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Boller and Chivens 16" Cassegrain

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#51 macdonjh

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Posted 16 August 2023 - 03:59 PM

Understandably, observatory class telescopes elicit high expectations of image quality, often they are disappointing.

Many decades ago, during a star party, I looked at M13 through the 74" Cassegrain reflector at the David Dunlap Observatory in the Toronto area.

I didn't think much of the image. Outside the view through a 12" f/5 Newt at 75x was a lot more inspiring. Looking back on the experience, we were looking through the center of the globular through the big scope because of the ludicrously high power. The globular look like an open cluster...

 

"Everything is big in this telescope!" said my wife as we poked around the sky with the B&C 16". At f/18 and 7,300mm focal length the lowest power I can get is about 200x with my 35mm TeleVue Panoptic.

That's very high power for most people, and it will reveal all the telescope's deficiencies and amplify any mediocre seeing effects. One can hide a lot of sins at 50x. Originally this instrument was mounted atop a five story science building so the seeing conditions were rarely good enough to show off it's optical quality. Fortunately, now it lives in a well ventilated mountain top observatory with often ideal seeing conditions. As I mentioned before, we often use Ludicrous Power on appropriate targets with good results.

 

The optics are of the classical Cassegrain type: F/3 parabolic primary and a 4" dia secondary to amplify the focal ratio 6x.

 

attachicon.gif B&C16_optics_layout.JPG

 

In the drawing above the optical quality is specked to be 80% of the light within 1/2 arc second.

That's a bit strange because to the best of my knowledge the Airy disk produced by a 16" instrument is about 0.7 arc seconds across. The B&C is not limited by the laws of physics! Let's assume the speck was a cut and paste error from the 24" drawing.

 

Here is a drawing with details of the Pyrex (I assume) mirror dimensions and radii:

 

attachicon.gif 16_B&C_prim_sec_drwg.pdf

 

Naturally one of the first things I did was to assemble the OTA and test the optics. They weren't too bad, a little under-corrected and a little rough.

 

The clipped aperture was a bit weird, the baffle tube must be out of alignment. More on that later.

 

attachicon.gif OTA test 1.jpg

 

attachicon.gif Berea_16.jpg

I had to chuckle at this.  The modern classical Cassegrain I use is f/15, 5250mm focal length.  The lowest practical magnification for me is ~130x with my Pentax 40XL (I also have a 50mm eye piece, but the eye relief is so long it's difficult to keep my eye properly placed).  The sweet spot for my scope, in my opinion, is 200x so I spend a lot of time with a 26mm eye piece.  Here, seeing limits me to 300x most every night, so my eye piece choices are 40mm, 26mm and 17-18mm.  Once or twice a year I get to bring out my 12-13mm eye pieces with this scope.

 

There's a thread here on Cloudy Nights about a member who rescued a 24" classical Cassegrain.  I was initially jealous until I did the math.  That scope's focal length (which I don't remember) would have made it a one-eye-piece scope I could only get sharp images with a few times a year with the seeing around here.  Guess I'll have to go back to being happy with what I have.


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#52 MGAR

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Posted 16 August 2023 - 05:13 PM

There's one about 220 miles from me.

 

Clear Skies. 

Let's go rescue it.
 


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#53 Peter Ceravolo

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Posted 17 August 2023 - 12:28 PM

The optical tube assembly is comprised of four components:

The mirror cell/back plate, center section for bolting to mount, main length of tube and the secondary end section.

 

IMG_3930.JPG

 

 

The components are extremely well built welded aluminum and precisely machined. They don't build 'em like this anymore!

 

But...

 

Most modern high end telescopes have generously sized tubes and well ventilated cells to speed up thermal equilibrium. As I was working on the telescope I noted that the tube is really tight to the light path, the end of the tapered tube is only 16.5" dia.! The 3.1" thick Pyrex primary is encased in metal, although the cell does have some ventilation, I'm looking at this will this OTA ever cool down?

 

primary installed.jpg

 

IMG_3601.JPG

 

 


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#54 jgraham

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Posted 17 August 2023 - 12:53 PM

I wonder if the idea was more thermal/structural stability than rapid acclimation.



#55 Peter Ceravolo

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Posted 17 August 2023 - 01:03 PM

While working on the scope I started drawing up a list of things I wanted to change to better "optimize" the design.

 

here's the OTA layout with the airflow for what little ventilation there is marked.

 

OTA assembly small.JPG

 

The back of the mirror cell serves as the back-plate for mounting instruments and focusers etc. The many vent holes are visible at the edge.

