36 replies to this topic

[Jeff B]

I had at one point a "vintage" TMB/APM LZOS 175 F8, objective serial number 041.  When thermally stable it gave simply great views.  Despite it being a "LW" OTA it still weighed in at 41 pounds for the bare OTA (no rings, nothing) and was very nose heavy.  Fully configured for viewing as you see in the indoor picture, it was a rather porky 47 pounds.  This prompted me to re-tube the objective with the custom OTA I tinkered together that you see in the outdoor picture.  It was ~38 pounds as shown.  A considerable improvement and an easy, balanced lift for me at the time into the rings.   The objective had its thermal challenges, taking considerable time to thermally settle out in the cooler months, and, sometimes, never really "getting there" thermally.   I then acquired an older, pre-ED AP 178 F9.  And man, was the bare OTA lighter coming in at ~30 pounds bare naked, 36 pounds fully configured for observing with viewers.  And it is much better behaved thermally.  So, the TMB 175 departed.  That was in my pre-DPAC days too.  

 

But I always regretted letting it go.

 

 

 

 

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[Astrojensen]

I had an original TMB CNC 100/800 on loan, a number of years ago, and it had amazing optics, but took forever to cool down, despite being only a 4" scope. 

 

I dream of one day owning something like a 6"-8" scope with that kind of optical quality, but it would have to be mounted in an observatory. 

 

 

Clear skies!

Thomas, Denmark

[Jeff B]

Fast forward and I had the opportunity to acquire another, similar vintage (S/N 067) from a gent in a trade deal.  This sample OTA is a one or two-of-a-kind OTA that was/were specifically made by Markus to be "light weight".  And it is indeed LW, coming in at ~39 pounds as  shown on the Mach 1.  It also has a removable section at the tailpiece that makes the scope directly bino-friendly but is easily installed for mono-vision use...I leave it off.  The OTA is in excellent condition considering its age, with only some "normal", small paint chips and that's about it.  The glass is very clean.

 

OK, blah, blah, blah, so how does it test out?!  

 

More to follow on that.

 

Jeff

 

 

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[Jeff B]

Ok, some testing results.  

 

As usual,  with refractors and cats, I tested primarily with a white light LED, which, is really made up of three tiny individual red, green and blue LEDs whose output is blended together to form the white light.  My specific white LED has a slightly "warm" tone to it which helps me greatly as my camera's chip is very blue sensitive and prone to "blooming" if the LED is too bright and/or too "cool blue-ish" in tint.  I take "master" DPAC images in white light inside, at and outside of focus.  Using some software, I then isolate the individual R,G, B color channels for images in those colors.  The "yellow" images are a blend of the R and G channels. This technique I learned from a couple of very smart folks who hang out frequently on the ATM forum.  Many thanks to you all if you are looking in.    

 

One advantage of this method is that the R, G, and B channels are all at the same physical distance from from the objective, which gives me some direct insight as to where the individual colors might focus relative to each other (I specifically choose the green/yellow as my reference color), and, importantly, insight as to spherochromatic effects in the red and blue.   The at focus images are typically taken with a green LED to maximize contrast and as I find it's easier to find focus with just green light. 

 

I also typically display the individual channels for each side of focus as color montages, along with the white "master" image.

 

Right!.  More, blah, blah, blah, so how about the images. 

 

Funny you should ask....

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[SchrödingersCat]

Hi Jeff,

 

What's happening at the edge of the lens? Does it affect the image seen through the scope?

[ngc7319_20]

Beautiful scope!  Did this have the usual interferometer report from LZOS?  Does the zoney stuff in the green "knife edge" image show up in the Strehl ratio?

 

How are star images?  Any on-axis coma?

Edited by ngc7319_20, 07 January 2023 - 01:41 PM.

[EricCCD]

Beautiful, Jeff!

[Deadlake]

Why did you acquire another one when you let the last one go for the AP?

[Jeff B]

Why did you acquire another one when you let the last one go for the AP?

I wanted it.

[scoale]

I wanted it.

[Deadlake]

I wanted it.

That is one reason I bought my LZOS, but in your view the AP is still better?

[bobhen]

   I then acquired an older, pre-ED AP 178 F9.  And man, was the bare OTA lighter coming in at ~30 pounds bare naked, 36 pounds fully configured for observing with viewers.  And it is much better behaved thermally.  So, the TMB 175 departed.  That was in my pre-DPAC days too.  

 

 

Yes. Those early Astro-Physics triplets were light. I bought a new 1989 152 f9 from AP and although longish, it was definitely lightweight. No problem carrying or mounting that baby. And oil-spaced lenses just acclimate quicker in these large sizes. Excellent optics and at the time my AP 152 cost about $6,500 less than a Tak FS 152.

 

Both of those refractors are superb but that AP 178 will be hard to beat.

 

Bob

Edited by bobhen, 08 January 2023 - 11:54 AM.

