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More information about the new RASA 14 and its focuser

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#26 Exnihilo

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Posted 24 April 2018 - 08:53 AM

Is there any chance RASA could ever become a retrofit for existing Edge or other SCTs?  Presumably it would involve at least taking out the mirror cell?



#27 mclewis1

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Posted 24 April 2018 - 09:05 AM

No, I couldn't imagine any sort of cost effective upgrade from the current bits and pieces. I think most large SCT owners and prospective owners live in hope that some of the technology proven effective in the RASA could eventually trickle down to other products ... but I sure wouldn't count on it.



#28 akulapanam

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Posted 24 April 2018 - 09:46 AM

Is there any chance RASA could ever become a retrofit for existing Edge or other SCTs? Presumably it would involve at least taking out the mirror cell?


No the entire tube length is different. There is also already a retrofit called hyperstar which is pretty awesome!

#29 WadeH237

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Posted 24 April 2018 - 04:33 PM

That is exactly what I describe.  It does not use the stars in the imaging sensor to assess focus - it uses a signal from a completely different sensor at a different focal point for reference.  It assumes that the system has been focused properly in the first place - but it doesn't actually confirm that by going through focus and measuring star size in the imaging sensor.  And then it uses a model for how to adjust the focus based on a measurement that is not directly based on the size of the stars in the imaging sensor.

Honestly, it sounds very different from what you describe.

 

Full disclosure:  I've not used FocusLock, so I am going by what I've read of it, and by reports from a friend that uses it with a Lacerta and OAG.

 

That said, are you *sure* that it assumes that the system has been "properly focused in the first place"?  The fact that it's using astigmatism would make it possible to not only assess focus quality on each exposure, but also to understand whether the actual focus is intra or extra.  Such a system could certainly move to proper focus from an out of focus position.  And presumably, that's exactly what it's doing with each guide exposure.

 

That is, unless you are saying that the system assumes that the main sensor and guide sensor are parfocal.  I agree that it makes this assumption, and I will add that this can be difficult to do with high precision.  But that is a very different thing than assuming that focus is correct at the start of a session.

 

Also, I'm not sure how you focus your system, but I use FocusMax.  FocusMax definitely does not "confirm that by going through focus and measuring the star size".  It determines the best focus position by assessing an *out of focus* star and then calculating it based on a set of vcurves.  If the vcurves are off, you will not get optimal focus.  It does typically take some images at the end of the focus routine, but that is only for reporting the end result.  I never checks anything by going through focus.  It would be possible to write software that does this, but nothing that I've used works that way.

 

There are pros and cons to everything that we do in this hobby - and I'm sure that FocusLock is not perfect.  But from what I've seen of it, FocusLock is the real deal.



#30 jhayes_tucson

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Posted 24 April 2018 - 04:35 PM

I honestly don't understand the point you are trying to make. Yes, using the star profile astigmatism in the guide camera using the ONAG is indeed a proxy for "real" focus. But it is a very good proxy. There is nothing wrong with a good proxy. You seem to be saying that using a more traditional v-curve focus routine is better, but you just aren't very convincing. While it is true that the v-curve routine measures actual star size on the imaging sensor, it only does so for the instant that the final focus image was taken. For the remaining time until the next focus routine (potentially the duration of many frames) there is absolutely no indication of focus quality other than a posteriori examination of subexposure frames. Of course by that time, if you find that focus has moved, it is too late and the frame is lost. You are advocating that a process that is completely blind during the course of image collection is superior to one that provides closed-loop continuous feedback of focus quality and corrections during image collection based on some strange idea that a good "proxy" is somehow wrong, or inferior, or something. 

 

Many people have had extraordinary success with the ONAG. It works, and works very well despite your repeated insistence to the contrary.