 

OTA backplate.jpg

 

The problem is when mounting the OTA counterweights (the system was designed with an old time heavy payload in mind) the vents are mostly covered up

 

OTA counterweights.jpg

 

I had my doubts about the system every reaching thermal equilibrium in a reasonable time.

 

But, thinking about it further, if I start messing with the system it'll be a long time before we got to use it. After a few deep breaths I decided to give the opto-mechanical engineers at Boller and Chivens the benefit of the doubt and try using the instrument as designed. It turns out I was not disappointed.

 

Of course the telescope is an observatory instrument - It is not transportable. Consequently it won't see the huge temperature swings portable instruments are subject to. Our 3m Observa-Dome on a 10 foot square observatory is very well ventilated. I typically open the double door and dome slit early in the day to let air circulate, being careful to shield the scope from any direct sunlight.

 

I'm pleasantly surprised that I've had no show stopping problem with tube thermals. One evening, a few hours prior to sunset, I was able to view Eta Crb, a 0.57 arc second double and see blue sky between the Airy disks at 1200x. The seeing was amazing, and OTA thermals did not interfere.

 

One problem we have in winter is frosting of components, including non warmed optics. I will have to ultimately install a secondary heater, at least, and would also ad fans to the OTA vents while I'm at it. The big primary should not be a problem, I think...


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#56 Peter Ceravolo

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Posted 17 August 2023 - 03:08 PM

I had to chuckle at this.  The modern classical Cassegrain I use is f/15, 5250mm focal length.  The lowest practical magnification for me is ~130x with my Pentax 40XL (I also have a 50mm eye piece, but the eye relief is so long it's difficult to keep my eye properly placed).  The sweet spot for my scope, in my opinion, is 200x so I spend a lot of time with a 26mm eye piece.  Here, seeing limits me to 300x most every night, so my eye piece choices are 40mm, 26mm and 17-18mm.  Once or twice a year I get to bring out my 12-13mm eye pieces with this scope.

 

There's a thread here on Cloudy Nights about a member who rescued a 24" classical Cassegrain.  I was initially jealous until I did the math.  That scope's focal length (which I don't remember) would have made it a one-eye-piece scope I could only get sharp images with a few times a year with the seeing around here.  Guess I'll have to go back to being happy with what I have.

 

I think observers need to have a healthy sense of denial, otherwise we wouldn't have any fun. The 24" is the one that John Briggs acquired I think. The challenge with such an instrument is the shear size of the beast. One would have to have a forklift to manage the parts and rent a crane to install. Then there's the observatory....

I was not expecting to be able to use the 16" at Ludicrous Power, but this is the most optimal time of the year for great seeing. It's true, most large instruments cannot reach their potential unless they are located on a mountain top or a low lying area of good seeing. I would gladly trade ultra dark skies for excellent seeing.


Edited by Peter Ceravolo, 17 August 2023 - 03:32 PM.

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#57 jgraham

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Posted 17 August 2023 - 03:18 PM

My 16.5” has a mirror that is 2.5” thick, which at the time I made it was considered thin. Weighing 45 pounds it suffered from thermal problems until I quit fighting it. Rather than store the telescope indoors and try to acclimate it using fans, I stored it outdoors in a vertical garden shed under a patio awning, keeping it out of any direct sunlight. This moderated the temperature swings and made the temperature of the mirror relatively stable. After that the performance greatly improved.

Food for thought.

 

Love following this scope!


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#58 macdonjh

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Posted 17 August 2023 - 04:01 PM

I think observers need to have a healthy sense of denial, otherwise we wouldn't have any fun. The 24" is the one that John Briggs acquired I think. The challenge with such an instrument is the shear size of the beast. One would have to have a forklift to manage the parts and rent a crane to install. Then there's the observatory....

I was not expecting to be able to use the 16" at Ludicrous Power, but this is the most optimal time of the year for great seeing. It's true, most large instruments cannot reach their potential unless they are located on a mountain top or a low lying area of good seeing. I would gladly trade ultra dark skies for excellent seeing.

Mark Dearborn, actually.  I am pretty sure he's since sold it.  He had plans to take it to an observatory in southern GA.



#59 Peter Ceravolo

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Posted 18 August 2023 - 12:53 AM

The telescope has that old school heavy industrial equipment feel to it, it's nothing like what is available to even high end amateur astronomers today. Make no mistake, the telescope will hurt you if you do not handle the heavy components correctly. I've had a few close calls.... Consequently it's a big deal to do any maintenance on it. No dovetails for a quick mount/dismount of the OTA, which is probably a good thing as it's too heavy to handle in one piece. I'm not a huge fan of dovetails anyway...

