Jump to content

  •  

* * * * *

TMB 130 f/9.25 with Custom Tube


Discuss this article in our forums
TMB 130 f/9

TMB 130 f/9.25 in Custom Tube

My apologies for what may be a somewhat lengthy lead-in to clarify what led me to this particular refractor. It will be as brief as possible.

I spent a great many years, perhaps about 35, with a 4.5-inch reflector on a less than stable mount that could just barely get the job done for visual use. Although I loved astronomy and was fascinated with the night sky I didn't have the time to indulge the way I would have liked. Finally, when the mid to late 1990's arrived I was in a position to better indulge in the hobby I love.

I finally fulfilled a lifelong ambition by buying my first refractor, a Meade 127ED. It provided excellent views once I had fought my way through the typical lens centering, focuser squaring, and collimation issues that are often encountered with these particular scopes. At the same time it was also an excellent learning experience, and it had me totally hooked on refractors. In particular, it had me hooked on 5-inch refractors due to the ease in handling and excellent performance. However, I knew there had to be something out there that was a level or two above the Meade in performance, and I wanted to know that I was getting all the performance I could possibly get from 5 inches of aperture. I did what most normal people doÉ. I got on the Astro-Physics waiting list for a 130 f/8.35, and of course for a 155EDF at the same time.

In the meantime I was in decent shape, astronomically speaking, since I had also acquired a 20-inch Obsession and a TMB 105/650 to cover both ends of my observing desires. I might add that the TMB 105/650 amazed me in comparison to the Meade 127ED. I had never been so impressed with a scope's quality, particularly one with such a fast f/ratio. I was actually able to see more low contrast detail in objects with an inch less aperture. This led me to the purchase of a TMB 175 f/8, without the slightest doubt about what a fantastic scope it would be, and I could never say enough about its' excellence. The worries about waiting for an indefinite period of time for a scope were over.

However, as the years crept by I found myself becoming overly anxious for a 5-inch scope with the same level of excellence as the TMB scopes, yet the only thing available in the f/ratio range I wanted was the AP 130 f/8.35, which I was already waiting for.

Finally the day of reckoning arrived.. Astro-Physics announced that the final run of 130 f/8.35 scopes was closed, and no more production of them was planned in the future. At that time I was about 7 weeks back on the waiting list and now knew that the dream would not come true. The really upsetting thing was that there was nothing else available anywhere that could fill the void.

I then attempted to convince Thomas Back of the need for a longer focus 5-inch scope since the AP 130 f/8.35 would no longer be available. After many conversations and a huge amount of e-mail (mostly begging and pleading on my part) it was finally decided that he would design one. I honestly don't know how much influence I had in his decision but I was very happy to hear that it would become a reality. In addition, I was very happily #1 on the list for the new lens.

Due to the fact that I had become very partial to the original silver/white TMB CNC tubed scopes, and the "originals" are no longer available unless special ordered for a much higher price, I decided to build my own optical tube for the new lens which would include some features that simply aren't found in most, or perhaps in any, other optical tubes. Of course the new style TMB CNC tubes ARE available, and come with the 3.5-inch Feathertouch focuser. I still prefer the silver/white original TMB CNC tubes, but this is nothing more than personal preference on my part.

I immediately began the design and construction of the optical tube, and wanted it to be capable of anything and everything I might ever want an optical tube to do.

After working on the design and fabrication of the optical tube for about 7 months I received what I consider to be an obscene phone call from Astro-Physics. They had made some "extras" in the final production run of the 130 f/8.35 and had reached my name on the list. What to do, what to do? As any sane person would do, I bought the scope, which I believe is one of the last 4 ever to be produced. The scope is just as fabulous as I ever could have imagined it would be.

So, who needs two utterly fabulous performing 130mm scopes? I guess nobody, but after so much time and effort had gone into the optical tube for the new TMB lens (that I had begged for) I decided I couldn't quit the project.

