35 replies to this topic

[RandyD]

Purchased this wonderful scope about 2 years ago.   I had found a list with all of the owners who had received one of these (13?) scopes.   Contacted one owner who had this scope still boxed in his garage, never having seen light yet.   We settled on a price, but then owner wavered and decided to maybe hold on to it a little longer.    I called this gentleman once a year for over 15 years, until one day I received a text from him....he was ready to sell.    By then I didn't realize it, but had moved within four hours of his home.   So, I finally finished out the deal, drove the four hours and picked up a like new 1980's astromak.

The scope is a dream.   Has incredible visual performance as well as imaging.    This mount has a large disc drive that works very well.  I was able to adapt the built in drive corrector to accept any ST-4 guiding source, which really helped making it a modern viable imaging platform.

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[petert913]

That's a beauty !  I assume it is a Cassegrain?  So f/5 doesn't sound right

 

Maybe f/15?  Please advise.  You'll have a lot of fun with that one

[ngc7319_20]

That's a beauty !  I assume it is a Cassegrain?  So f/5 doesn't sound right

 

Maybe f/15?  Please advise.  You'll have a lot of fun with that one

Yup these are F/5...  Big secondary mirror!   Primarily designed for wide-field imaging.  3.5 degree, 3.5 inch image circle....

 

https://www.astrobin...astromak-12-f5/

Edited by ngc7319_20, 13 April 2019 - 10:29 PM.

[petert913]

Wow !  That is very cool and unique.

[gnabgib]

RandyD
Welcome to the small group of Astromak owners. I myself own the 11.5 inch/ f5.6 prototype Astromak and agree with you about the incredible views! I am currently rebuilding a ccd camera based on the Kodak kaf16801e sensor to take advantage of the wide, flat field of view. Enjoy the scope for they are truly a fantastic expression of the opticians art.

Kevin

[deepwoods1]

More pics please......

[Bomber Bob]

Wow!  Just Wow!  Congratulations!  Your patience paid off -- Big Time.  It sure doesn't look 30 years old to me.  Even more amazing & interesting to me is how you've integrated modern tech into a vintage mount.

[RandyD]

RandyD
Welcome to the small group of Astromak owners. I myself own the 11.5 inch/ f5.6 prototype Astromak and agree with you about the incredible views! I am currently rebuilding a ccd camera based on the Kodak kaf16801e sensor to take advantage of the wide, flat field of view. Enjoy the scope for they are truly a fantastic expression of the opticians art.

Kevin

Kevin.....I sold you the 11.5/F5.6!

Randy

[vahe]

Yup these are F/5...  Big secondary mirror!   Primarily designed for wide-field imaging.  3.5 degree, 3.5 inch image circle....

 

If I am not mistaken these have very large, 50% co., primarily for wide field imaging.

.

Vahe

[ngc7319_20]

How about a pic looking in the front?  Want to see that big CO....

[Terra Nova]

How neat! Talk about a super-rare scope!!

[bremms]

THAT is a cool scope..  An imager's dream really.

[bratislav]

Those were designed for large format film, where 30-50 micron spots were 'as good as it gets'. It may not be so stellar paired with modern small pixel CCDs whose sensitivity extends well below 450nm/beyond 656nm. It also had a flattenner lens very close to focus (fraction of inch from memory), making it really hard to use with CCDs and their filter wheels. It may not be required for relatively small area covered by CCDs (even largest ones are small compared by 6x7cm film).

Nevertheless it would be interesting to see some digital images!

Edited by bratislav, 16 April 2019 - 04:09 AM.

[gnabgib]

Granted film had many issues (such as halation, reciprocity failure, and problems in devleopement, printing etc) but there were several

accomplished astrophotographers who mastered the films available.    The Astromak telescopes were designed to cover a 3 degree field with stars in the 20 micron range.  Thats a field about 3.5 inch diameter.  The large format camera used 4x5 plate holders with a flatenner lens about 3/4 inch in front of the film.  All other formats (35mm, 6x7 ) did not need the flattener and actually had images closer to the 10 micron size provided seeing and guiding were excellent.  These scopes are also interesting in that there are no collimation adjustments provided.  Jim Riffle had the opinion that if the optics could be out of alignment they were!  The Astromak tube assemblies and the optical elements were machined/ ground so that once assembled everything aligned correctly.  No small feat.  I do not believe ANY manufacturer to date has done this.  Jim Riffle used to blow up his "negatives" to 2x3 , 3x5 feet and display them at the telescope conferences.  They were magnificent.  I remember attending one such conference where Jim was displaying his images in the exhibits area while inside the lecture hall two well known astrophotographers talked for an hour explaining how the Astromak telescopes could not possibly perform.  Jim never said a word.   A true gentleman.  The attached image is a digital camera shot of a tri-x film negative illuminated by a fluorescent panel. Not the best circumstances but since I do not own a film scanner it is the best I could do to show the field circle on 4x5 film.  Remember this is a 60 inch focal length telescope and this required 40 minutes to expose.