[CHASLX200]

Kinda rich for my blood but if setup was not so hard i would try it out and see how it compares to my freaky sharp 826. I have had the worst seeing the last year and if i can't use a 2.5mm eyepiece in my Newts i hang it up fast.  Last nite everything was mush bombs in the 826 and even a 7mm was mush. 7mm is where i get started and on down to a 2.5mm.  

[Jeff B]

Ok, it's time for some "analysis", which is where I typically get into trouble with some very smart folks here. 

 

First, overall color correction looks great, right up there with the best, with little fringing around the lines in the white light images, which to me indicates scant longitudinal color error.  Attached is an inside of focus image montage, along with a similar image taken recently through my AP Stowaway.  Just superb.  You will see the normal APO spherochromatic effects though as the lines bow in opposite directions in the blue and the red with a bit more sphero in the LZOS 175 but, hey, I'm comparing a modern 92mm design with an "older" 175mmaperture design.  Excellent.

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Edited by Jeff B, 08 January 2023 - 11:01 AM.

[Jeff B]

I spent A LOT of time testing this lens and talking to some very experienced friends concerning this objective because there was one feature in DPAC that stands out in the images.  It's that "wagging tail" of the bottom of the right hand DPAC ronchi line, which you can easily see in the white light images.  

 

My initial impression was that I saw a mild ((( and then ))) character to the inside and outside of focus images respectively.  I immediately became concerned about astigmatism and coma, despite the focuser and objective being in excellent "collimation" with the laser and Cheshire eyepiece.  In DPAC, astigmatism can show itself as a rotation or clocking of the lines as I sweep from one side of focus to the other.  I did not see any of that.  So my concern was more related to coma, which, with this design, might mean decentering of the lens elements.  Which is not good.  At all. 

 

The more I looked at the images, did more testing and talking to a friend, I started coming to the conclusion that the ((( and ))) of the lines was really a bit of an illusion driven by a local area of undercorrection out towards the edge (that wagging tail).  This idea was supported when I simply blacked out that area as you see in the attached images. Then the figure looks really pretty good, slightly over-corrected.   I found I could also "walk around" that area in DPAC a bit by simply rotating the ronchi screen as you see in the green images.  After talking a bit more with my friend, he said that he's seen this type of thing before where the optician may have been trying to correct a local error and, perhaps, overshot a bit, with the wave front is no longer a figure of revolution, but still an excellent Strehl number (in this case, .980, which is the highest number of  any LZOS large lens I've had). 

 

You can also see a suggestion of this in the LZOS contour plot as well as the at focus green image (that narrow dark band in the lower left edge is real and, I believe, accounts for that little bit of funny business at the lower LH ronchi line.  I could walk around that a bit too).  

 

I started felling better about the objective, but my experience has been that DPAC and a star test together will tell me all I really need to know.

 

Jeff  

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[Jeff B]

Star Testing

 

I chose to do daytime artificial star testing as I've found that if I carefully choose my day, I can get truly excellent results.  I typically choose overcast days (to minimize solar ground heating) with mild/no wind (a very gentle breeze actually helps) and very stable temperatures.  I use a 100 micron star placed ~210' away which gives an ~.33 arc second size.  I also use 2x and 3x barlows on the 1.25" nosepiece of the star to further reduce it in size (but also brightness). 

 

I did two rounds of testing.  For the first round, I took the OTA from my 68 degree F basement to the 32 degree F outdoors (which stayed there for four hours). I wanted to also judge its thermal performance.  I did not use any barlows on the artificial star....forgot to really.  Well, the objective, like the last one I had, was a thermal mess for a couple of hours showing heavy undercorrection (sharp rings inside of focus, mush outside of focus) and "text book" tube currents.  But, at 350x, no astigmatism and the jury was out on any coma.  I saw what looked like coma when moving very slightly inside of focus, but none really at focus.  I have been fooled and tricked by tube currents before as they can, tome, mimic astigmatism away from focus and coma at focus.   Things started settling out at the three hour mark, when I saw no signs of astigmatism.  There was, maybe,  scant coma at focus at powers up to 400x as a local slight thinning & dimming of the first at focus diffraction ring on one side of the image and a mild thickening & brightening of on the opposite side.  The airy disk though seemed completely round, sharp and color free.  The image looked really good though and really pretty calm with excellent "seeing".  I was really pleased.

 

However, I became concerned that, even though at ~.33", and ~ 1/2 the resolution of the aperture (~.66"), it was not a true point source and may have affected what I was seeing.  So a few days later, I repeated the testing, with even better ambient conditions (steady 60 degree F), and using 2x & 3x barlows on the star.   Boy (!) the warmer ambient temperature really made a huge difference in how quickly the objective settled out.  I could quickly use powers of 350X and later up to 600X with what I would call Pickering 9+ "seeing".  Other than the obvious brightness difference, I did not see much difference between using the star at .33" or with the 3x barlow, except, with the barlow, the airy disk looked slightly smaller and there was no second diffraction ring visible at focus, both of which are probably related to the reduced brightness of the star. And man, did it look good with no hint of astigmatism and, again, maybe, a bit of "transient" coma at 400x and 600x, which was just not visible at 350X. 

