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More information about the new RASA 14 and its focuser

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#1 jhayes_tucson

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Posted 17 April 2018 - 12:39 AM

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:  

Attached Thumbnails

  • Screen Shot 2018-04-11 at 10.42.41 PM copy.jpg

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#2 rockstarbill

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Posted 17 April 2018 - 01:12 AM

Wonderful change! Do you know if this will make its way into all of the Edge scopes? 



#3 bilgebay

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Posted 17 April 2018 - 06:16 AM

This is awesome! thanks for sharing!



#4 leveye

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Posted 17 April 2018 - 07:32 AM

Amazing results. Congrats and thank you for the hard work on this. I hope it's implemented on all of their SCT designed scopes.


Edited by leveye, 17 April 2018 - 07:32 AM.

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#5 jhayes_tucson

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Posted 17 April 2018 - 11:29 PM

I'd love to see it make its way into the Edge product line but as I've said, I honestly don't know what Celestron will decide to do with it.  My own guess is that in production, it might add about the same to the price as adding a high-end external motorized focuser (e.g. FLI Atlas focuser) so it's not going to be inexpensive.  Unfortunately, the size of the market for telescopes shrinks pretty rapidly as the price goes up so we'll just have to see if it's a good enough solution to fit a market that Celestron might be willing to address.

 

John



#6 Bigdan

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Posted 18 April 2018 - 06:11 AM

Very interesting, John.... with mirror flop eliminated, and near-zero image shift..... do they still have the mirror locking knobs on it?



#7 WadeH237

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Posted 18 April 2018 - 08:15 AM

I'd love to see it make its way into the Edge product line but as I've said, I honestly don't know what Celestron will decide to do with it.  My own guess is that in production, it might add about the same to the price as adding a high-end external motorized focuser (e.g. FLI Atlas focuser) so it's not going to be inexpensive.  Unfortunately, the size of the market for telescopes shrinks pretty rapidly as the price goes up so we'll just have to see if it's a good enough solution to fit a market that Celestron might be willing to address.

 

John

I don't know if Celestron is listening, but this particular improvement would be enough to get me to buy a couple of telescopes.

 

I would immediately replace my EdgeHD 8 with a new one that has this focuser.  I would also either replace my 15 year old C14 with an EdgeHD 14, or perhaps keep the C14 and add an EdgeHD 11.

 

With the existing focus mechanism, I have no plans to buy any more SCTs.



#8 mclewis1

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Posted 18 April 2018 - 09:09 AM

Cue the discussion about the viability of Celestron doing a higher end series of scopes (although not here as that would obviously be off topic).

 

Speaking of on topic - I was going to ask John if he could characterize the mechanical complexity of the new focuser vs. the traditional one. Not asking about the mechanical details or anything like that, just an idea about how much more complex the new RASA focuser is to get an idea about it's potential in other SCT models. But I guess the comment above about the potential cost similarity to an external motorized focuser answers that question. 



#9 jhayes_tucson

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Posted 18 April 2018 - 10:45 AM

Very interesting, John.... with mirror flop eliminated, and near-zero image shift..... do they still have the mirror locking knobs on it?

No, they are not needed.  The shipping screw is also obsolete.

 

John



#10 Edd Weninger

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Posted 18 April 2018 - 11:42 AM

Ah Ha, so that was why you did the interferometric alignment of your C-14s, to validate the performance of the focuser alignment smirk.gif .  Good work.

 

Now that anyone can buy one, can you provide any engineering details of the focuser.  I liked Roland Christen's approach in his Mak 10, but it seemed quite costly.  And, there were other ideas about.

 

Did Celestron make any effort to reduce the mass of the C-14 mirror, which I always thought excessive.

 

Thx,



#11 jhayes_tucson

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Posted 18 April 2018 - 12:40 PM

Ah Ha, so that was why you did the interferometric alignment of your C-14s, to validate the performance of the focuser alignment smirk.gif .  Good work.

 

Now that anyone can buy one, can you provide any engineering details of the focuser.  I liked Roland Christen's approach in his Mak 10, but it seemed quite costly.  And, there were other ideas about.

