My New Scope: Meade LXD75 SN-6

I recently purchased a Meade LDX75 with the SN-6 OTA and thought I'd share some first impressions for anyone also considering this scope.  I bought it primarily for the mount because I was tired of lugging my 10" LX200 around as a tracking platform every time I wanted to do any kind of imaging.  I was looking for a portable astrophotography rig that was more affordable than the Losmandy GM-8 and had the astrophotography support features of my LX200; like an autoguiding interface, PEC, and a high precision Goto capability.  The LXD75 fit the bill perfectly and I'd heard that Meade had fixed the stability and tracking issues of the previous model so I felt it was worth a try.

I was also interested in the (F/5) Schmidt-Newtonian OTA. At 762mm it nicely filled a gap in my existing imaging scopes and lenses.  I went with the 6" because it's the lightest of the SN line at 12lbs.  The mount is rated for 30lbs. so this gives me the most weight margin for piggybacking other scopes and cameras.  The 6" also has a slightly slower focal ratio and a relatively smaller secondary so I thought it might perform a little better optically than the bigger SN OTAs for photography.

PROBLEMS (Right out of the box):

1. The eyepiece on the polar scope was loose and the polar scope required centering.
2. There was a small scratch on the finder scope
3. Excessive grease on the focuser
4. The tapped holes in the OTA rings were each different (6mm and 1/4-20)
5. Backlash in the DEC motor

THINGS I LIKE:

1. Portable but solid tripod and spreader bar design
2. Smooth motion when slewing
3. AutoStar controller
4. Finish and look
5. The price

THINGS I DIDN'T LIKE:

1. No level on the tripod
2. Sloppy mount declination adjustment
3. Optical shift when changing direction with the focuser
4. Poor camera T-adapter rotation design
5. Residual coma in the SN optics when using large sensors or film for imaging - (see an example below)

SN-6/300D unguided 1min exposure of M17 - Click the image for a 500MB full-resolution version.

Look carefully at the stars in the corners of the full resolution image to see the residual coma that the corrector plate doesn't remove.  Meade states that the SN design results in about half the coma of a standard Newtonian telescope.  I imagine this is only a problem photographically when using large sensors or 35mm film cameras.  This image was taken using a Canon 300D, which has a 15mm x 23mm sensor and the central 10-12mm exhibits tight stars. Baader makes a Newtonian coma corrector called the MPCC that is said to virtually eliminate the residual coma.  At $139.00 this may be a good solution.  Stay -tuned...  Here's an interesting modification to virtually eliminate coma in these scopes.  This could be a  future project. Here's a link to a comparison of coma for various newtonian hybrid designs.

8-13-05  I've only had the scope out for two nights under marginal conditions but so far I'm pleased with both the mount and OTA.  The tracking performance looks promising but I'll have a better feel for how good it really is after I train the PEC.  There's some significant backlash in the DEC motor, similar to that of the DEC motor on my LX200 and I'm currently having trouble getting the Alt/Dec Percent adjustment to compensate for it.  I modified my Losmandy dovetail plate to mount on the SN-6 rings and set up a guidescope and webcam for some GuideDog control error measurements.  The seeing conditions were marginal but I was still measuring less than +- 5 arc-seconds of error and I'm sure it will do even better when I get things tuned.  As far as goto testing,  I had one instance where the mount took off in the wrong direction but that may have been due to some configuration mistake on my part.  Other than that, it's been fairly accurate.  I haven't tested the high precision pointing yet.    More to follow...

8-28-05 I ordered the Baader Multi Purpose Coma Corrector (MPCC) from Alpine Astronomical and was able to test it under marginal conditions last night.  There is some concern that the MPCC might overcorrect the schmidt-newts because they exhibit only half the coma of a standard newtonian and the MPCC is designed to correct standard newts with a f/4 - f/6 focal ratio.  Click the images above to examine the full resolution versions (275KB.)  This was a preliminary test under poor imaging conditions (breezy with poor seeing) and the tracking and collimation weren't perfect but the improvement is dramatic and at first glance I don't think I see evidence of over-correction.  I plan to do more testing as time and conditions permit.       

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