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A&M 152mm f8 APO Refractor


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Review of A&M 152mm f8 APO Refractor

with carbon fiber tube, TMB triplet optic and 3 ½” Feather Touch Focuser



Fully compressed and ready for transport to your favorite observing site.

I first heard of the new A&M telescopes through their web site http://www.astrotechengineering.com/refr.htm. I had been looking for a 6" APO refractor telescope for a few years and had previous experience with friends TMBs, Astro Physics, Televues TECs and TAKs in several apertures ranging from 70mm through a 10”TMB as well as my own TV 85 APO and 4" and 6" home made APO scopes. As an astrophotographer I wanted a refractor rather than a reflector since besides imaging from my own home observatory I do a lot of traveling to star parties and I wanted to eliminate the issue of collimation. I also needed a scope with a larger focuser and the ability to take a flat field corrector to use with my SBIG STL 6303e large chip CCD camera and medium format film.

After researching the availability of various telescopes and finding long waits I decided to contact the company directly through Giovanni Dal Lago. I received a quick reply that told me that they were about to attend the NEAF show in NY this year and that if I wanted one they could send the show demo model to me direct from NY after the show. I decided to go for it and ordered it with a set of CNC rings . I also had them include a case for it.

I also ordered a flat field lens and A&M custom machined the adaptors for the flat field lens so that it screwed directly to the 3 1/2" focuser tube without any step down. Rather than using the 2" nosepiece of the camera they machined the adaptor so it bolted directly to the camera’s stock bolt pattern. This will eliminate any chance of vignetting in the optical path. The flat field lens can cover a 82mm image circle with pinpoint stars to the edge...sufficient to cover a 6x7cm Pentax camera if I ever decide to go back to film. It took a bit longer for them to machine the flat field lens cell and adaptors but I now have it and it is impressive. It is a 1x device...no enlargement or reduction of the neither FOV nor scope speed. If I want a faster/wider setup I made a homemade focal reducer that works OK (f5) but doesn't give the edge performance of the dedicated unit. With the flat field setup the FOV on my ST 6303e is 1.29 x .87 degrees.

A few days after the show the scope arrived in perfect condition. I told Tom Trusock of Cloudynights I would review the scope...but would not do it until I had a lot of time to check the scope out. I know that often when you first get a new piece of equipment you are smitten with the thrill of the new equipment and often don't find the "gremlins" until you have used it a lot. In fact my friends know that one of my favorite sayings is that "All telescopes and mounts are basically a kit...you buy it and than finish it."

The telescope features a TMB Triplet APO in a temperature compensating cell, carbon fiber tube and a 3 1/2" Giant Feather Touch focuser. It also comes with a test report that showed a Strehl ratio of .96 as obtained with a Twyman-Green Interferometer (p.t.v. .219 and RMS .033).

In an earlier review of a A&M 80mm f6 telescope for Cloudynights, Tom Trusock was impressed with the elegance that the Italian "touch" gave that scope and I have to agree that as a visual work of art you would have to go a long way to find a prettier scope. But beauty can be skin deep and so I did a lot of testing to find any "warts".


SETUP IN VISUAL MODE

First some specifics...the tube only weighs 27# stripped and it is about 30# with the rings and Astro Physics dovetail bar. Length fully compressed with the dew cap slid down is 39 1/2". Tube diameter is 6 1/2". The tube has 5 knife-edge baffles with black felt between the baffles to further absorb any stray light. The focuser has 12 machined baffles. The dew shield is about 14" long. I opted for the pricier CNC rings rather than the standard water cut rings. This proved to be a good choice as I found I didn't have to do any shimming of the rings to get perfect orthogonality on my AP 900 GTO mount. I use it with the 15" AP dovetail plate and 15" bar. Balance of the front heavy scope is such that with my AP diagonal and heavy Pentax eyepieces the scope would balance with the scope slid down in the rings about 4" from the tubes travel limit. With the 5# SBIG STL camera and the 3# flat field corrector I slide the tube up in the rings so it is about 4" from the rear limit. In other words...able to balance for visual or astrophotography with no problems.


Feather Touch wireless focuser head shown with CCD setup w/Flat Field Lens adaptor to STL 6303e. Note engraved scale on focuser tube. The label on the focuser is a home-made eyepiece/magnification/FOV scale.