 

There is provision for fine adjusting the OTA balance without fear of the the thing coming apart or sliding down a dovetail unexpectedly. The center section is clamped in place by two metal bars and big allen head bolts, it can slide up and down the Dec saddle plate by rotating a captive lead screw with a hex nut driver. One cannot mount the OTA on the Dec saddle plate as a fully assembled item, it bolts on in pieces starting with the center section.

 

The Dec saddle plate has a large aluminum block with a 1/2" threaded hole affixed to it. The center section is clamped to the Dec saddle plate using the two bars and then the lead screw is screwed in place. The lead screw is a 1/2-13 rod held captive by a large brass nut that threads into the center section. The drive part of the lead screw protrudes many inches past the center section so as to be accessible when the mirror cell is bolted in place.

 

Primary assembly drwg.JPG

 

IMG_7469.JPG

 

IMG_3411.JPG

 

IMG_7322.JPG

 

IMG_7483.JPG

 


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#60 CHASLX200

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Posted 18 August 2023 - 05:53 AM

I think observers need to have a healthy sense of denial, otherwise we wouldn't have any fun. The 24" is the one that John Briggs acquired I think. The challenge with such an instrument is the shear size of the beast. One would have to have a forklift to manage the parts and rent a crane to install. Then there's the observatory....

I was not expecting to be able to use the 16" at Ludicrous Power, but this is the most optimal time of the year for great seeing. It's true, most large instruments cannot reach their potential unless they are located on a mountain top or a low lying area of good seeing. I would gladly trade ultra dark skies for excellent seeing.

Way too big for me. Even the 16" is too big. But with my super warm nites and seeing it may give a good view and get close to my Zambuto Newts.



#61 tim53

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Posted 18 August 2023 - 07:05 AM

The optical tube assembly is comprised of four components:

The mirror cell/back plate, center section for bolting to mount, main length of tube and the secondary end section.

 

attachicon.gif IMG_3930.JPG

 

 

The components are extremely well built welded aluminum and precisely machined. They don't build 'em like this anymore!

 

But...

 

Most modern high end telescopes have generously sized tubes and well ventilated cells to speed up thermal equilibrium. As I was working on the telescope I noted that the tube is really tight to the light path, the end of the tapered tube is only 16.5" dia.! The 3.1" thick Pyrex primary is encased in metal, although the cell does have some ventilation, I'm looking at this will this OTA ever cool down?

 

attachicon.gif primary installed.jpg

 

attachicon.gif IMG_3601.JPG

That's when you air condition the observatory to the nighttime forecasted temperature.


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#62 jragsdale

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Posted 18 August 2023 - 11:51 AM

That's when you air condition the observatory to the nighttime forecasted temperature.

I've wondered what temp operating observatories shoot for? The temp as night begins, or cooler so that the mirror slowly warms up as the temps cool and they meet in the middle somewhere?



#63 starman876

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Posted 18 August 2023 - 12:54 PM

I am sure when you are done with the scope It will be an excellent performer.  It is great to have all that focal length.  How would you say a scope like this can perform compared to what is available to most amateur astronomers.  



#64 CHASLX200

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Posted 18 August 2023 - 06:18 PM

I've wondered what temp operating observatories shoot for? The temp as night begins, or cooler so that the mirror slowly warms up as the temps cool and they meet in the middle somewhere?

My mirrors always gotta warm up.



#65 Peter Ceravolo

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Posted 18 August 2023 - 09:29 PM

I am sure when you are done with the scope It will be an excellent performer.  It is great to have all that focal length.  How would you say a scope like this can perform compared to what is available to most amateur astronomers.  

The 16" B&C not really going to offer much more visually or photographically than what can be seen/imaged in a really good 16" reflector on any reasonable go-to equatorial mount. It's just a telescope.

 

But... that huge mount is great! It's always desirable to have more mount than one needs. That extra margin means you're not going to operate at the limit of the mechanics, the B&C offers HUGE margin.

 

Even at Ludicrous Power, that is anything higher than 800x, (my whimsical definition) I can literally hang on to the telescope to brace myself in awkward positions as I'm looking through the eyepiece.

 

The down side is the need to move carefully in the observatory, I've whacked my head against the scope a few times - it doesn't move...