 

Did Celestron make any effort to reduce the mass of the C-14 mirror, which I always thought excessive.

 

Thx,

 

 

1)  No, I did the interferometric measurements on the C14 to establish its wavefront accuracy and to better align the optics.  The mechanical stability of the system insures that after the system is aligned on the bench, it will stay aligned when it is pointed at the sky.  Since I did that alignment, I have transported the telescope many times all around the west and I have never once had to realign it.

 

2) I can tell you about the benefits, features, and performance of the new focuser; but, I am not going to talk about any engineering details.

 

3) To my knowledge Celestron hasn't changed the design of the C14 primary mirror.  It is tricky business to light-weight a mirror so that you don't introduce problems in manufacturing or in the optical performance of the mirror.  The Celestron mirror is a tapered rib design that strikes me as a good compromise between a very heavy simple disk and a much more expensive ultra-lightweighted design.  It certainly could be made lighter but I suspect that it might add quite a bit to the cost.  As it is, the C14 is actually reasonably light for its size.  I didn't mention it, but the new focuser probably adds less than a pound to the weight of the telescope.

 

Ultimately, it's the optical accuracy that's most important and that's where I believe Celestron has made the most improvement in recent years.

 

 

John


Edited by jhayes_tucson, 18 April 2018 - 04:05 PM.


#12 psandelle

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Posted 18 April 2018 - 07:06 PM

Fantastic! At those fast speeds, the focuser was always the question with Hyperstars and the 11” RASA. Great news and job well done. Now I have to go see one in person.

 

Paul



#13 Edd Weninger

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Posted 19 April 2018 - 11:54 AM

"We used a C14 Edge as the test-bed prototype and here are the features and benefits that we achieved:"

 

Sorry for the mistake.  I made an assumption based on the above statement.

 

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 likely not a customer since I commissioned a custom 16" Rumak several years ago.  However, before I did so, I did review the Meade and Celestron products.  One of my thoughts was "why don't they do whatever to make some of them better".  I ended up buying neither.  

 

Surely there are a number of astro imagers who would enjoy a quality OTA vs. seeking out and fussing with the visual scope.  They would be mounted on quality GEMS and fitted with quality cameras.  So why not a quality scope?

 

Thx,

 

edit: Oh, don't forget a better conductive heat path from a warm mirror vs. the dependence of radiance to the internal air of the closed tube.


Edited by Edd Weninger, 19 April 2018 - 12:02 PM.


#14 jhayes_tucson

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Posted 19 April 2018 - 12:49 PM

Ed,

I totally agree that a higher end product line is a good idea.  As an engineer, I always worked on high-end, precision products so I'm not the guy to ask about how to make it cheaper or how to make a "low-end" product.  My focus has always been on developing better solutions.  Regardless, the whole thing will be a business call for the folks at Celestron and I don't play any role in that decision.   It seems that the owners in China have historically focused on volume sales and they have been very successful with that approach.  Whether they can transition to a higher level in the market remains to be seen.  There are already a number of other players in that space so it's a tricky proposition in such a small but highly competitive market.  I'm glad to see them taking the first step with the RASA 14, which is aimed mostly at professional applications such as university, research, and government customers.

 

John


Edited by jhayes_tucson, 19 April 2018 - 12:50 PM.

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#15 JoeR

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Posted 22 April 2018 - 09:32 AM

That is a big improvement. I was paying close attention to my focusing last Friday and the image shift in my 14 is exactly as it is in the YouTube video. I installed Zapsteel Flop Stoppers which do help stabilize the mirror for imaging but does nothing to correct focus shift. In fact when I lock them down the image does shift slightly. Would it be theoretically possible for a third party to offer an upgraded rear cell with this design or would that require too much modification to an existing OTA?



#16 akulapanam

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Posted 22 April 2018 - 10:49 PM

It was pretty impressive very smooth, no backlash. That said I didn’t see an easy way to motorize but I could be wrong.

Will we see this added to the 11” RASA?

#17 sink45ny

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Posted 22 April 2018 - 11:02 PM

Really interesting John. I hope Celestron adopts your recommendations.