More than another two years passed before the new TMB 130 f/9.25 became a reality but that only gave me more time to try to perfect the optical tube.

FINALLY, on July 23 of 2005, TMB 130 f/9.25 lens #2 arrived. On paper, lens #2 was the best of the first batch of lenses. Although having lens #1 would have been a nice touch it was hard to resist going with the interferometer report, even though we all know there is no way I would ever have been able to tell the difference in lenses at this level of quality.

The description of the lens and optical tube is as follows:

Clear Aperture 130mm (5.1 inches)
Focal length 1202mm (47.3 inches)
Focal Ratio f/9.246
Tube 5.5-inch aluminum (with four baffles)
Focuser Type 2.7 inch Astro-Physics with Feathertouch dual-speed Micro Unit
(With precision rotation mechanism ahead of focuser)
Telescope Length 1118mm (44 inches) with dewshield retracted
Dewshield Length 432mm (17 inches)
Dewshield Diameter 197mm (7.75 inches)
Weight 35 Lbs. (tube alone, with no accessories)

The lens, of course, came through the hands of Thomas Back, and I asked him to sign the nameplate since LZOS had failed to add "TMB design" to the nameplate. And, I very much liked the idea of Thomas having his name on it.




Everything was ready on the optical tube, with the exception of finding the exact optimum length that was needed. I was only willing to determine that with an actual physical measurement with the lens in place. So, with the lens finally on the tube, out we went for the first trial run. Amazingly enough, it was a decent night so I didn't need to pray for good weather.

After a reasonable cool-down period I swung the scope toward Vega to finally try a star test that I had been waiting almost three years to do. Keep in mind that I was by that time fairly familiar with some Ôpretty good' optics in my other refractors.

It didn't take but a few minutes for me to realize that up until that time I only thought I knew what an "apochromat" was. Of course, in a sense this is hair splitting, as it usually is with optics at or near this level of perfection, but I don't care how hard you try you will not find even the slightest hint of color either in or out of focus with this lens. It is honestly so color free that the only reason you would know you weren't using a reflector would be due to the lack of a central obstruction messing up the defocused patterns. It honestly has to be seen to be believed.

The tube was then shortened to the desired length and the full complement of baffles were installed and positioned to give an un-vignetted image circle of 45mm at the focal plane. Now the scope was complete and all that was needed was some good weather.

Finally a night came along that even the Clear Sky Clock said would be 5/5 seeing. I honestly don't think I've ever experienced another night that good from my location. Well, perhaps one other night, and luckily I was using the 20-inch Obsession at that time.

Since it was two or three weeks prior to Mars reaching opposition I kept myself entertained early in the evening by looking at what I perceived to be almost perfect star tests. I say "almost" only because I know nothing is supposed to be perfect. Time was also spent splitting doubles, viewing M13 and becoming locked to the eyepiece in each case. Every time I would switch to a different target it was almost like seeing them for the first time. The splits were superb and clean and individual stars in M13 were resolved better than I had ever seen them in a 5-inch scope. I'm not even sure how much credit should go to the optics and how much to the seeing conditions, but the optics certainly didn't stand in the way, which is the way it should be.

Finally, after a few hours, Mars was high enough in the sky for me to take a first look. Atmospheric refraction was a bit of an issue initially, but eventually it was not a factor as Mars rose higher in the sky. By the time it was within an hour of reaching the meridian I had slowly increased the magnification to very acceptable level. I had NEVER been able to observe from my location with that same magnification with any other scope, so I have to attribute the good fortune in large part to the fabulous seeing conditions of that night. The most amazing thing is that the optics were easily able to handle everything I was throwing at them, and I would have increased it more if I had an easy way to do so. However, it's silly to mess with success and I was enjoying the best views I've ever had of Mars under any circumstances.

For the record, I was using a Baader Mark V bino-viewer, 1.7x corrector and a pair of TMB Super Monos at that time. There was no problem at all merging the images and I've never had much trouble in that area with a bino-viewer.