 

Peter


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#66 jgraham

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Posted 18 August 2023 - 09:34 PM

Heh, heh, I like a scope that I can steady myself on in the dark. :)

 

Wonderful!


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#67 deSitter

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Posted 18 August 2023 - 09:37 PM

Heh, heh, I like a scope that I can steady myself on in the dark. smile.gif

 

Wonderful!

I have done this successfully with my 60mm Monolux 4380 from 1962 :) Yes that has a steady mount! I did have to aim carefully!

 

-drl



#68 deSitter

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Posted 18 August 2023 - 09:44 PM

The 16" B&C not really going to offer much more visually or photographically than what can be seen/imaged in a really good 16" reflector on any reasonable go-to equatorial mount. It's just a telescope.

 

But... that huge mount is great! It's always desirable to have more mount than one needs. That extra margin means you're not going to operate at the limit of the mechanics, the B&C offers HUGE margin.

 

Even at Ludicrous Power, that is anything higher than 800x, (my whimsical definition) I can literally hang on to the telescope to brace myself in awkward positions as I'm looking through the eyepiece.

 

The down side is the need to move carefully in the observatory, I've whacked my head against the scope a few times - it doesn't move...

 

Peter

That's great!! In the 1960 film "Universe" (see link) the astronomer Donald MacCrae steps inside the big reflector at the David Dunlap Observatory and walks up a truss to remove the secondary cover :) That's a solid telescope!

 

https://youtu.be/48gIN4hGOdI

 

-drl


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#69 macdonjh

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Posted 19 August 2023 - 07:41 AM

That's great!! In the 1960 film "Universe" (see link) the astronomer Donald MacCrae steps inside the big reflector at the David Dunlap Observatory and walks up a truss to remove the secondary cover smile.gif That's a solid telescope!

 

https://youtu.be/48gIN4hGOdI

 

-drl

Why haven't I seen that scope at any star parties?


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#70 PaulEK

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Posted 20 August 2023 - 12:03 AM

That's great!! In the 1960 film "Universe" (see link) the astronomer Donald MacCrae steps inside the big reflector at the David Dunlap Observatory and walks up a truss to remove the secondary cover smile.gif That's a solid telescope!

 

https://youtu.be/48gIN4hGOdI

 

-drl

What a great film! I had not seen nor heard of it before. Pretty fascinating to see the current view of the universe from the year I was born. Some is completely outdated, but much is still spot on. And the special effects were more impressive than many I've seen from decades later. Also, in the first few comments: the director went on to invent IMAX, and the narrator is the voice of HAL from 2001. I know it's off-topic, but so cool. Thanks!



#71 Peter Ceravolo

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Posted 25 August 2023 - 03:39 AM

When the Boller and Chivens 16" Classical Cassegrain was designed and built, in the early 1960's, it was intended to be a workhorse observatory instrument capable of carrying hundreds of pounds of instrumentation without stressing the telescope.

 

This was accomplished with complex aluminum castings, weldments and lots of precision machining of very large components.

 

To get a sense of the enormity of the work go to: https://bollerandchi...m/?page_id=1174

 

The optical tube assembly is a wonderful example of precision work on thin skined, long tube components that can be difficult to handle without the right equipment.

 

IMG_6938.JPG

 

The OTA's critical component is the center section, (my not completely accurate label) that mates the optical tube assembly to the mount.

 

Lip.jpg

 

It's a double walled, gusseted piece of precision welded and machined art the is incredibly stiff. The other OTA components bolt onto the center section via a precision machine lip to ensure centration.

The OTA components have alignment marks to ensure reproducible assembly.

 

IMG_4125.JPG

 

Centration is a big deal in Cassegrain telescopes. Tip and tilt of the primary and secondary are easy to achieve, but each mirror is aspheric and has its own discreet optical axis, these axes must line up in order for the combination to form ideal images. What separates a fabulous Cassegrain telescope from a pain in the backside is the OTA's finesse of construction. In my decades of experience designing, building and using telescopes I have found that an instrument with too many collimation adjustments will likely be a failure, a source of great frustration.

 

There is nothing worse than having to fiddle with complex collimation in the dark and cold. In the next post I'll detail how easy it is to collimate the B&C 16" in the shop.

 

 


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#72 deSitter

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Posted 25 August 2023 - 06:32 AM

When the Boller and Chivens 16" Classical Cassegrain was designed and built, in the early 1960's, it was intended to be a workhorse observatory instrument capable of carrying hundreds of pounds of instrumentation without stressing the telescope.