 

I also used my cell phone to capture some crude shots of the star test at 350X, at focus.  They are single frame, heavily cropped with some noise, (the live visual view was sooo much better, beautiful actually) but they tell the story.  Image Fa was taken without the barlow, the other two with the 3X barlow as you see in the pictures.

 

Jeff

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Edited by Jeff B, 08 January 2023 - 01:32 PM.

[Scott99]

Yes. Those early Astro-Physics triplets were light. I bought a new 1989 152 f9 from AP and although longish, it was definitely lightweight. No problem carrying or mounting that baby. And oil-spaced lenses just acclimate quicker in these large sizes. Excellent optics and at the time my AP 152 cost about $6,500 less than a Tak FS 152.

 

Both of those refractors are superb but that AP 178 will be hard to beat.

 

Bob

I lusted after the blue-tube 178 for years, even asking Jeff if he planned to sell - the lightest 7-inch apo out there - but I finally retreated/retrenched to the 21-pound f/9 Star 155ED and couldn't be happer.   Those early 90's days of 20-pound f/8 and f/9 6-inch apos were the glory days for me, I think the f/8 FS152's were 22 pounds as well.

 

Thanks for all the test reports on these Jeff, I always wonder what it's like to use these big apos in real world conditions and it's so hard to find objective user reports like this, let alone DPAC tests
 

Edited by Scott99, 08 January 2023 - 01:58 PM.

[Jeff B]

So was the star test perfect?

 

No.

 

At 400X and above when I would move very slightly inside of focus, a little "hot spot" would develop between the airy disk and the first ring. As the airy disk  expanded and the first out of focus ring really started forming, the spot would start to "melt" into that first ring, disappearing entirely at one wave of defocus.  At one wave, inside, the ring was somewhat oval, becoming circular by the time the second ring showed up.  This pattern repeated moving slightly outside of focus, except there was no "hot spot" and that small oval at one wave out did not flip 90 degrees as you would expect with a small trace of astigmatism.  Me thinks it might be thermally related or from that small area of under correction.   My buddie, predicted I might see a hot spot close in to focus, and I did.....above 400x.

 

Moving inside/outside to 3-4 waves of defocus showed basically identical patterns except for tint differences.

 

This is a very good objective......when thermally  stable.

 

Jeff

[Jon Isaacs]

 

Jeff:

 

Another nice piece of work. It's not really down my alley but I enjoy reading about your discussions of what you did and what you saw as well as seeing the test images..  Very objective.. and your conclusions, they come at the end like they are supposed to in any test report.  It reminds me of the many test reports I did working in a research lab.  

 

No "saber rattling", just an honest test I feel I can trust.. 

 

The real question:  Is this telescope good enough to give away or is it better just to bury it? 

 

Jon

[Jeff B]

Thank you Jon for the kind words.

 

And that's a very real question.

 

But I do know it goes nowhere until I've looked through it at real stuff.

 

Jeff

[CHASLX200]

Jupiter tells me all i need to know and it's moons on a dead still nite at around 100x per inch.

[C. Evangelista]

There is a tiny little zone, corresponding to the central topography of the objectiv that is displayed in the test report from LZOS (upper right).  

[Jeff B]

There is a tiny little zone, corresponding to the central topography of the objectiv that is displayed in the test report from LZOS (upper right).  

Yes, and there are other features that are common to my at-focus shot and the old LZOS contour plot.

 

For example, in my image, note that little dark spot at the top of my image as well as the thin darker line spanning the edge from about 6:00 to 8:00.  They are real and show up on the contour plot.  Similarly, there is a mild, smooth ridge that spans between those two, which you also see in the contour plot.  Then there is that smooth plane to the right of center that spans the RH side.

 

This is three out of three times so far where my DPAC results, especially the in-focus images, are substantially correlated to what the LZOS contour plots show me.

 

Jeff

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[Darren Drake]

That artificial star device is the same one I use to test scopes indoors using another telescope to produce the parallel beam. If you have a scope known to be excellent optically and insert it into the scope you can aim the scope being tested into it and test but no thermal issues or large distances are involved. I've tested many scopes this way. Here's the null image of a C14 I recently tested and it's null result. It's clearly suffering from some issues...

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Edited by Darren Drake, 10 January 2023 - 07:23 PM.

[C. Evangelista]

A test report of LZOS 175 #071 from the retired german astro-optics guru Wolfgang Rohr:

 

http://r2.astro-fore...75-1400portugal

Edited by C. Evangelista, 10 January 2023 - 04:12 PM.