One of the things that convinced me to go with the A&M scope is that it has a Feather Touch focuser. I already have three scopes with smaller versions of the Feather Touch. They have always impressed me with their smoothness and the 2-speed feature is very handy. I use a wireless focuser for the Feather Touch focusers from Starizona that uses radio rather than IR and with Focus Max running under Maxim DL the combo has proven to be a very fine automatic focusing setup. One nice feature of the wireless unit is that you can use the same transmitter/receiver for all your gear. You only need to get an additional motor drive for the larger unit since that motor and housing is larger than that for the rest of the Feather Touch line. The new focuser is easily the nicest large focuser I have used...it is extremely smooth, has user adjustable glides, has an adjustable brake and a very precise oversized rack and pinion. While a Crayford focuser eliminates the rack and pinion... I much prefer the exact positioning a rack and pinion provides for photography. One hugely useful feature is an engraved metric scale on top of the focuser. Once I determine the normal position of the focuser with the camera I can than set it to that point and balance the scope. On other scopes I resort to marking the focuser tube with a fine point marker...but this is a lot more elegant. The focuser has 4 1/2" of travel plus it has a unique lock ring that allows one to rotate the focuser/camera to best frame an image. When not using the camera and flat field lens Feather Touch supplies a thread on 3 1/2" to 2" adaptor. Here attention to detail was impressive. First...if you wish to lock that adaptor in place it has three small Allen screws. Each is tipped with nylon so it won't dig into the mating aluminum part. It also has a unique clamping arrangement. Rather than screws that did into your diagonal...or the much better brass compression rings better diagonals now use...it has a large compression collar that ensures the diagonal is on the optical dead center. It has a large lock ring with machined finger "grabs" plus toggle bars to lock this...very nice when I am wearing gloves in the cold Michigan weather. Included is an additional 2” extender to give another 2” of travel if needed. It has a brass-locking collar.

UNDER THE STARS
The test of a telescope is how it performs for YOU. While I am an astrophotographer...I also love to visual observe. I use the various star testing methods recommended by Suiter in his book "Star Testing Astronomical Telescopes". As indicated by the optics test report I expected it to have excellent performance. Patterns on both sides of sharp focus were identical, showed NO color at all and showed no problems. Running at higher power on the limb of the moon there was no color. Next...same power on Vega... there was not any spurious color in or out of focus. I found the focus to snap sharply into focus. I ran it on the double-double and both pairs cleanly separated at lower powers. I than ran it up to higher power placing the pairs so that they would be equally spaced from the center of the FOV. This is a good test of astigmatism...both star pairs should be cleanly separated at the same focus point and equally sharp. If have found this is a better test of astigmatism than the traditional comparing of the out of focus image on each side of focus to see if there any elliptical shapes out of phase. A good test of a scope is the triple star Iota Cassiopeia. The separation of the closest star to Iota was very sharp and there was no merging of the two stars. This is a testament to great optics as well as the efforts taken to reduce flare within the scope tube and focuser.

Next I tried my Burgess binoviewer. Back when the planets were up I was blown away with the views of Saturn and Jupiter. If you have a refractor you owe it to yourself to get a binoviewer...the color and contrast to my eyes were so much better than with a single eyepiece it was almost a religious experience! However, note that this may be problematic with many refractors. Quite often they will not have enough in-focus to achieve focus without using a relay lens, which is a small Barlow that attaches to the front of your binoviewer. In my case I wasn't able to get it in focus without the relay lens and a 2" diagonal but with my 1 1/4" diagonal I was. That being said...if you have a refractor you may want to research which binoviewers will work with your setup. You could have A&M make the tube a bit shorter but in my case that would have been problematic as I would not be able to achieve enough back-focus travel to focus my CCD camera setup without an extension tube. While Feather Touch does make 3 1/2" extension tubes...I prefer to not use them. In my setup I am at focus with the flat field lens and camera at a point about 15mm from maximum back-focus.

While it is very impressive for the planets and lunar observing...it is less so on dim objects. That brings me to another misconception...many feel that a top-level refractor can match a much bigger aperture reflector in performance on deep sky objects. The bottom line is that you can't beat bigger aperture for deep sky. I had the 6" setup alongside my friends new 6" TAK TOA APO refractor and also had my 11" Celestron SCT alongside. There wasn't any DSO that we looked at that didn't reveal more structure through the SCT. Yes the stars were pin points in the refractors...but the light gathering of nearly twice the aperture is a huge equalizer. This also begs the question why pay that much money for a telescope when you can see more with a big mirror? It all comes down to what you want to do with the scope. If I were purely a visual observer I would have bought a Starmaster or Obsession 18" DOB and been in visual heaven...but for doubles and variable stars, planets and lunar observing it is hard to beat the image of a fine refractor. For me the only reason to get the big refractor is astrophotography...longer duration shots easily makeup for a large aperture. And as I mentioned early on...a refractor doesn't require collimating. Those who collimate RCs know what I am talking about.

DRAWBACKS?

Remember my comments about a telescope being a kit? This proved to be one of the best turnkey scopes I have owned. Only two minor issues surfaced. One is that the material they used to coat the inside of the rings suffered from migration of the glue to the surface of the material and stuck to the tube of the scope. I fixed this by pulling the material off and replacing it with felt strips from the local fabric store. I used a spray on glue to adhere it to the rings.
The other issue was actually due to a feature they use that I favor. The dew cap is sized to fit over the lens cell instead of over the dew cap. Moisture can be absorbed into the inside anti-reflection coating of a dew shield and if the cap was over the dew shield it would trap this moisture in the optics and could cause mold or mildew to form on the lens. By having the cap on the lens cell this is avoided. A&M and my Televue 85 APO both do it the correct way whereas my friends TAK has it over the dew cap. The problem in the A&M is that with the dew cap extended you can't grab the cover to remove it without sliding the scope up in the rings to gain access. My solution was to drill a hole and use a pop rivet in the cap to attach a small piece of nylon strap as a grab to remove it. The picture shows the simple modification I made.


