the older versions of the Meade..

Cheers guys

Simon