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Clement Focuser on C11 - Review

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#1 novalab

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Posted 09 February 2013 - 01:42 AM

Over the years I tried just about every focuser out there. Somehow, I was also fooled by the hype that the Crayford is the thing to have. Perhaps this is true for visual work, but for AP? So, I dutifully tried them all – with the same results more or less. From what I observed, the biggest issue I had was the lack of position repeatability due to inner tube slippage. Basically, if you go back and forth a number of times, there will be some positional “loss”. As I do temperature compensation on my system, and prefer not to have to refocus every session, this is a bad thing. To remedy this I tried modifying them in every conceivable way, but to no avail. A secondary issue is that under the weight of the camera, the tube will slightly tilt the bearings, and thus there will be some image shift as you move the focuser. I do a lot of photometry, and to me it is important to keep the star centroid on a pretty much the same pixel over a several hour period. Everything I tried failed in one way or other.

Then I finally ordered a Clement Focuser (http://www.clementfocuser.com/) . See the attached photo in my initial setup. As an engineer, I must say that it is an ingenious approach. The focuser is well built, solid, and inspires confidence as soon as you see it. It does look different, but then it really is different. It weighs around 3lbs, but can easily carry 20lbs, with no issues. It is a very low profile, allowing you to bring your camera closer to the OTA. I use a Starizona SCT corrector/reducer on my C11, and this allows it to actually go inside the C11 OTA. Difference is F7.6 down to F7.2 now. BTW, Starizona corrector really works well. Best of all I tried, with pin point stars across the whole KAF8300.

What a difference in performance!!! No image shift at all, backlash of less than 3 microns (measured in my lab), a perfect repeatability, and it can carry more load than I could possibly ever throw on it. Optical path is totally enclosed, and no dust will ever enter your optics (no Crayford can make that claim, as there is a gap around the tube). Most of all, it is a true absolute screw driven focuser. I know that many will think it expensive at $1000+, but in reality, if I had it to start with, it would have saved me a ton of money and frustration. Don Clement sells the focusers, and will also provide a RoboFocus at an extra cost, but in reality, Robo motor is a very bad solution for it, providing mere 0.14um steps (which is way to slow). Not Robos fault at all, as it was meant for something else, and is heavily geared. Instead, I attached a NEMA-17 1.8 deg stepper, in a 2:1 reduction drive – direct drive would work too, but this was simpler for me. This gave me 1.4um steps, which is appropriate on a F7.2 system like my C11. I had to make a custom C11 focuser knob, and to machine down the focuser mandrel, as I wanted the focuser as close to the OTA as humanly possible.

Since the pic was taken the C11 has gotten a CF tube and a set of tube rings, which made TC control work perfectly. The Al tube worked pretty good too, but I think that there was a mismatch between the dovetail and tube TC, which caused issues. The Parallax tube rings solved that.

I drive the focuser with a SharpSky from http://www.dt-space....k/SharpSky_Kit/ from my good friend and a fellow EE, Dave Trewren from UK. It is far superior in features and performance to RoboFocus, Moonlite or Starlight (I do have them all). Direct USB control, no drivers needed. I used Moonlite and RF on it before, but I wanted some features from the SharpSky, so I switched.

So, after I have been using it for a while, I thought to share my experiences with you. Suffice it to say is that my observatory Scope has not needed refocusing in past two weeks. I turn on the system, the focuser senses the temp and automatically assumes the appropriate starting position (thanks Dave). It all works seamlessly with MDL, and a filter wheel. You will need to measure the thermal coefficient of your tube over few nights.

I am getting another Clement focuser to go on my CDK17”. I cant think of a better way to focus.

I hope you find this useful.

Chris Krstanovic
NovaLab Observatory
Windham, NH, USA

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  • 5670045-Focuser (Medium).jpg


#2 JM La Galette

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Posted 09 February 2013 - 04:52 AM

Looks nice... Thanks for the report and also for SharpSky, I need to have a look.

Have you received your CDK17? Is it from OO UK?

Thanks,
JMarc

#3 novalab

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Posted 09 February 2013 - 06:03 PM

Hi JMarc,

I already have the CDK17, but will be waiting for spring to put it up in my observatory. Winters in New Hampshire are brutal, even in a heated observatory. I got the CDK from HK, along with the FR. 17" at F4.5 effective, 40mm diam flat field. I will have to make few mods, but will be doing a post on it. I did a quick test when it arrived, and the optics are very good.

I will do a write up in SharpSky Product (not a kit) soon. It is a new company, and takes time to get things going in distribution.

Now, I have to go and dig myself out of 0.75m of snow we got last night.

Chris K

#4 smallscopefanLeo

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Posted 09 February 2013 - 06:58 PM

Wow. I had no idea such a focuser existed! Gives one food for thought for the future... :thinking:
Enjoy your exotic, finely specified equipment there! :waytogo:

#5 wolfman_4_ever

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Posted 09 February 2013 - 07:17 PM

I was going to go the Clements focuser route on my HD11.. The robofocus and the .14 steps through me for a loop. I ended up going with an FLI focuser... Very nice focuser the clements... I really like it..

#6 novalab

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Posted 10 February 2013 - 12:01 PM

Hi Dave,

The main reason I posted this was to spread the word on the Clement. Although it is not a brand new thing, not too many people know about it. I guess the price has something to do with it. If you are into AP, I feel it is a must. Like I said, the RF motor is not the best control solution due to tiny step size. However, it is a pretty simple thing to direct attach any moderate stepper to it (NEMA17 or bigger). Another advantage is that can still retain the manual “knob” control, while having the motor in place – no need for the clutch or similar.

One thing I forgot to mention is that I noticed the autofocus seems to be more accurate in doing its thing. While temperature tracking on exposures, I noticed that the stars are considerably smaller too. I have a feeling that this may be due to the image shift when the Crayford is moved. I think that the stress the of Crayford tube on the bearings due to the weight of the camera+FW causes image shift of some microns. Not much, but enough. I could prove it, but frankly have no desire to pull out my old CFs.

As for the controller, SharpSky was my choice, but ML controller works too. I don’t want to appear partial, since I know Dave Trewren. The trouble I had with the ML is that the temp resolution is only 0.5C, which is not adequate for good TC. SkarpSky has 0.0625C resolution, and I adjust focus every 0.13C. Works like a charm.

Chris

#7 JM La Galette

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Posted 10 February 2013 - 03:01 PM

Thanks for the answer Chris, enjoy your new toy!

JMarc


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