Recessed lens cap with pull-tab modification I made. Two-time Astronaut and Senator John Glenn’s signature doesn’t come with the scope!

Tom mentioned the price of the scope on his review of the 80mm. I would check the current price but when I got mine it was $5500 Euros. You can figure about 1.2x that amount for the exchange rate. The flat field lens was expensive since I had custom machining done...it amounted to about $1100 by the time I was done. I would recommend the CNC ring upgrade...they are that nice. Also...the case they supplied was a nice one that was made by an Italian company called Amabilia. The scope can fit in the case with the rings and an AP dove tail on both the top and bottom of the rings. The scope/rings/case weighs 53# and that makes it very manageable. It has wheels on one end.

The finish on mine was flawless...one of the most striking touches on the scope is the orange/gold on each end of the tube. After seeing the basic white/black tubes of other competing telescopes it is nice to have a bit more attention paid to cosmetics. The A&M scopes are available in a few different accent colors...the website shows blue, red and gray besides the orange/gold on mine.


THE BIG QUESTION

Some have asked if the carbon fiber is a good or bad feature for a refractor. It is much more rigid and stronger than an aluminum tube but that is not a big issue in the small cylinders used in a scope. Some have questioned the performance of the scope for maintaining focus as the temperature changes. Well...those questioning it have never had the actual scope to test...like me (I believe there are only a couple of the A&M refractors in the US and mine may be the only 6"). First, the scope uses a temperature compensating lens cell. As I understand it as the temperature changes the cell compensates for the change in the glass keeping it at the null point. I do know that the earliest TMBs had a problem with pinched optics in cold weather...but the new setup cures that. As an engineer (now retired) I do know about the temperature coefficient of carbon fiber that is much better than aluminum. That being said...all this is theoretical and only hands on use will test these features. I have done several all night imaging sessions here in Michigan. I do recognize that at f8 there is more of a zone of focus than at a much faster optical system. Still I have been very impressed with the lack of temperature-induced change in focusing. I did one test run with the scope operating at f5 through my home made focal reducer and imaging through Astrodon Narrow Band filters from about 11:00pm to sun up. I intentionally didn't touch the focus for any of the filter changes and the image is sharp and there aren't out of focus colored rings around stars. I plan on doing some additional testing using the auto focuser to give me some sort of quantitative test. But based on my comparison of a SCT with aluminum tube and an identical scope with a carbon fiber tube I expect there to be less focus change with the carbon fiber. On the aluminum tubed SCT it would only go 2 degrees max before the focus would change. With the carbon fiber it would go for at least twice that before it changed. In any case it may be academic since most of us normally always focus with each filter change and confirm focus throughout the night.

FOOD FOR THOUGHT

I also use the scope with my Coronado 60mm Halpha filter set and a BF 30. It makes for a fine solar scope although the TV 85 APO with the same filter set is a lot more convenient to setup! That being said I noted that the black surface of the carbon fiber when turned so the sun shines on the side gets very hot. While that is a product of the black color...black aluminum on the focuser etc. is far cooler. While I haven't had a chance to measure the temperature inside vs. outside the scope (I do have the equipment to do it) I would be careful to avoid leaving it setup in the direct sun and with the sun shining directly on the tubes side.

I also noted that while the black surface of the carbon fiber dews up on the scope tube, the dew cap (14" long) does an amazing job of keeping the optics free from dew. I have had my finder scope and piggybacked TV85 completely dewed up yet the A&M optic was still bone dry. It is rare when I need to use my dew heater.

Cool down seems to be reasonable. I will try to do a comparison to my friends TAK 150 TOA in the future but that is hard to quantify. Suffice it to say that it gets cooled down MUCH faster than my SCTs.

FINAL NOTE

One of the most impressive things about the whole scope was the wonderful help I got from Gio at A&M. They are very responsive and proved to be very knowledgeable and honest. I feared that the Italian/American language issue might cause some difficulties but Gio speaks and writes English very well and there wasn't a single problem in all the transactions and agreements.
Tom Trusock mentioned in an early post after seeing the A&M scopes at the NEAF show that they were the Ferrari of telescopes. While they certainly have the Italian beauty of a Ferrari…they also possess something else the Ferrari means to me…fantastic performance. And as if to state the obvious…every A&M telescope comes with a sticker that says “A&M Advanced Telescopes, Italian Style and Performance”. In this case the manufacturers claims are valid.




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