 

Tim

 

Tim,

Let's be clear, that any discussion about "proxies" is a red herring.  We are talking about beamsplitters, which are widely used in all kinds of precision optical instruments.  Beamsplitters can be both very accurate and very accurately aligned.  They are widely used with great precision in DSLR cameras for focusing.  They are very widely used in optical data storage devices for focusing.  And, they are very widely used in all kinds of optical equipment for precision metrology.  Frank has taken a position on this that leads me to conclude that he either don't believe that beamsplitters can be very accurately aligned or that he doesn't understand how they work--I can't tell which.  On top of that, for a long time he has disagreed with me about the basic optical principle of how an astigmatic wavefront is affected by the addition of power.  I believe that Frank knows a lot about optics so I'm left scratching my head over this.  I want to be fair to Frank so I have to add that it's hard for anyone who hasn't actually tried astigmatic focusing to fully understand it or to appreciate all of it's characteristics. 

 

This whole discussion of auto-focusing is certainly interesting but it isn't why I started this thread (and it is stating to feel highjacked.)  I will say that the focusing system that I've developed helps quite a bit with FocusLock by removing focus shift and backlash.  It will also improve V-curve focusing by removing the need to compensate for backlash and image shift.  The system was tested with V-curve focusing and I found that the system will find the correct position on the curve and move directly there.

 

John


Edited by jhayes_tucson, 24 April 2018 - 04:38 PM.

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#31 spokeshave

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Posted 24 April 2018 - 04:56 PM

John:

As I understand it, Frank's "proxy" straw man has nothing to do with the beamsplitter. He's arguing that since focus is determined by the guide camera image, it is a mere "proxy" for the imaging camera image. While I suppose that's true, it is a downright silly argument. What he calls a "proxy" is a principle used in just about every scientific instrument in one form or another. I spent much of the day today setting up an atomic force microscope. It images individual atoms, but does so using the "proxy" of the Casimir force, not measuring the atoms themselves. Nonetheless, if the instrument is set up and calibrated properly, it works wonderfully. The same is true of the ONAG. The whole "proxy" argument makes no sense at all.

Your DSLR analogy is probably the most apt. DSLRs use a focusing sensor as a "proxy" for actual focus at the imaging sensor essentially the same process as the ONAG. Somehow, despite the enormous failings of such a system in Frank's esteem, the system manages to work perfectly in millions of DSLRs.

At any rate, we all digress. The new focus systems sounds great and I look forward to learning more about it. I recently added the Optec SMFS to my Edge 14 and am very happy with it, but I also look forward to the day when the very good optics in a scope like this are paired with a similarly good focus system.

Tim

Edited by spokeshave, 24 April 2018 - 04:57 PM.


#32 Corsica

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Posted 25 April 2018 - 06:51 AM

It seems there maybe some confusion about FocusLock (FL) and how it works:

 

FL does not require that both cameras are perfectly par-focal to begin with. What is required is that when FL is calibrated (for each filter, if any) the imaging camera was at its best focus, by using any means which fits the job (mask, V-curve, ...).

The only assumption made when using FL is that the both camera sensor plane relationship remains the same after calibration, this is a similar assumption one usually makes for a guide scope, OAG or ONAG, however differential flexure is not an issue here, instead one wants both sensor planes remaining at the same location (distance from the ONAG scope port flange).

The guide star astigmatism (shape) may be different from one filter to the next, unless they are perfectly par-focal which is seldom the case. Therefore in this context FL offers a calibration per filter, basically the guide star shape at best focus (its relative roundness, or astigmatism value) is saved for each filter and used as the target value by the FL close loop system when this filter is in used. This is similar to filter offset values when using a FW, excepted it is an active process since the focus is monitor and correct, if necessary, live for each filter.

 

Also FL is a patented technique for live focusing in astronomy, astigmatic auto-focus has been used since many decades in video microscopy and CD/DVD optical pick up heads, among others, where live critical focus is required, using beam splitter (dichroic or not).

 

The basic idea is to extract a pilot signal from the same optical system under analysis, even on the same target when using an ONAG. As a matter of fact since the ONAG generated astigmatism is across the field one can also use the all guider frame to retrieve focus information (with direction) from the guider image as well without any assumption about the pattern or image content (a bit like software Bisque @focus3 in TSX using the all imager frame contrast, but live with FL and the guider frame).