[RandyD]

Here is shot of front corrector.  (sorry for dust, time to clean it!).    Inside the corrector you can see the large secondary.

Randy

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[RandyD]

Here is image of last lunar eclipse with the astromak and Nikon D5300

Randy

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[RandyD]

Here is shot of M45 with astromak and D5300

Randy

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[bratislav]

The Astromak telescopes were designed to cover a 3 degree field with stars in the 20 micron range.  Thats a field about 3.5 inch diameter.  The large format camera used 4x5 plate holders with a flatenner lens about 3/4 inch in front of the film.  All other formats (35mm, 6x7 ) did not need the flattener and actually had images closer to the 10 micron size provided seeing and guiding were excellent.  

 

I'd love to share the enthusiasm, but reality is somewhat different from advertizing brochures. I do have extensive experience with film (used it for many years), and I have NEVER measured 10 micron star images on film (yes, I used a calibrated microscope). Closest I've seen is a negative of hypered TechPan out of a 4" f/2 Schmidt camera; smallest stars on that measured around 15 micron. Even professional scopes (I've seen plates from AAO Schmidt) have images in 20-25 microns range on very best negatives, but typically in 30-50 microns range. 

Simak (I do have its design in Zemax, from Mike Simmons) is not capable even theoretically producing on axis spots (450-800nm) that are 10 micron RMS. Without flattener, it is good (but not great) over about 25 mm (1 inch) circle, showing approx 20 micron RMS images in corners. Over that, curvature of the field takes over and 20mm off axis (40mm circle, approx corners of 35mm film) it is already well over 50 micron RMS. Perhaps "good enough" in film era, but far from what modern CCDs can resolve.

[bratislav]

Main problem in Simak (and all fast Maksutovs) is high order residual spherical aberration (fixable by aspherizing either R1 or primary - both VERY tough jobs, trust me), and unavoidable spherochromatism, which cannot be fixed. 

Here I've "figured" R1 in Zemax to a perfect null in green (again, much easier than with pitch), yet residual spherochromatism is easily half a wave; on-axis spot also attached (note 50 micron scale).

 

Edited by bratislav, 17 April 2019 - 05:45 AM.

[gnabgib]

I'd love to share the enthusiasm, but reality is somewhat different from advertizing brochures. I do have extensive experience with film (used it for many years), and I have NEVER measured 10 micron star images on film (yes, I used a calibrated microscope). Closest I've seen is a negative of hypered TechPan out of a 4" f/2 Schmidt camera; smallest stars on that measured around 15 micron. Even professional scopes (I've seen plates from AAO Schmidt) have images in 20-25 microns range on very best negatives, but typically in 30-50 microns range. 

Simak (I do have its design in Zemax, from Mike Simmons) is not capable even theoretically producing on axis spots (450-800nm) that are 10 micron RMS. Without flattener, it is good (but not great) over about 25 mm (1 inch) circle, showing approx 20 micron RMS images in corners. Over that, curvature of the field takes over and 20mm off axis (40mm circle, approx corners of 35mm film) it is already well over 50 micron RMS. Perhaps "good enough" in film era, but far from what modern CCDs can resolve.

So basically you are telling me that the best resolution I can expect is 3 to 7 arc seconds. (20 to 50 microns)   I guess the arc second double stars I spit visually are theoretical also. 

[bratislav]

No. This really is optics 101. Many excellent achromats split close double stars, yet noone claims they work well as astrographs (even with film!). Visual performance depends mainly on null in green (which can be excellent, as shown). It is once we go outside c-F wavelength range (as seen by CCDs) that problems show up.

[bratislav]

It is very easy to prove me (and Zemax) wrong. Show us CCD (or even better, film) images that support your claim (10 micron star images).

[ngc7319_20]

It is very easy to prove me (and Zemax) wrong. Show us CCD (or even better, film) images that support your claim (10 micron star images).

Can you show the optical layout?  Are there correction lenses in the back?

 

I think if you drop the 8000A spots and re-focus for green light, it will look much better -- maybe 10 microns?

[bratislav]

It took exactly three words into Google to find it : "optical design simak". How hard was that?

 

 

 

No lenses, just corrector and two mirrors (flattenner only does that - flattens the residual curvature (only affects off axis images).

 

But here's the prescription as well, so feel free to drop any wavelengths and refocus as much as you like. Just a reminder that CCD won't drop anything; 450-800nm is even fairly conservative.

 

[ngc7319_20]

 

But here's the prescription as well, so feel free to drop any wavelengths and refocus as much as you like. Just a reminder that CCD won't drop anything; 450-800nm is even fairly conservative.

 

The typical IR/UV blocking filters that color imagers use passes roughly 400nm to 700nm.    That and maybe a violet blocking filter drops the spot size down to about 25 microns.  Probably will still look a bit fuzzy with typical 4 micron pixels.