14.5" F/9 Ritchey-Chrètien Truss by RCOS
Brad of RCOS loaned me a 14.5" Truss R-C design for evaluation. The optics for this scope were made by Aries Optics and do not require indexing. Also, the primary has a plano back, which allows a more secure mirror support. Ths OTA weighs in at 71 lbs. This truss design is the "big brother" of the 10RC I had been evaluating and has more extensive baffling. There is also a removable light stop mounted in the back plate. The scope was received uncollimated. The TCC controller and Instrument Rotator were installed from the 10RC.
Just barely fits in to close the roof
Mounting in my observatory proved to be a challenge. I had to set the scope back in the ME dovetail plate and offset the weight imbalance via additional weights on a dovetail plate extending out the front of the ME VersaPlate. The added weights can be seen near the secondary support ring. The ME had 70 lbs. of counterweights and had no problem handling the load. An STV was mounted on the top dovetail plate.
Speaking of Dovetail plates, it is somewhat disconcerting having a scope of this size and value secured by only two dovetail screws. I would suggest that the bottom dovetail be extended sufficiently so that at least 3 clamp screws can be used to secure the OTA. Alternatively, some kind of stop could be fashioned to act as a safety mechanism in case of slippage.
Bench collimation proceeded smoothly using the Takahashi collimating scope. The back plate light stop was removed for collimation. In the evening I performed a slight touch-up on a star, after the primary mirror was within 2° C of ambient. Final collimation consisted of less than 1/8 turn of one of the primary tip-tilt screws and the pattern looked as good as my seeing would allow. M1 was pretty well positioned before I did the touch-up collimation so I took 10 unguided 3 minute exposures for a total exposure time of 30 minutes. I combined them in Sigma, did a 2 iteration of CCDSharp and some curves , levels and mild USM in Photoshop.
And this was before final collimation. The stars deconvolved to around 2.2 arc-sec. Subsequent to collimation, I was seeing star sizes as low as 2.1 arc-sec. during FocusMax focusing. There is no doubt that size does matter! The image scale with my lowly ST-8E was .56 arc-sec./pixel with this scope.
The specified back focus distance for this design is a very generous 10.30". With my camera combination (ST-8E, CFW-8, Muscle Plate, SBIG T-thread/SCT adapter), the ADA-204 and IR, extension tubes of 2.5" and 1.75" got me within .04" of the specified back focus distance. A plate solve via TheSky Image Link gave a scale of .559 arc-sec./pixel. Assuming a 9 micron pixel, this resulted in a focal length of 3320 mm. Assuming a primary mirror diameter of 14.5", this resulted in F/9.02 - pretty close.
The removable back plate light stop used was 2" in diameter. This calculated to be sufficient for ST-8/9/10 chips without vignetting.
FocusMax beta 3.0.12 was used for focusing. 6 V-curve runs were taken and averaged. The resulting focus point was very repeatable, as expected with the TCC.
The evening was cooling off fast. My focus point during FocusMax calibration averaged 12773 while the temperature was around 5° C. Later in the evening, the focus point averaged 12732 at 1.9° C. I have had similar changes in focus position with the 10RC, leading me to believe there is a slight temperature coefficient of focus with the truss design.
Initial indications are very positive - this appears to be a gorgeous set of optics. There seemed to be good uniformity of focus with the seeing conditions. I am hoping for better seeing tonight.
2/7/2003: I wanted to try some high resolution imaging and decided on a small galaxy, NGC2357 in Gemini. Click on the image for a link to the detail page.
NGC2357 in Gemini
The galaxy is 3.4 x 0.3 arc-min. The flat field vignetting measured a respectable 3.6% fall-off and was easily flat-fielded out.
Just for the heck of it, I tried to image Saturn at the native F/9 focal length of 3320 mm. Here is the result:
Image scale: 0.56 arc-sec./pixel
I was pretty pleased with the results for a non-Barlowed arrangement.
2/12/2003 update: As I sit here under what has seemed to be continuous clouds with the possibility of clearing by Sunday(!), I decided to add some color data obtained previously to the above M1 image. The color data came from an M1 image acquired 11/13,14/2002 with a 10" OGS RC operating at F/7.3. The data was scaled to match the above monochrome M1 and here is the result.
This image has been downsized to 800 pixels wide. Here is a link to the full sized image (273K). It may be worthwhile to compare this image, with a mere 30 minutes of luminance, to this image taken with the 10RC at F/7.3. The outer regions of nebulosity are more clearly visible and the resolution is considerably higher.
2/15/2003: Well, there was enough of a break in the clouds to open up and give the scope a whirl. It was around 48°F with 90% humidity with some spotty thin clouds. First I decided to check the collimation by imaging a star field, M35 in this case, guided by my newly self-repaired ST-8E/AO7. (Removed the ferrite beads on the ST-8E digital board that power the AO7. Seemed to eliminate the AO7 problem.) a 5-minute exposure showed stars with FWHM's of 2.1 to 2.2 arc-sec. at the center and extreme corners of the field so it looked pretty good.
With that kind of seeing, even though variable, I had to take a look at Saturn visually. Started with my 12 mm Radian, which was my standard starting point with the 10" RC. I momentarily forgot I was using a 3300 mm FL instrument and was shocked by the size and clarity of the image. The view was breathtaking! The Cassini division was jet black, colored bands on the planet were visible and many of the moons were in the field of view. And this was at 275x. I pushed it to 550x with the 6 mm Radian and the image still held together well. Six of the Trapezium stars were clearly visible. What a great view!
Since everything was looking so good and the scope was close to thermal equilibrium, I decided to check intra- and extra- focus patterns with the 12 and 6 mm eyepieces. I went plus and minus 200 counts with the TCC hand controller. For the first time with an RC, I saw symmetrical patterns either side of center! I had heard of this being a feature of the Aries optics but this was the first time I saw it for myself. And not only that, they were symmetrical, at least as nearly as I could tell. These patterns, coupled with the M35 image, leads me to believe collimation is spot on! The brief activities of last night also lead me to believe this is a very special set of optics!
2/16/2003: Had another humid (for Tucson) night. I had some 5 min. exposures with 1.9 arc-sec. stars, and in all four corners, too. But then of course it was full moon time, so that figures!
In any case, I decided to look for stray light intrusion. Here are two images taken east of the meridian shortly after the moon had crossed the meridian.
South is up in both images. Neither image was flat fielded, hence the hot spot in the center. However, note the light intrusion in the lower left corner of M108. At the time, M108 was 41° from the moon and M51 was 56° from the moon. There was no light spill detectable in the M51 image, regardless of image stretch. The light spill in the M108 image was very pronounced. The light spill is in the opposite direction of the moon. By that I mean the light spill maps to the NE and the moon was SW from M108. My guess is an internal reflection off the primary baffle. Brad told me he did not put in the primary baffle honeycomb intentionally as was done on the 10RC to see if it made a difference. I suspect it does and will attempt to confirm it when the 10RC is reinstalled later.
For me, these optics appear outstanding. There were negligible artifacts surrounding bright stars, the collimation patterns were exceptional and collimation was relatively straightforward.