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Skywatcher 10" Flextube for NV use.

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#1 Eddgie

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Posted 21 September 2020 - 10:34 AM

Having had to move away from my 12" due to the difficulty of moving it to my front yard (back yard sky has become obscured over the last decade by neighbor trees) it was time to go to a smaller, lighter, easier scope to move.

 

I had some paralysis.  I had first considered a ES Truss but the 10" version Truss assembly weighs in at 44 lbs.   Now the Truss assembly on my 12" dob was only 50 lbs, so that did not seem like much of a weight saving.

 

I then considered a closed tube dob and was almost ready to buy the Apertura. There was a lot about the Apertura that I really liked on paper, and I thought I had my mind made up, but at the last minute, I reconsidered a telescope that I had thought about at one point but dismissed and that was the Flextube 10".  The one great appeal of the Flextube was that due to the ability to easily change the lenght of the truss poles, it would give me the ability to reach focus with my ASA reducer/Corrector.  Also, the the Skywatcer starts out a little bit faster (f/4.7 vs f/4.9 with the Apertura) so reduced, this gets me down to a little under f/3.45. 

I really love the ASA corrector because it produces a very crisp field.  I have tried simple reducers but they tend to have bothersome (to me) aberrations at the edge of the field, but the ASA just makes are a wonderfully sharp visual experience. 

 

So, I am not going to go to a full review on the Skywatcher, but I will say this.  I dislike it.  LOL..  It does not balance well with the reducer, filter wheel, and Mod 3 even without a finder.  It is quite horribly balanced in my opinion. Worse than that, to keep the scope from nose diving, the little pressure knob had to be cranked up so high that it made altitude movement stiff.  Azimuth Movement is very poor, requiring considerable pressure to get it to move. I already have the Rockler bearing on order for an azimuth cure and I guess I will have to resort to three pounds of counterweight to correct the altitude issue, but on the whole, just as a telescope for telescoping, I give low marks to the base and to the balance issue. 

 

But this post is focused on using it for NV and in particular, to NV using the ASA reducer because that is at the end of the day, the main reason that I made the last minute decision to switch from the Apertura (which I confess that I kind of regret in many ways) to the Skywatcher. 

 

Last year when I first considered the 10" Flextube, I was able to get a lot of figures from another CN member, Will_S.  Will was kind enough to make a lot of measurements on the flange to focal plane distance.  The ASA corrector requires 90mm of back focus. It seems that several of the 10" dobs today have a lot of back focus and I guess this is for camera mounting, but the Flextube seemed to have quite a lot, with Will giving a figure of about 70mm.  (So kind of him to measure this for me..).  Will also had a number of 8mm for a fully illuminated field.

 

This meant that on paper, I would be short by about 20mm. In fact, that is just about the amount I had to shorten the trusses to reach focus with the reducer.  The math then says that the fully illuminated field will be reduced down to 3.75mm, and the reducer will narrow this to 2.73mm at the photocatode due to the reduction.  I have estimated, that at f/4.7 and 25% obstruction for the primary, the illumination falloff is probably only about 20% at the edge of a 20mm circle but since I am starting with a small fully illuminated circle due to the pole shortening, my guess is that on paper, the falloff is probably in the 15% to 20% over the small area of the photocatode.  

Visually, this translated into a field that appeared to be quite evenly illuminated.  The sky was so bright that I could not do any nebula, but on several star fields, with the reducer, the view was very pleasant without any real sign of illumination falloff, suggesting that the falloff curve is mild and gentle.

 

The design of the detents is such that it will be easy to add new detents.  They are simple holes in the steel tubes with spring loaded ball detents.  Once I have ensured that I have set the trusses to the optimal length, I will mark the tubes and drill new holes at the appropriate spots so that I have this new length available on demand.

The reason I sold my 8" f/4 (also able to work at f/2.8) was that I deemed that the tripod was harder to take out than a 10" telescope OTA and when reduced, the scope simply did not have that much image scale (too close to the 6" Boren Simon) and unreduced, it did not have that much more limiting magnitude, and that was what was pushing me to a 10".    I wanted portability, and more aperture, but I still wanted the ability to get the same wide field views that I could get in the 8" f/4.  With the 10" in reduced mode, I have a nice 876mm focal length (less than 10% smaller than the 8") and while there is some illumination falloff, even at the edge of the field, I will get considerably better limiting magnitude than with the 8". 

 

One of the goals of the larger scope is more image scale than I could get with the 6" and 8" scope, and for this, I would of course use the scope at full extension with no reducer (I use the MPCC for coma correction). This would be my primary configuration for galaxy season and globulars, but for most things, I will probably use use the scope in the reduced mode. An added benefit is that in reduced mode with the trusses shortened, balance is much better. 

