Over the years there has been a lot of discussion about the focusing system used in Celestron telescopes here on Cloudy Nights. The basic focusing system that has been used in the Celestron scopes for a long time is simple, reliable, inexpensive, and generally works well for most applications. However, it also has some well-known side effects such as focus shift, mirror flop, and backlash issues that can be a bit more problematic for astrophotography.
These forums are full of stories from folks who have tried to improve the Celestron focusing system on their own and as many have learned, it’s not an easy problem to solve. The problem becomes increasingly more difficult as the aperture size increases culminating with the C14 systems. One of the things that makes the solution so difficult is the optical magnification factor provided by the Cassegrain secondary mirror. Any image shift caused by tilt in the primary mirror is magnified directly by the secondary mirror, which in the case of the Celestron systems is a factor of about 5x. Remember that the image will shift by twice the amount of tilt in the primary so if you want to hold the image stable to within 1 arc-second, the tilt of the primary must be controlled to within 0.1 arc-second as the mirror is translated. That is a VERY tight tolerance and it’s the reason that most simple fixes are unlikely to work very well—if at all.
A few years ago, I became involved with Celestron as an optical engineering consultant to design a new focusing system. We succeeded and the system that I designed has recently been introduced in the new RASA 14" astrograph. Up until now, I haven’t been able to say anything about it but I’ve recently been given permission to talk a little bit about my involvement and to show some results. We used a C14 Edge as the test-bed prototype and here are the features and benefits that we achieved:
1) It is a completely internal focuser with no loss in back working distance or focusing range.
2) The system is preloaded for zero backlash in any orientation. There is no need to approach focus from a preferred direction.
3) There is near-zero image shift with focus.
4) Primary mirror flop is eliminated. There is no mechanical "slop" in the mirror mount.
5) It has high mechanical stability--completely eliminating the need for mirror locks or shipping screws.
6) It allows both motorized and manual focus (at the same time.) The motor can be used for fine focus or automatic focusing even though the user can manually adjust focus using a knob.
7) It provides high precision, fine focusing capability eliminating the need for a third-party dual speed focuser.
8) It has a firm, smooth feel that solidly places the focus where you want it.
9) Inherently reliable operation that requires zero maintenance.
10) Eliminates grease and any possibility of outgassing in the OTA.
I measured the focus shift on one of the prototypes and the image shift was at or below 1 arc-second of lateral deflection of the image per mm of focus shift at a back working distance of 146.05 mm. Even at moderately high magnification, the focus shift is not visually noticeable.
In order to demonstrate and compare the performance of the new system to a factory standard system, I made the following movie using a Canon 6D running in 720p video. This clip shows the video slightly cropped (about 2/3). Please understand that the amount of focus shift present in any given factory system will vary. The system that I used in this video was purchased new from a retailer and is generally representative of a number of other new systems that I've seen. I’ll post a picture showing the amount of crop below. You can find the video on YouTube using the link below:
The operation of the new system is silky smooth and it feels like a Leica microscope. I’ve been using this system on my C14 for imaging for a while and it works quite well. The night-to-night stability of the system is very impressive. The focus simply stays where you leave it. I've found that to be true even when the telescope is moved from location to location. As long as the focus knob is not turned, the focus stays where you leave it.
In the RASA application, there is no secondary magnification factor, so the optical stability of the focuser will be greatly improved over what you see in the video (by about 5x.) Again there is no backlash so it will be very easy to produce V-curves for precision focusing. The new RASA 14” system is not inexpensive but it contains some very significant improvements in mechanical stability that I believe will impress new owners. The RASA 14" has mated world class optical performance with best in class mechanical performance that I believe will be well received in the market.
Finally, I want to emphasize that I do not work for Celestron. That means that I don’t know what they will do with this whole thing going forward. They have kindly agreed to let me talk publically about my involvement with this project and to share my video. I want to add that the folks at Celestron were a delight to work with and I want to thank them for the opportunity to work on this project.
Here's the crop used on the video: