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.