 

While I hugely dislike the balance and poor motions of this scope, it does shine at being light and easy to move, and the ability to tune the truss system at will to convert back and forth to reduced mode was the ultimate decision point for me.  I just hate that the base and bearing design is so primitive, but I am working on ironing some of those issues out.  

 

I had thought that I might make a new base for it some day, but my track record with following through on big projects is poor.  There is always house maintenance, or yard work or something else so it is hard to get motivated to do a big project. 

 

In some ways, I do regret not getting the Apertura though, but some of the problems I am having are simply a consequence of the fact that the scope can do some things that the Apertura can't and the ability to run my ASA reducer was a powerful incentive to just bear with the issues, improve them as I can, and enjoy the easy of deployment of this scope, which is far easier to get out the door than the 8" on the big Alt-az mount.    A lose, win, win, so the wins had it.  I have not owned the Apertura, but my frustrations with the Flextube base are enough that unless the bulk was a big issue, I would say that the Apertura might be a better scope for Non NV people, but if this was a review specifically of the telescope and not in the context of NV use, I would have posted it on the Reflector forum.  

 

I hope to mod the trusses this week as weather permits me to get out to make an accurate measurement of the correct length. 


Edited by Eddgie, 21 September 2020 - 04:01 PM.


#2 gatorengineer

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Posted 21 September 2020 - 08:22 PM

wondering could you increase your secondary size a bit to get a better fully illuminated field?



#3 Eddgie

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Posted 21 September 2020 - 10:15 PM

wondering could you increase your secondary size a bit to get a better fully illuminated field?

I have actually considered this but I probably won't.  At f/4.7 the illumination falloff over such a small circle probably is not really worth the cost.  Visually I could not see the falloff so I am not inclined to worry about it yet, but as I get more experience, I will re-evaluate that decision. 

 

I will try to remember to measure it next time I use the scope.  If it is substantially more than I estimated, I may rethink that.  I would only need to go maybe 70mm and that would only increase the obstruction to about 28% so still not terrible.   This is my only planetary scope though, so while I do not do much planetary, I do enough that I would like to keep the 25% obstruction if possible.  



#4 Eddgie

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Posted 21 September 2020 - 10:34 PM

I also learned today that Astro Devices is working on a new encoder kit for this scope model.  

 

One of the reasons I was leaning toward the Apertura was because the encoder kit was designed so that the scope could simply be lifted out of the base. On the 10" Flextube, the encoder kit required a bolt that went into the side of the mount in place of the handle.  This meant that to remove the telescope from the cradle, you would have to remove the encoder bracket and remove the bolt and this just seemed like kind of a pain to do every time the scope needed to be moved. 

 

I had been investigating ways to circumvent this and I had an idea for it so I contacted Asto Devices and showed them my idea, but as it turns out, they had already been in the process of producing a kit that used a setup very similar to the design I was going to do on my own, so lucky me, they will have their new design for the encoders out in then next couple of weeks.  Serge was nice enough to show me the instructions for it, so very excited about that.

 

With the new encoder kit, there is no retaining bolt on the Alt axis and the scope just sets directly on to the glides as it did before, 

 

I also ordered a 4 lb magnet today, so that should take care of the balance issues.  I will pretty it up a little (handle) and put some felt on the scope side to keep it from direct paint contact and so it can be slid.  Only $15.  Orion wanted $50 (plus $10 for shipping) for a 3 lb magnet and I thought this was worth the small extra effort to save $45.  I am pretty sure I have some stick on felt around.   

 

 

 

https://www.amazon.c...0?ie=UTF8&psc=1

 

I had already ordered the Rockler Lazy Susan so this is sorting out OK. 

 

It was my intention to maybe go to custom made mount, but now that an encoder kit will be available I doubt that I will do that.

 

Your question was a good one though, and it is something I have considered, but initial impression is that the falloff is within my tolerance threshold (and I have what I think is a pretty low tolerance for stuff like this) but I have to use the scope a bit more before I make a final decision. 

 

So, I with the new encoder kit and these other actions I think I can make myself happier with it.  We'll see. 



#5 gatorengineer

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Posted 22 September 2020 - 08:13 AM

how are you connecting your asa reducer to eyepieces?  or do you just use it at prime focus on the scope?



#6 Eddgie

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Posted 22 September 2020 - 10:53 AM

how are you connecting your asa reducer to eyepieces?  or do you just use it at prime focus on the scope?

I really only do prime focus so the reducer is attached to the bottom of the filter wheel with a T2 extension, then the filter wheel, then a T2 gender changer (the output thread of the filter wheel is female, and the gender changer ring makes it male), then the T2 female to C mount male, and finally the device. 

 

XQE_00991.JPG

 

For the MPCC, I just unthread the reducer and extension from the filter wheel and then just thread on the MPCC. 

 

Just for the experience, I tried using a couple of eyepieces behind the reducer and it was pretty ugly. Most eyepieces are not going to digest an f/2.8 light cone without showing some astigmatism.  Maybe something like an Ethos would, but since the longest focal length Ethos is 21mm, then what would be the point?  I could get lower power and faster speed out of prime focus. 

 

Anyway, on a rare occasion I will do afocal, (usually just holding the device up to the eyepiece) but 98% of the time I use afocal.  This was why the Flextube was interesting to me.  It would allow me to do the reducer or the MPCC and still have a scope for planetary because this is now my only planetary telescope. 

 

I am actually down to just three telescopes now, the 6" f/2.8 at 420mm focal length, the 10" at either f/3.43 and 876mm, or f/4.7 and 1200mm, and a Lunt 80. 

 

The 8" was just too close to the 6" when reduced and not enough added limiting magnitude, plus it was not the best for planets.  Also, an 8" f/4 imaging Newtonian takes a robust Alt-az mount, and that mount was the bigger issue.  It was pretty heavy and hard to move. The 10" Flextube is pretty easy to move by comparison.  I mean I could have folded up the legs on the Tri-pier to make it easier to get outside, but it was tedious to do that.  As much as I like the Tri-pier for stability, if you had to fold it up, it was kind of time consuming, and with the mount head on, it was pretty heavy.   I mean it was rock solid though, but not worth it for 8" of aperture. 


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#7 gatorengineer

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Posted 22 September 2020 - 04:46 PM

Is that an ASA or a baader MPCC, I presume its the ASA....  thinking about getting one for my 24" but as I am afocal not prime not sure how to set up the ep.



#8 Eddgie

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Posted 22 September 2020 - 07:47 PM

I have the ASA .73 and the Baader MPCC but in the picture it is the ASA.  The Baader just screws on to the filter wheel in place of the ASA and extension.

 

As far as using the ASA, most reflectors will not reach focus with it.  It requires almost 90mm back focus.  The corrector needs to be separated from the focal plane by about 66mm (depending on the exact model of reducer/corrector.)

 

Some of the correctors have M48 at the top, so you would just need to get the correct spacers to go between the rear flange and the eyepiece.  Since most eyepieces have a 30mm or so barrel to field stop, then you would have to come up with another 35mm for the MPCC or 46mm for the ASA (these figures are approximate). There is no reason why some of that space could not be occupied with a filter wheel so in lieu of spacers, you might be able to get a filter wheel in there.

 

But the ASA takes a lot of back focus. That was why the Flextube was interesting to me.  The ability to shorten the truss ensured that I could use the ASA with it. 

 

On the plus side, the MPCC actually gives you about 10mm of outward travel (you would rack the focuser out 10mm when using the MPCC vs where it would be when you were not using the MPCC). 



#9 Eddgie

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Posted 25 September 2020 - 08:46 AM

Had the 10" out last night.   The first thing is that the 4 lb. counterweight was the best $15 I have ever spent.  The balance was just about perfect. I did not even need to use the carry handles.  Even with the scope just sitting on the bearing supports, the scope was in perfect balance in all positions from horizon to zenith, even with binoviewers!  Not only was the altitude motion better, but even the azimuth motion was better and I am guessing that this was because the pressure on the three nylon glides was more evenly distributed.  It is still a bit heavy in azimuth, but improved over running the scope with a lot of imbalance. 

 

The reducer works super-well.   While only a small circle at the center of the field is 100% illuminated, there is no sign of excessive illumination falloff.  It is quite even across the field, so my guess is that the falloff is simply very gradual and steady so that it does not intrude.   Along the Milky Way in Cygnus, the star fields were very rich. 

 

The Barlow built into the filter wheel worked well at the center of the field, but at f/3.44, the outer portion of the field shows a lot of astigmatism.  It is still quite useful for a quick bump in magnification.  The difference on M11 was enoght to make it worth having the barlow, but mostly this would be used for lower power work and the Barlow will work better when I am using the MPCC at full focal lenght.

 

I am getting ready to drill in my new detents on the truss poles for my reducer position. That way it will be a bit easier to go back and forth. I set the extension last night and did not move the scope when I brought it in, so I am pretty much ready to drill the new holes.

 

There are mixed reports on the Rockler bearing but if that does not work, I will probably try to find a good laminate and new PTFE bearings or maybe metal plates.

 

So, the scope is shaping up.  The counterweight was an absolute home run.  I am well past my original buyer's remorse because the scope is meeting my expectations for NV use and is very easy to move out for viewing, which were the two primary goals for it.  The Counterweight was the key.  Transformed the scope. 



#10 Eddgie

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Posted 25 September 2020 - 12:01 PM

I just finished drilling the new detents for the reducer position.  Offset was just shy of 25mm, so this means that only a very small circle at the center of the field is 100% illuminated, but the as reported earlier, the falloff is very difficult to see.  Most scope designed for visual use will have much smaller fully illuminted fields when used with a reducer but again, this scope has to do double duty for ocassional planetary work, and mostly I think it will be used at f/4.7, but it works quite well at f/3.44.  Even under a half moon and less than good transparency, the Crescent nebula looked great last night.

 

I measured my holes super carefully and there is zero collimation shift at the new detent or the full extension and the Binoviewer extension, so all good.   

 

The altitude movement could be left alone because the 4 lb counterweight solved that problem pretty much completely and the detents are set for reducer use, so the only thing left it to improve the azimuth motion and add DSCs, though I have been using the green laser finder and it works fine for the well known stuff like Ring Nebula or Wild Duck.  I can nail those easily with the laser and the 1.15 degree field when running the reducer, but I know I will want the Nexus II for galaxies.

 

The Rockler bearing has shipped as have some laminate samples for the altitude bearings (I still want to try to improve those but at this point the counterweight is working so well that it seems to be more of an academic desire to see if the laminate helps with the altitude, because I probably won't bother with the handles now that I have seen how effective the counterweight is.)  

 

A work in progress, but the progress to this point leaves me with a telescope I am very satisfied with even if nothing else is done. 


Edited by Eddgie, 25 September 2020 - 12:04 PM.


#11 Eddgie

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Posted 26 September 2020 - 11:16 AM

Update on the Rockler Lazy Susan bearing.   There were mixed experiences with these with some reporting improvement and others saying it did not work all that great.  Put me in the not all that great column.  $15, so worth trying.  Apparently it can be improved by sandwiching between metal sheets, but I would be worried about very thin material being flat enough. 

 

Telescope Express sells the whole setup with the bearing and two metal sheets, which I think are the same as used on the Apertura and similar telescopes for 51 Euro but I don't want to spend that much at this time.

 

For now, I have ordered a 6" x 6" x 1/16th sheet of PTFE.  It was only about $9.  I will try new glides and be on the lookout for some textured laminate. Sometimes the old school ways are still the best ways.


Edited by Eddgie, 26 September 2020 - 11:17 AM.


#12 Eddgie

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Posted 27 September 2020 - 04:58 PM

Update on the altitude bearing. Out of the box, the round nylon bearings do not extend out far enough from the sides of the rocker box to span the bearing race on the plastic altitude bearings and these bearings have some stiction and now that the scope is in almost perfect balance, they were still kind of stiff.  

 

Out of the box, there is about a 5mm space between the box sides and the bearing, and as you use the handles to tension the clutch, the tops of the box pinch in and this will move the round nylon supports further under the bearing but the engagement of the friction ring is always at the top as the walls bend in and the nylon glides do not ever fully extend under the bearing.

 

 

 

I have not done the laminate spacers yet and these will fill in the gap between the bearings and the rocker box sides so that the sides to not bow in so badly, and with that in mind, I decided I should re-space the nylon supports.   To do this, I 3D printed four 5mm wide by 20mm diameter spacers to move the nylon supports out so that they fully support the bearing. While I was at it, I decided to put some bar soap on the altitude bearing races.

 

XQE_01091.JPG

The difference in feel and function was dramatic.  Movement is now exactly what I would expect from a good dob.  It takes only fingertip pressure to start the scope moving.  The scope glides and costs to a stop rather than just kind of grind to a stop.  Now I am guessing most of this is due to the lubrication from the soap but even with just moving the support glides out a bit further I thought the motion was improved, but with the soap the motion is now premium dob smooth. I was shocked at the difference it made.

 

If anything it is now too smooth and based on that, if there is even three or four ounces of imbalance the scope will start to move on its own (well gravity..You know what I mean) and this will actually make the laminate spacers necessary so that I can get a smooth parallel engagement of the friction ring.  This is where large diameter bearings really pay off. The can hold an attitude with a little imbalance, but they move with light pressure.   I may try 3D printing some larger bearings but for now, with the scope in balance, the motion is simply perfect. 

 

I took out the Lazy Susan bearing. Went back to the Nylon, but I have a sheet of Teflon on the way and will replace the nylon azimuth glides with it as soon as it arrives. 


Edited by Eddgie, 27 September 2020 - 04:59 PM.



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