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RCX400 a winner!

Posted by Mark de Regt   07/02/2005 07:00AM

RCX400 a winner!
A Bit of Background

I bought a 10” LX200 in 1998, intending it for use only as a visual instrument. Over time, I became less interested in losing sleep to see fuzzy things in the sky (I live in a light-polluted, sea level suburb of Seattle), so I started thinking about astrophotography.

I tried using my film SLR camera, but found that to be an exercise in frustration, since you don’t find out if everything is working well until some days later.

So, I bought a dedicated CCD imaging camera (an SBIG ST-7E, with a color filter wheel). I was instantly hooked. Over the following four years, I slowly upgraded my camera and other equipment, while continuing to use the trusty old LX200. In order to make the LX200 a more imaging-friendly scope, I had to do some improvements. First, I got a Crayford-style focuser. Then, I got one of those bolt/springs for locking the main mirror, to reduce mirror flop. Then I got a Robofocus motor for the focuser, to be able to use the auto-focusing routine in CCDSoft, since I have to refocus frequently because the aluminum tube of the scope contracts with the falling temperature. Then I got a dew-heater with a controller. I got thumb screws to collimate, which makes an almost impossible task (using a tiny allen wrench in the dark on the other end of the scope to make minute adjustments) merely very difficult. This picture shows the scope in my garage; you can see the dew heater (without the wire which attaches it to the battery or the one which attached it to the controller); the external focuser, which adds length to the scope back, and the rail on the bottom, with the holder for weights to balance the tube.

I started dreaming about a scope which wasn’t so much work to image with, but I really didn’t want to spend $30,000 for scope and mount, especially for imaging in a place where the weather is so horrible so much of the time.


Then Meade announced the RCX400 line of scopes, and (in the interest of full and fair disclosure) I was enchanted with the prospect of getting much better optics for a relatively modest price. And (there’s that full disclosure thing again) I prefer working with fork mounts to GEMs, for the sheer simplicity (and no meridian flip).

I got a 12” RCX400 in early June, just in time for a long stretch of dreary weather. Although I was not able to use it outside, I set it up in the garage, to get familiar with Autostar, and the physical aspects of the scope. There were many early impressions, most quite favorable; Meade listened to the complaints about the shortcomings of the LX200 scopes, and did a fine job of making this a much more user-friendly imaging instrument; some examples:

--The center bolt on the tripod, which secures the scope on the tripod in alt-az mode, has a spring attached, so you can gently place the scope on the tripod, and then center it, without worrying about poking a hole in the bottom of the mount.

--By its design (more or less a Ritchey-Cretien), it focuses by moving the secondary mirror instead of the primary mirror (as on Schmidt-Cassegrain scopes). The primary mirror is fixed, so there’s no mirror flop, and practically no image shift (often absolutely no image shift).

--The focusing mechanism is quite exact, allowing for the most minute of adjustments.

--There is a great deal of clearance, both for the far end of the scope, and for the scope back, with camera/filter wheel/adaptive optics device on, to clear the base of the fork arm. This allows one to image any object anywhere in the sky, and eliminates the most significant criticism of fork mounts. These pictures show the clearance on both the front and the back of the tube.

--It has a carbon fiber tube, so temperature changes should necessitate few or no focus changes.

--Collimation is a dream, taking no more than a minute to get perfect collimation.

--It has a built-in, variable dew heater.

--It has a number of memory positions, allowing the user to store the focus position for, e.g., several eyepieces and camera setups, so the lack of parfocal status is much less of an issue.

--The Autostar interface is not particularly difficult to learn.

--The tube is weighted so that, with camera/filter wheel/fast guider attached, the balance is perfect--with just a slight load on the gears.

There also are some negatives:

--The handles need to be bigger to be really useful; as it is, anyone who is strong enough to lift one of these will have hands too large to comfortably use these. But replacement handles are available on the market for reasonable prices.

--I miss the ability to focus by hand; the telescope must be up and running in order to use the focuser. It’s not a big deal, and I don’t know that there’s any good way to have a manual focuser with this scope, but, for viewing, I prefer a precise manual focuser to using an electric focuser, even a very precise one.

It is also nice that the scope fits just fine on my existing wedge, so that anyone with a permanent setup or semi-permanent setup for their LX200 need not mess with the pier or tripod already being used. However, I do not think that a wedge for an LX200 will fit optimally on the very nice tripod which comes with the scope.

Finally, outdoors

After several weeks of garage astronomy, we had a slight break in the weather. No opaque clouds; steady skies, but terrible transparency and a very bright moon. Good enough; I set up.

The scope is remarkably easier to use than the LX200 was. After years of struggling with the collimation of my LX200, it is an indescribably pleasure to fix collimation in seconds; no muss, no fuss. As much as I could tell from the conditions, the optics, for visual use, are excellent, probably much better than my LX200. It looks like it has more contrast (it has better coatings, and probably has better baffling), and the ability to easily get (and store) perfect collimation should not be forgotten; I no longer have to settle for “good enough” collimation because of frustration.

It slews as quickly as the LX200 (although I always set the fastest slewing speed much slower than the maximum, to save wear and tear on the motors as they get the massive OTA in motion from a full stop). It slews a bit more quietly than the LX200 Classic, I think, but it’s still quite loud. The slower the speed, the quieter the slewing. It’s goto is very, very accurate, putting objects in the center of the FOV of even fairly high power eyepieces. Although there is quite a bit of vibration when the OTA is touched, it dampens out quickly.

But I got the scope for imaging, and its qualities as an imaging scope would determine whether or not it is worth the extra cost over an LX200.

Right away, I was pleased at how much quicker it is for me to set up the RCX for imaging than it was for the LX200—built-in precisions focuser and built-in adjustable dew heater eliminated a number of steps, and wires, from the process, a welcome simplification. As mentioned, collimation was a breeze. I attached the camera, cooled off the chip, and started focusing.

I use @Focus in CCDSoft, an automated focusing routine which requires (i) very good repeatability in focuser position, and (ii) that one find appropriate “large steps” and “small steps” for the focuser’s motion. After a bit of fiddling with the step sizes, excellent focus was achieved. Because of the conditions (excellent seeing, poor transparency, full moon), and because star shape and size was a primary concern for me with the new scope, I had chosen M92, a nice globular cluster, as my target.

So, I told the scope to go to M92, took a quick image, and there it was. How nice! I proceeded to take a series of images, four minutes each (11 of them) for the luminance layer (the one which shows the detail), and ten minutes each (3 for each) for each of the three color channels. I was looking in particular for small, tight stars, and whether the focus degraded as the temperature dropped (it typically will drop at least 20 degrees F after I start imaging).

On both counts, I was very pleased. The scope gave very tight stars, and I never had to adjust the focus, all night.

Finally, I took flat frames at dawn, and it was nice to see how uniform the exposure was across the chip, compared to the LX200.

I am very impressed with the scope, which performed flawlessly. It was remarkable to me that it produced a credible image on the first night I had it outside. You can see it at:

--Mark de Regt

Click here for more about this subject. -Ed.