This technique is now available in SkySurveyor using the guider full frame, here is more information:

 

https://www.innovati...uidingfocusing/


Edited by Corsica, 25 April 2018 - 08:55 AM.

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#33 cbwerner

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Posted 25 April 2018 - 09:36 PM

Folks - this is the topic. Stay very closely on it, no further hijacking, intentional or not, with tangential issues or else we're in lockdown mode.

 

Any permutations of this topic *must* go to new threads.

 

But thank you for being civil in the exchanges. That is very much appreciated.


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#34 Freud

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Posted 29 April 2018 - 02:38 PM

Thank you very much John for sharing the information.

 

As mentioned on previous discussions, I hope that Celestron team will make this RASA focuser available as an upgrade kit for my C14 XLT.

 

- How will it be possible to motorize it?
- Will it be possible to install motors similar to the Feather Touch HSM or FocusLynx Optec systems?
- Will it be easy to get the Critical Focus Zone if that is installed on a C14/Hyperstar?

 

Regards
Frederic


Edited by Freud, 29 April 2018 - 04:43 PM.


#35 jhayes_tucson

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Posted 29 April 2018 - 03:31 PM

1)  It can be motorized with a specific kit made by Starlight express and they know what kit is required.

 

2)  Not any motor kit will work.  (It will also not work with a two speed Feather Touch focuser; but a two speed focuser isn't needed.)  The motorized focuser kit available from Starlight Express will work fine with the Optec FocusLynx driver and software.

 

3)  I don't know if Celestron will ever put this system on the C14 system (but I hope they do.)  I hate to break the bad news, but it will never be available as an upgrade.  I have it on my C14 Edge and it provides very fine control over the focus position (at the micron level) so it is very easy to achieve and maintain focus to within the CFZ.  It would work equally well with the Hyperstar system.  The current version in my scope has a minor limitation for use with Hyperstar but I'm not using HyperStar right now so I haven't fixed it.  Regardless, that issue could be very easily resolved so that the system would work quite well with Hyperstar.

 

 

John


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#36 Freud

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Posted 29 April 2018 - 04:46 PM

Thank.

Frederic



#37 charlesgeiger

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Posted 18 August 2018 - 01:21 AM

Jon,

It appears that you may be in some kind of contractural agreement with Celestron.  It would seem to me that you could produce these focussers  as an aftermarket item for the various scopes for a beta if the units are tested and bolt up anddd test wel.l



#38 jhayes_tucson

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Posted 19 August 2018 - 12:28 AM

A)  I supplied the design to Celestron and yes there is a contract.

 

B)  As I've said a number of times, this is not a design that could simply be bolted onto an existing scope--there are too many modifications that would require rebuilding the whole scope.  In my view, it's not economically feasible to retrofit this particular focuser to an existing system.  That means that there is no aftermarket option--even if someone (Celestron, me, or a 3rd party) wanted to do that.  I am happily retired and I have little to no interest in starting a business modifying scopes.  Trust me:  No one here could afford what I would charge to make it worth my while!  lol.gif lol.gif lol.gif

 

C)  Let's just hope that Celestron incorporates the new focuser system into more scopes.  It works pretty well...

 

 

John



#39 classpath

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Posted 04 September 2018 - 10:09 PM

Looks like Celetron adopted the new focuser system for all new RASA's lineup? I cannot find product page from Celestron but backpage AD in Astronomy magazin shows 8" F/2.0 RASA. And one of its feature is 

 

"NEW Ultra-Stable Focus System - six precision sealed ball bearings virtually eliminate focus shift"


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#40 HxPI

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Posted 06 September 2018 - 10:47 PM

I’m waiting for this new focuser put on the 9.25 before purchasing. Not holding my breath though!