#18 jhayes_tucson

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Posted 22 April 2018 - 11:05 PM

That is a big improvement. I was paying close attention to my focusing last Friday and the image shift in my 14 is exactly as it is in the YouTube video. I installed Zapsteel Flop Stoppers which do help stabilize the mirror for imaging but does nothing to correct focus shift. In fact when I lock them down the image does shift slightly. Would it be theoretically possible for a third party to offer an upgraded rear cell with this design or would that require too much modification to an existing OTA?

I won't go into details, but this is something that is better built into the scope in the first place.

 

John



#19 jhayes_tucson

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Posted 22 April 2018 - 11:40 PM

It was pretty impressive very smooth, no backlash. That said I didn’t see an easy way to motorize but I could be wrong.

Will we see this added to the 11” RASA?

 

This system is actually easier to motorize than the original factory design.  As I've said, one feature is that both manual and motorized focusing can be done at the same time.  It is either/or or both (if you like.)  I made the movie by manually turning the focus knob but it works equally well with the motor.  I have the motor speed set to be very low for precision focusing so the demo movie would have been in very slow motion using the motor, which wouldn't have been as impressive.

 

I don't know what Celestron will do with the 11" RASA.  Most Celestron customers are accustomed to very low prices so a key question is how would the market react to a higher price?  Let me to turn it around and ask how much more you would be willing to pay for a focusing system with the benefits and features that I listed in the first post?  I'm pretty sure that everyone would pay an extra $10 and that no one would pay an extra $10,000.  That bounds the answer.   So the first question for the marketing folks is:  What's it worth in the market as a function of how many can be sold?  The next question is:  Where does profitability happen on that curve?  That's a question that I imagine the folks at Celestron might try to figure out.   I would guess that the price sensitivity goes up pretty fast for the smaller scopes so I personally wouldn't expect to see it on the C8 Edge scope.  It's more likely to go into the larger system and that's probably why it appeared in the RASA 14, which is aimed more at professional applications. I could speculate extensively on this stuff, but I really don't want anyone to get the idea that I have any insight into what Celestron might do because of my work on this project--I do not.  So, I'll stop here.

 

 

John



#20 akulapanam

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Posted 23 April 2018 - 12:51 AM

This system is actually easier to motorize than the original factory design.  As I've said, one feature is that both manual and motorized focusing can be done at the same time.  It is either/or or both (if you like.)  I made the movie by manually turning the focus knob but it works equally well with the motor.  I have the motor speed set to be very low for precision focusing so the demo movie would have been in very slow motion using the motor, which wouldn't have been as impressive.

 

I don't know what Celestron will do with the 11" RASA.  Most Celestron customers are accustomed to very low prices so a key question is how would the market react to a higher price?  Let me to turn it around and ask how much more you would be willing to pay for a focusing system with the benefits and features that I listed in the first post?  I'm pretty sure that everyone would pay an extra $10 and that no one would pay an extra $10,000.  That bounds the answer.   So the first question for the marketing folks is:  What's it worth in the market as a function of how many can be sold?  The next question is:  Where does profitability happen on that curve?  That's a question that I imagine the folks at Celestron might try to figure out.   I would guess that the price sensitivity goes up pretty fast for the smaller scopes so I personally wouldn't expect to see it on the C8 Edge scope.  It's more likely to go into the larger system and that's probably why it appeared in the RASA 14, which is aimed more at professional applications. I could speculate extensively on this stuff, but I really don't want anyone to get the idea that I have any insight into what Celestron might do because of my work on this project--I do not.  So, I'll stop here.

 

 

John

That's a good question.  Seems to me that about $1200 is the max point on an Edge HD because at that point you can just lock your mirrors and buy a LEO or moonlite.  I haven't used a RASA but it would seem more critical.  Then again I never ran into this issue with my C9.25 and hyperstar and @focus3 seems like it would handle this issue pretty well as long as your box was reasonably sized.



#21 freestar8n

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Posted 23 April 2018 - 04:03 AM

That's a good question.  Seems to me that about $1200 is the max point on an Edge HD because at that point you can just lock your mirrors and buy a LEO or moonlite.  I haven't used a RASA but it would seem more critical.  Then again I never ran into this issue with my C9.25 and hyperstar and @focus3 seems like it would handle this issue pretty well as long as your box was reasonably sized.

I think this system would only appeal to a particular cross section of users - and it would have a downside for almost all by increasing the cost and possibly weight.

 

A key point is that there is no evidence this actually would improve results for deep sky imaging either at f/10 or with hyperstar - as long as automatic focus is done with the primary and backlash is removed.  Backlash and image shift have no role in realized performance - it's only the repeatability and consistency that matter.

 

There would be a clear look and feel improvement - but there are people who are so picky about this that they would probably prefer a separate crayford anyway - and not just any crayford but the exact one they prefer.

 

For planetary imagers, they don't mind tweaking collimation on every imaging session, while pointing right at the object - so they would probably be collimating anyway.

 

The entire OTA is flexing anyway - so improving only the mirror will not make the system suddenly perfect - and beneficial tweaks are always possible.  This would only reduce the size of those tweaks.

 

For the particular case of the RASA - that is a very specialized instrument and people would be more willing to pay more for improved focusing - but again if they spend a ton on a large sensor hanging off the front - they will probably need to tweak the alignment anyway if they want perfect imaging across the sensor.  And since they already have wires going across to the camera over the corrector - they are more suited to an expensive and precise focusing/alignment system built into the camera attachment - which means they could leave the primary fixed.

 

And for crayford people who don't intend to use hyperstar - you could glue the primary in place and not even provide a focuser - and let the user choose a 3rd party crayford to attach.  This would decrease weight and reduce cost - but would lose the ability to focus nearby.

 

And there are other changes that could be made to the sct - including a longer tube and smaller secondary for high res work.

 

But the sct with primary mirror focuser is a very simple system that keeps the cost very low while remaining very versatile.  And with a simple stepper motor on the primary knob, combined with autofocus, you will be oblivious to the backlash and imaging shift - and it will have no impact on the result.

 

In summary I think it makes sense to have the focuser improved on the RASA - but I don't consider it important for most other sct applications.  A separate crayford and locked mirror has inherent advantages when that is a suitable solution, such as planetary and visual.  And for deep sky imaging the existing system works perfectly well with the right technique.

 

Frank


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#22 jhayes_tucson

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Posted 23 April 2018 - 11:36 AM

As I said, the added weight is no more than about 1 lb (and it's probably closer to 1/2 lb) so it's not significant.  I completely agree that the biggest downside is in the cost.   I agree that the current system is workable but all of the things that need to be done to make it work are Band-Aides.  If cost is the biggest concern, the current system is certainly workable; but, in my view, it's not the right way to do things with a precision optical system.  The new system holds optical alignment quite well in all orientations.  Furthermore, I've shipped the scope many times between Bend, LA, and New Mexico and I've never once had to realign it.

 

Eliminating focus shift and backlash allows continuous focusing in real time with the shutter open.  I'm always amused when I'm at the observatory at night and I hear all the scopes clicking away with motors running every time each system has to refocus.  It's an almost continuous cacophony of shutter clicks and focus motors among the 20+ scopes.  My system focuses within a minute after I start it and it just holds focus all night long as I image.  

 

Another key advantage of this new focusing system is that it doesn't eat up any back working distance--and it works for both the Cassegrain focus and for Hyperstar.  Fixing the primary and adding an external focuser to the back of the scope chews up a lot of BWD.  Fixing the primary and adding a focuser to the secondary is probably the next best solution, but it requires wiring through the pupil and it won't work with Hyperstar.  Still it's a solution that's available today and I recommend it.

 

While it's true that flexure within the OTA might be an issue, my job wasn't to try to fix all that stuff.  In principle, this solution can actually make the system considerably more mechanically rigid, but I didn't accomplish that on the system that I'm using.  I'll admit that on my particular system something is flexing quite a bit more than I'd like.  Right now, the main suspect lies in the whole camera package hanging off the back of the telescope.  I can't go into details but I designed a new baffle nut to help reduce this possibility and to better insure that the optical axis is properly carried through to the camera, but a machining error meant that I couldn't use the one that I had made.  At that point, I couldn't get it fixed so I was really disappointed that I couldn't test it.  As it is, I can grab my camera package and flex it without exerting a lot of force so I know that it sags much more than I'd like as the scope moves around the sky.  Everything is really tight (all the screws and interfaces) but that whole package is pretty heavy and the mechanical interfaces aren't very stout so it's just due to mechanical flexure.  This is probably the lowest hanging fruit in terms of cleaning up flexure on my system--and I suspect on a lot of systems with heavy camera packages.  I don't see optical alignment issues as the scope changes pointing angle so I believe that the internal optics aren't flexing very much but I'm still doing some additional testing to better verify that conclusion.

