I mentioned previously that I had switched over temporarily to a borrowed 12″ F/8 LX850 OTA. I didn’t mention that it was a prototype incorporating a proposed new feature – installation of a forced-air setup comparable to previous 7″ and 16″ LX200 models. I’ve been using it and trying to get a feel for how effective the cooling is compared to the three 12″ F/10 Meades I owned previously, though of course without a controlled, side by side A-B test the result won’t be definitive. The fan has been running during all of my imaging runs, to take advantage of whatever improvement there is.

The other night, I tried something different. I took a series of ten 5 second exposures with the fan off. I then engaged the fan and took ten more. Then I repeated both sets. That gave me 40 exposures, 20 each with and without the fan spinning. I knew that over such a short term I wouldn’t see any cooling effects but I wanted to establish that there was no more issue with fan vibration than in any of the other cooled Meade tubes I’ve owned. There was a lot of variation from frame to frame due to early evening urban environmental cooling effects so I examined the data in sets. I ran the sets through CCDInspector and derived the following FWHM values for each set:

first series without fan 2.26 arcseconds

first series with fan 2.32 arcseconds

second series without fan 2.46 arcseconds

second series with fan 2.38 arcseconds

average of all with fan 2.35 arcseconds

average of all without fan 2.36 arcseconds

I conclude from this that fan vibration is having no effect on star images.

Let me preface this post with a warning not to read too much into the above images. They are indicative of a software estimation of the effects of field curvature under the conditions at the time of the test images, using the particular camera and OTA. They are most useful for comparing different instruments if the test images contain similar distributions of stars and if the camera chip sizes are similar, not for deriving an actual measurement of anything.

In this case, the OTA is a Meade 12″ LX850 tube, the camera is an QSI683WSG-8, the starfield is centered around Albireo, and seeing was mediocre (see the FWHM measurements). The charts were generated from calibrated but unprocessed FITS images by CCDWare’s CCDInspector. BTW, I strongly recommend CCDI; I don’t shoot without it. It lets you know just how everything is working. There’s still a bit of focuser tilt (in MY focuser, not Meade’s!) but it’s not bad enough to affect what the charts are trying to show.

I have old images from a conventional F/10 Meade SCT somewhere around here (they’d be ten years old by now). I was using a camera with a fairly similar chip size – and if I want to I can crop them from APS size to 8300 size to make then the same. Some should contain appropriate starfields (the results are distorted if a bright DSO or too many bright stars are in the field). When I find them I’ll post the same analyses for comparison.

Bottom line? The ACF (and especially the F/8 ACF) tubes are said to be flatter than the conventional F/10 SCT of the same aperture – but they obviously aren’t FLAT flat. That’s fine if the end result is curvature you can live with – or if you have a suitable external flattener. I might have something suitable here.  The next step is to take similar images using a couple of flatteners I have lying around and see how they do.

It’s a matter of design priorities. The Edge HD starts with a conventional SCT (spherical mirrors plus a Schmidt corrector) and then adds even more refractive optics to correct for coma and to flatten the field. Neither is (or can be) perfectly effective – but the results are a very large improvement and folks like them. Meade’s approach was to accept the necessity of the Schmidt corrector, but not to add any more correctors inside. They instead selected mirror curves that don’t generate coma in the first place; more of a purist attitude. I’m just barely willing to accept the Schmidt corrector (my preference is really for a pure Cass reflector like the RC, though the more I shoot through this thing the more I like it) and prefer a design that doesn’t pile on more elements. If I need a flattener I can add one of my choice – and if I don’t need one I can skip the extra stuff entirely. What I don’t know yet is whether it is really flatter than a conventional SCT or if just seems to be because the lack of coma makes the corners look better anyway – and if so, how is that achieved? I intend to find out. Watch for updates on this subject.

p.s. Has anyone seen the RiDK 305 by Officina Stellare? It’s similar to a CDK and has internal correcting elements. I’m forced to admit that I was VERY impressed by the one I saw. Of course, it costs 50% more than the whole 12″ LX850 combo…

I had an opportunity to borrow a 12″ LX850 OTA so I snatched it. I’ve previously owned three 12″ F/10 Schmidt-Cassegrains and reviewed a 12″ RCX back in the day – and my current primary instrument is a 12″ F/8 RC so I have a pretty good idea what to expect. I haven’t done any serious analysis yet but I’ve had it on the mount for a few sessions and can make some comments.

All my leftover accessories from the old F/10 scopes fit the new model. I was going to re-use my tube rings but after taking a look at the beefy radius blocks on the new OTA I decided to stick with the supplied dovetail plate. It’s very strong and I trust it to remain rigid more than I trusted the original Losmandy plates on my old tubes (hence the tube rings). Plus the new plate has adjustments to eliminate cone error! I haven’t bothered with that but if I owned one of these and planned to use it long term I would. The metal dewshield dropped right on, though of course it’s the old shiny finish. Speaking of finish – that pebbly new finish grows on one after a while. I didn’t think I’d like it. I even found the old 3.25″ Meade flange for my Moonlight focuser so I was able transfer that to the new Meade tube.

About the focuser: This is as good as I hoped it would be when I first heard about the new design. Firm, smooth, no backlash and little or no image shift. I haven’t noticed any but will watch it with a camera when time permits so I can make a definitive statement. I wouldn’t have bothered with the Moonlight but I rely on being able to focus remotely. I believe that with one of the old belt-drive Robofocus motors this focuser could be controlled just as accurately as my stepper-equipped Moonlight. I checked image scale with the Moonlight in place, by the way, and was a little surprised at the modest focal length increase. It’s running at 2530mm (F/8.3); I’m used to seeing a larger increase from adding  a focuser to an SCT. Makes sense after some thought, though. The F/8 tube uses a secondary with a smaller multiplier. The good news is that this probably also means less focus shift with temperature, but of course that isn’t eliminated (thus my need for remote focus capability).

This OTA is probably about as close to what the RCX  could have been as anything we are likely to see. It has the same great optics set and solves the traditional focuser issues, though in a different way.  The RCX did have some other nice features. I won’t miss the internal dew heater much; that’s easy enough to do otherwise. The remote collimation was very cool, but my gear isn’t moved around so I don’t need to collimate frequently. I did really, really like the through-the-tube airflow with rear fans and front vents; that is a great way to end tube currents.  I think that when this one goes back home I’ll miss it as much as I missed the RCX.