#41 akulapanam

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Posted 06 September 2018 - 10:51 PM

Looks like Celetron adopted the new focuser system for all new RASA's lineup? I cannot find product page from Celestron but backpage AD in Astronomy magazin shows 8" F/2.0 RASA. And one of its feature is 

 

"NEW Ultra-Stable Focus System - six precision sealed ball bearings virtually eliminate focus shift"

Hopefully they put it on the 11" one too.



#42 coinboy1

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Posted 07 September 2018 - 12:51 PM

I too would like a confirmation whether the new 8" RASA uses John's focusing system! 



#43 jhayes_tucson

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Posted 07 September 2018 - 04:09 PM

At this point, I don't know but I can probably find out.  Stay tuned...

 

John


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#44 Jaimo!

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Posted 07 September 2018 - 05:44 PM

Is there an advantage of moving the mirror for focus over an aftermarket Crayford?  I do understand that is not how the Rowe-Ackerman works, but for a SCT or MCT.   I am no engineer, but it does not make a whole lot of sense (to me), especially as other high end cassegrains also go that route...  Unless it is a cost savings measure.

 

Jaimo!



#45 jhayes_tucson

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Posted 08 September 2018 - 12:21 PM

Is there an advantage of moving the mirror for focus over an aftermarket Crayford?  I do understand that is not how the Rowe-Ackerman works, but for a SCT or MCT.   I am no engineer, but it does not make a whole lot of sense (to me), especially as other high end cassegrains also go that route...  Unless it is a cost savings measure.

 

Jaimo!

 

It's all about back working distance.


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#46 akulapanam

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Posted 08 September 2018 - 02:09 PM

Is there an advantage of moving the mirror for focus over an aftermarket Crayford? I do understand that is not how the Rowe-Ackerman works, but for a SCT or MCT. I am no engineer, but it does not make a whole lot of sense (to me), especially as other high end cassegrains also go that route... Unless it is a cost savings measure.

Jaimo!


Many high end scopes use a moving mirror system as well either on the primary or more commonly on the secondary
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#47 Rayastro

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Posted 03 October 2018 - 07:13 PM

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:

 

https://www.youtube....h?v=QKK7LB08Olw

 

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.

John

 

 

Here's the crop used on the video:  

Nice to see this demo.  I am not really sure about the focusing shift.  But, I end up testing quite a bit on Celestron 8SE SCT with Hyperstar.

I created three different videos so far with my learning on Autofocus and experience.

I will really appreciate and provide me some feedback since you knew this area in and out.

Part- 1:  https://youtu.be/YEcBEejzfIU

Part - 2:  https://youtu.be/GmjAuWM40CU

Part - 3: https://youtu.be/Kp7hLTmwMqU

Regards,

Ray
Ray's Astrophotography



#48 Rayastro

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Posted 03 October 2018 - 07:16 PM

Thank you very much John for sharing the information.

 

As mentioned on previous discussions, I hope that Celestron team will make this RASA focuser available as an upgrade kit for my C14 XLT.

 

- How will it be possible to motorize it?
- Will it be possible to install motors similar to the Feather Touch HSM or FocusLynx Optec systems?
- Will it be easy to get the Critical Focus Zone if that is installed on a C14/Hyperstar?

 

Regards
Frederic

Check my post on this topic.  I provided John the details with my testing and work on automatic focuser. 

 

Regards,

Ray



#49 GlendaleGuy

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Posted 25 February 2019 - 05:36 PM

I am encouraged that Celestron is doing some work which might possibly result in an up-scale 'astrograph' version of the 11 and 14 inch SCTs.  As others have mentioned, I believe there is a marketing opportunity there.  

 

    I'm very interested in this as well. If they offered a C11 HD with this focus system update they would have a sale from me. Maybe a C8 HD as well.



#50 classpath

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Posted 25 February 2019 - 11:36 PM

    I'm very interested in this as well. If they offered a C11 HD with this focus system update they would have a sale from me. Maybe a C8 HD as well.

Same here. If they offer C11 HD with new focuser, I'll upgrade from my C8 HD definitely. I have been using FocusLock with my small refractor only but I highly want to use it for my SCT too!! :) 




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