 

John


Edited by jhayes_tucson, 23 April 2018 - 02:09 PM.


#23 freestar8n

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Posted 23 April 2018 - 05:27 PM

I know that you like focusing while imaging - but it has the inherent downside that it is blind to what is actually happening in the image.  At the same time, periodic focusing tells you exactly what is happening and provides a direct measure of how much drift there is between exposures.  If you detect negligible change in fwhm over 30 minutes after re-focusing, you know there would be no benefit from focusing during the exposure - and in fact it could make things worse since it is not using the actual stars in the actual image.

 

So - although making the mirror motion more controlled shouldn't have a downside in terms of performance, realtime focusing with a proxy actually could hurt the fwhm.  It will only have benefit if the system is well focused in the first place and the focus proxy is well calibrated - and that is not guaranteed.  It would be better to call it a precise focusing system rather than accurate - and it is only precise in terms of tracking the proxy - and not in terms of maintaining a fixed focus error.

 

Making the system more rigid also likely has benefit - but again you have only addressed one element.  Professional systems don't aim to make things perfectly rigid and instead assume things are flexing all over the place - and install active optics controls to compensate.  So for the RASA, a meaningful comparison would be align the system well when aimed at 20 degrees altitude - and then aim it at 80 degrees and see if any tweaking is needed.  If tweaking is needed even when you have your focusing installed - then you still would want to tweak the alignment anyway.  And if you are willing to spend much more on a focusing system - I would put it on the secondary and allow automatic alignment also - with a fixed mirror.

 

And if you re-focus with the primary as normal and re-load the mirror - you will remove much of the tilt incurred by the change of altitude - and there may not be much difference from having the mirror fixed in the first place.

 

My main point is - none of these systems will be perfect and have zero slop - and a critical factor is the tolerance a user has to those errors.  Some people are perfectly happy with the current focusing system and have never even collimated their scopes.  Others will collimate on every session.  Some will immediately attach a crayford and wouldn't think of focusing with the mirror.

 

I think it's great you are exploring these alternative mechanics - but keep in mind many of us are taking a stock OTA and exploring simple approaches to get optimal results from them - without modification - and getting excellent results that can be directly transferred to other users at 0 cost.  Your changes should make it easier for people to get good results with little skill and effort - but skill and technique can help overcome deficiencies in most any design.

 

Frank



#24 jhayes_tucson

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Posted 23 April 2018 - 07:26 PM

I know that you like focusing while imaging - but it has the inherent downside that it is blind to what is actually happening in the image.  At the same time, periodic focusing tells you exactly what is happening and provides a direct measure of how much drift there is between exposures.  If you detect negligible change in fwhm over 30 minutes after re-focusing, you know there would be no benefit from focusing during the exposure - and in fact it could make things worse since it is not using the actual stars in the actual image.

 

Let me get this straight.  You think that by checking focus every 30 minutes, you know more about what's going on than by monitoring focus continuously in real time?  Are you serious...really?  If so, I just don't know what more I can say about it.

 

 

So - although making the mirror motion more controlled shouldn't have a downside in terms of performance, realtime focusing with a proxy actually could hurt the fwhm.  It will only have benefit if the system is well focused in the first place and the focus proxy is well calibrated - and that is not guaranteed.  It would be better to call it a precise focusing system rather than accurate - and it is only precise in terms of tracking the proxy - and not in terms of maintaining a fixed focus error.

 

Have you looked at the focus in my images?  I analyze FWHM on every sub and the focus accuracy is limited primarily by local seeing conditions.  I often monitor the seeing quality in real time using the guide camera so I can easily predict how the FWHM will be affected for each and every sub.  That's how I can tell what effect the seeing is having--in real time.