After nearly 5 fabulous hours of observing Mars I saw that Saturn was almost 30 degrees above the horizon (3:30 a.m.) and I needed to have my first look of the season. Once again I was utterly amazed at the view. Even though I was looking through quite a bit of atmosphere the view was still splendid. Particularly since I had never observed Saturn at that magnification before. It was a sight to behold and I continued to observe Saturn until almost 5 a.m.. Mainly because I didn't know if I would ever see conditions like these again. I might add that I haven't seen conditions like that since.

Toward the end of the observing session I began to think that it was just as well that I hadn't tried to push the magnification any further. I seemed to notice that I was probably borderline for running out of light, even though there wasn't a sign of image breakdown. I was actually a bit surprised that I hadn't noticed it earlier. Then it hit me that perhaps dew was arriving on the scene. I quickly checked the lens with a red flashlight and saw that there was no dew on the lens whatsoever. The Kendrick dew system was doing its' job, as always. I decided that it was about time to wrap things up for the evening/morning so I began to put things away.

I soon noticed that the prism diagonal for the Baader bino-viewer had started to slightly fog up internally. I had never encountered that before but it was a welcome explanation for the image not being a bright as I would have preferred. Now I have to have another great night to experiment with before I'll know whether I run out of light before I run out of optical performance. I suspect the light will be the first to go.

There is no point in trying to guess at the magnification level this scope is capable of when conditions permit. Suffice to say that your only limitation will be the seeing, as it should be.

All in all, I simply don't know what else to say to describe this fabulous lens. For my observing needs it is a cut above anything else that is available in this size aperture.

This is not to take anything at all away from the fabulous AP 130 f/8.35. It too is a fantastic scope. Aside from an extremely small hint of color in the out of focus pattern it is every bit the equal of the TMB 130 f/9.25ÉÉ. however, I prefer the TMB for the greater image scale due to the longer focal length, the more forgiving compatibility with simple eyepiece designs, and the fact that it is in a tube that meets my needs 100 percent. In addition, the air-spaced lens should be closer to perfection in color correction if everyone does their job properly. And of course the somewhat slower f/ratio is also an aid in that area.

If someone is in the market for the absolute best performing 130mm planetary lens available then the TMB 130 f/9.25 is for you. Get something else for your wide field views but use the TMB 130 f/9.25 when you want to see all the low contrast planetary and lunar detail you can possibly see with a 130mm scope.

For those of you who may have interest I'll describe what is different about this optical tube.

I had a few simple things that I wanted the scope to be capable of doing. I wanted to build it so I never had to worry about insufficient backfocus when using a bino-viewer and I wanted the focuser to rotate with total precision. In addition, I wanted it to function as a normal scope with regard to focuser drawtube position when being used in a normal (Cyclops) mode.

Last but not least, I wanted a 45mm un-vignetted image circle at the focal plane to insure that film or CCD sizes up to 35mm would work perfectly.

To quickly summarize what I did, the scope was built with 3/8" more backfocus than the AP 130 f/8.35. This allows full use of a bino-viewer with no corrector when using my eyepieces that require the most inward travel. When the thin AP 2" adapter is used there is ? inch of inward travel left.

The rotating section is ahead of the focuser and is quickly and easily removed if more backfocus is required. This allows me to use a SolarMax H-alpha filter with a BF-30 added to the system and have adequate backfocus for solar viewing with a bino-viewer, and with a full sized 2-inch diagonal if I prefer.

The scope uses the Astro-Physics 2.7 inch focuser with the Feathertouch Micro upgrade, and the dew shield is longer than normal to improve the ability to block stray light and reduce the chance of dew forming.

This is a close up of the rotating mechanism ahead of the focuser.




The TMB 130 f/9.25 after completion
.





View of dew shield and baffles.



This was a difficult project but it has been easily worth the wait and the effort. The TMB 130 f/9.25 takes a back seat to nothing of similar aperture.

Harvey Gryttenholm










  • dawsonian2000 likes this


0 Comments



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics