35 replies to this topic

[starman876]

I have seen these pop up for sale over the years and always wanted to try one. Seems with that fast focal length exposure times are very short.

[Jim Riffle]

Hi All,

 

Mike's original Simak (Simmons+Maksutov) design was for the prototype 11 1/2" f/5.6 with a smaller field which suffered from too short of a back focus to barely use visually, even with a 1" thick helicoid focuser.  With a 2" star diagonal, I used to joke you had to be a primate to get your forehead situated.  Don't get me wrong, I fell in love with that Simak as I built and used it.  It did require figuring on the convex side of the corrector to null the system.  Without that required aspherizing the all spherical images were as mushy as has been mentioned.

 

The Astromak is a different design with a 1.5X thicker corrector, larger secondary, a little faster @ f/5 and a couple inches more back-focus.  The field flattener lens was mounted in the 4x5 camera back along with a custom minus violet filter with a nominal 440 nm cutoff.  The original Simak flattener had a flat surface facing the film which ghosted badly esp. since I never got it AR multicoated back then.  The Astromak version has both sides curved and AR coated to eliminate multiple reflections off the film during very long exposures.  90 minutes was the killer exposure for dark sky with gas hypered 2415.  That was before autoguiders happened.  A couple of 90 min exposures on the same night was a very Zen like period.  Then waiting with anticipation till the next day to develop the 4x5 negs.  Focus, guiding, seeing, clouds, the odd klink from the telescope, jamming the film holder slide/shutter, needing to nod off while trying to get lucky was some challenge.  With a 80mm FOV, object centering wasn't an issue even for all the North American/Pelican Nebula.  You can see how M31 just floats in the center of the image circle on a negative..

 

The Amak optics were all made by J.R. Cumberland in Maryland, same optician that Questar had used.  The glass blanks were custom molded/generated at Corning for me.  Fine annealed BK7 corrector and Pyrex mirrors.  I can't recall the glass type for the 4x5 camera field flattener.  I did the pattern making and aluminum castings in house along with machining and the rest.  So my only subcontractors were opticians and gear makers.  I had the printed circuit boards designed and stuffed locally with my purchased components.  A 555 timer was the most exotic part on the RA boards.  40-80 Hz was your speed range (60+ 20).

 

Yep, I was heckled about spot size back then, but nobody else was producing those caliber images with small amateur stuff.  When you examine the near grainless Kodak 2415 negs, the lower brightness small stars take on a Gaussian like intensity to where it is had to find a sharp edge to call the edge.  That was before many of us understood what FWHM was or how to quantify it.  This was during the analog film days with push-to telescopes. 

 

Who cares if your stars are or aren't smaller than there's?  Only the PC TWEAKERS  with the nicknames.

 

So fast forward to imaging on 6.4u pixels and yes, almost all the 12" f/5s an 24 1/2" f/6.2 SCTs wouldn't cut it now for small format CCDS .  Their size is not even necessary anymore. Enter the compressed/condensed/efficient/ full color APO-CCD possibilities delivering the numerous breath-taking amateur astro images I've seen.

 

Then there is the 14 bit CMOS one shots that can deliver fully baked color images with 30 seconds on a Hyperstar 14.  Attached is one from an old cheap Fujicolor X-A1 unmodded on my H-14.

 

I have to count my blessings when I think about the "old wet darkroom times" I endured after designing, casting, machining and using the darn scopes to get that definitive "ASTRONOMICAL image" on the wall.  32x40 inch to be exact.  Just an 8x enlargement on 4 sheets of 16x20. Lot of paper.

 

Jim Riffle

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[Chuck Hards]

Jim, it's a work of art, like everything you ever did.  Thanks for the background info!

[Jim Riffle]

I'm baaack,

 

I have attempted to measure actual AstroMak 4x5 negative star sizes this afternoon.  The most definitive means was using a 10, 20, 30 micron line width reticule with a modest microscope and hand held camera.  My judgement looks like the 20 micron line will blot out some of the faintest/smallest stars and the 10 micron line shows some fuzz on each side of that line.  

 

Understand that this star size measurement is after the sum of 70 minutes hand guiding by me, seeing, focus, perhaps a breeze, etc included.  Focus was indirect using a ground glass and loupe before the film holder was inserted.  The film was sucked flat with the custom vacuum film holder so curl didn't add in.  The Astromak focuser was fitted with a .001" dial indicator to allow average rack and pinion position to assist in best focus.

 

But for me the bottom line was that many customers liked their scopes considering to total integrated package or as Mike Simmons always said, "all things considered".  So I go all-in on 20u.  

 

I still build complete desktop Maksutov Cassegrains for fun.  These are visual miniatures complete with tiny eyepieces. They are not CCD friendly and they are precision analog push to technology ala the 60s.  I also have some smaller 1 1/2" Newtonians with a 1/2" minor axis diagonal (f/5s)  

 

Haven't gotten to a 2" SCT yet.  But that would put me in the smallest SCT category along with the current largest at 24 1/2".  Not a very big range when you think about it, but mass goes by the cube of the scale difference.

 

12x12 x12 (1728) in this case.  Actually that is about right considering these 2" Maks in that type calculation.  BUT their cost is more closer to linear and not 37 bucks.

 

Jim

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[Jim Riffle]

Star size, telescope size, grin size, wallet size.  Attached is a portion of a 4x5 negative. Probably one of the sharpest I did for a long exposure test on a customers 12" f/5 Astromak  telescope before it went out the door.  The AstroMak optical design has a Mak corrector that is over 1.5X thicker than the one on prototype 11 1/2" Simak that I finished and owned.  It also has a longer back focus to allow for a proper focuser and 2" star diagonal for wide field visual.  Also has a larger diameter oversize primary mirror and a bigger secondary ratio.  So while they are related, the AstroMak is to be considered a further refined/tweaked Simak.  The attached image line is set over a tighter star in the FOV.   "Armchair theory" can really blow dust all over nice optics when some folks chose to recite such.

jr

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[Mopman]

I know this is ancient history ...  But I am looking for a AstroWorks 12" F5 Mak OPT.  I placed a "Wanted" Ad in the Classifieds. Thanks !!!  Gary

[Tom Stock]

I still build complete desktop Maksutov Cassegrains for fun.  These are visual miniatures complete with tiny eyepieces. They are not CCD friendly and they are precision analog push to technology ala the 60s.  I also have some smaller 1 1/2" Newtonians with a 1/2" minor axis diagonal (f/5s)  

 

Those are so very cool. Can you actually look thru that tiny eyepiece?

 

It never occurred to me to build a small telescope simply for the purpose of making it small. What a fun idea.

[luxo II]

Can you actually look thru that tiny eyepiece?

Yes you can, I have one of Jims MikroMaks, it is f/12. The eyepieces are Huygens in 3/8" barrels. The lenses aren't fully polished on mine, but they do work surprisingly well.

It's for the man-cave, to dream of on a rainy night.

Edited by luxo II, 05 April 2025 - 03:29 AM.

[R Botero]

Those are so very cool. Can you actually look thru that tiny eyepiece?

 

It never occurred to me to build a small telescope simply for the purpose of making it small. What a fun idea.

I made my own OTA for Jim's mount a couple of years ago:  https://www.cloudyni...ope/?p=12317435

 

Jim sold me the mount but had no OTAs to sell.  He gave me instructions of what mirrors to buy and where (here in the UK).  I tried my best to emulate his work.   I reduced the barrels and made the eyepiece adapters also.  The optics are surprisingly good and I've looked at the planets with it!

 

 

 

Roberto

Edited by R Botero, 05 April 2025 - 04:36 AM.

[charles genovese]

Jim- I have 3 or 4 of the 60mm f/20ish Maks used for a guide scope on a line of Russian 6" fork mounted Maks Tom Dobbins was selling in the 90's. PM me if you are interested

 

Charles

[Fingolfin]

Awesome stuff guys.. I learned a LOT! Very interesting. What I love about Cloudy Nights!