 

Making the system more rigid also likely has benefit - but again you have only addressed one element.  Professional systems don't aim to make things perfectly rigid and instead assume things are flexing all over the place - and install active optics controls to compensate.  So for the RASA, a meaningful comparison would be align the system well when aimed at 20 degrees altitude - and then aim it at 80 degrees and see if any tweaking is needed.  If tweaking is needed even when you have your focusing installed - then you still would want to tweak the alignment anyway.  And if you are willing to spend much more on a focusing system - I would put it on the secondary and allow automatic alignment also - with a fixed mirror.

 

I don't know what you mean by "professional systems" and I haven't seen any data from you on how much flexure you are talking about so there's only one thing that I can say in response.  Telescopes with an aperture less than 24 - 30" should not require any active system(s) to maintain optical alignment as the telescope changes pointing angle.  I believe in good mechanics first and in correcting residual, higher order errors with active controls--if it's needed.  There is something wrong with any 14" telescope that can't maintain optical alignment when it is pointed at different points in the sky.  The new RASA 14 will not require realignment when it changes pointing angle.

 

And if you re-focus with the primary as normal and re-load the mirror - you will remove much of the tilt incurred by the change of altitude - and there may not be much difference from having the mirror fixed in the first place.

 

Right...that's a workable fix but it's a Band-Aide that doesn't eliminate mirror flop--among other things.  If all you have used is a Celestron, you cannot appreciate the advantages of having a focusing system that has zero image shift and zero backlash.  When you pass focus, you don't have to start all over again to get back to the same place on the hysteresis curve.  Focusing is quick, accurate, and stable.  Sure, you can work around the problem, but all that backlash and image motion make a lot of operations on the scope (both manual and automated) a lot more time consuming and difficult. 

 

My main point is - none of these systems will be perfect and have zero slop - and a critical factor is the tolerance a user has to those errors.  Some people are perfectly happy with the current focusing system and have never even collimated their scopes.  Others will collimate on every session.  Some will immediately attach a crayford and wouldn't think of focusing with the mirror.

 

Yes there are a lot of work arounds.  Call me crazy but when I buy a new car, I expect that it will drive straight without first changing the tires and installing a new steering control module.  I don't think that it's asking too much to expect it to work right as soon as I drive it off the lot.

 

 

I think it's great you are exploring these alternative mechanics - but keep in mind many of us are taking a stock OTA and exploring simple approaches to get optimal results from them - without modification - and getting excellent results that can be directly transferred to other users at 0 cost.  Your changes should make it easier for people to get good results with little skill and effort - but skill and technique can help overcome deficiencies in most any design.

 

Patience, care, and perseverance are essential in all aspects of this endeavor no matter how good (or bad) your equipment might be.   It is a complete fallacy that anyone can "get good results with little skill and effort."  You have tossed out this notion in a number of other discussions with me and it is incorrect.  You've figured out some effective ways to get around a bunch of issues and that's great; but, I think that you've just normalized the problems to the extent that you don't think of them as problems anymore.

 

 

Frank


Edited by jhayes_tucson, 23 April 2018 - 08:59 PM.

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#25 WadeH237

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Posted 24 April 2018 - 08:28 AM

People have been doing "realtime focus" with sct's for years - and it always has the same problems:  1)  It is relying on an indirect measurement of something unrelated to stars in the image sensor 2)  It relies on a model that is assumed to be accurate and trustworthy and 3) It isn't actually "focusing" in time but instead somehow tracking focus relative to some initial focus that is assumed to be perfect.

I don't think that you understand how FocusLock works.

 

Assuming that I understand it correctly, it uses astigmatism on actual stars in the guide image to assess focus on every guide exposure.  This works naturally with ONAG, which has some slight astigmatism in the guide image.  For OAG, Optec offers the Lacerta, which induces astigmatism in the guide image just for this purpose.

 

The initial setup requires that you get the guide camera exactly parfocal with the main imaging camera.  Once this is done, the system will maintain focus using actual stars in a closed loop system.  It does not depend on any models or assumptions.




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