 

This was accomplished with complex aluminum castings, weldments and lots of precision machining of very large components.

 

To get a sense of the enormity of the work go to: https://bollerandchi...m/?page_id=1174

 

The optical tube assembly is a wonderful example of precision work on thin skined, long tube components that can be difficult to handle without the right equipment.

 

attachicon.gif IMG_6938.JPG

 

The OTA's critical component is the center section, (my not completely accurate label) that mates the optical tube assembly to the mount.

 

attachicon.gif Lip.jpg

 

It's a double walled, gusseted piece of precision welded and machined art the is incredibly stiff. The other OTA components bolt onto the center section via a precision machine lip to ensure centration.

The OTA components have alignment marks to ensure reproducible assembly.

 

attachicon.gif IMG_4125.JPG

 

Centration is a big deal in Cassegrain telescopes. Tip and tilt of the primary and secondary are easy to achieve, but each mirror is aspheric and has its own discreet optical axis, these axes must line up in order for the combination to form ideal images. What separates a fabulous Cassegrain telescope from a pain in the backside is the OTA's finesse of construction. In my decades of experience designing, building and using telescopes I have found that an instrument with too many collimation adjustments will likely be a failure, a source of great frustration.

 

There is nothing worse than having to fiddle with complex collimation in the dark and cold. In the next post I'll detail how easy it is to collimate the B&C 16" in the shop.

This is a great point and is also true of our humble Newtonians. My 10" has a massive spider cast into the end ring that yes, makes prominent spikes, but which holds a secondary which hasn't been touched for years. I was determined to have a scope I would not have to adjust every trip out. When I see Bob's Knobs protruding out of various places on a telescope I wince.

 

-drl



#73 Peter Ceravolo

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Posted 26 August 2023 - 03:24 PM

I'm lazy... I truly hate having to do a lot of finicky collimation etc in the field and will go to great lengths in telescope design and construction to ensure optimal performance with minimal fussing.

It's clear to me that the engineers at Boller and Chivens must have felt the same way. They went to great lengths to make collimation easy, but there is a procedure to be followed and I'm wondering if any of this has been documented in a user manual. Despite extensive searching online I've not found anything of substance. What follows is the collimation procedure I've been able to figure out by pondering and playing.

 

To recap, here is a snippet of the primary assembly drawing illustrating how it all does together. As I mentioned before, I'm not crazy about how they mounted the big, thick primary on a 6" annulus of thin material, but so far I've not found fault with it. The optics stand up to Ludicrous Power in excellent seeing. Take that you 18 point flotation mirror cells!

 

There is one key aspect to the arrangement that may not be obvious, the centering boss (a raised lip circled in red on drwg) on the primary mounting plate that defines the primary's optical axis centration relative to the cell/OTA. That raised lip slides into the big, double walled, finely machined weldment of a mirror cell decisively locating the primary. That plate is then adjusted in tip and tilt via four sets of push pull screws in a 90 degree, orthogonal arrangement to align the primary's (already centered) optical axis. The hole in the primary cell is, of course, precision machined. You will see that all this precision machining is what makes the telescope a joy to work with...

 

mirror centering boss.JPG

 

mirror support plate.jpg

 

IMG_3605.JPG

 

The primary's cell allows centering the mirror via four push pads that bear against the side of the hole in the mirror, this is a critical mechanical refinement that makes adjustments easy. I've heard of other instruments that simply use set screws pushing directly against the primary hole, the B&C method is way more refined but obviously costlier to fabricate.

 

Large, fast primary mirror's are almost always figured face up with sub-diameter tools. The mirror rests on a rotating turntable, it's standard practice to ensure the mirror's edge is centered while rotating so the parabolic figure is also centered on the primary. I'm assuming this was done.

 

With the primary resting face down (coated surface protected of course) and the plate is positioned. I use a digital vernier to measure the plate's centration relative to the primary's edge. One uses the push pads to translate the plate until it's centered realtive to the primary's edge.

 

primary centering-coll.jpg

 

 

 


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#74 Peter Ceravolo

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Posted 26 August 2023 - 03:35 PM

Next I carefully rest the mirror cell over the primary, lining up the holes and inserting the push pull screws.

Then the whole assembly is tilted up with gloved hands bearing against the mirror surface so the primary does not slide off the mounting plate. This is a heavy assembly, one needs to be careful.

 

With the mounted mirror face up the retainer is bolted in place.

 

primary installed.jpg

 

The heavy center section is then placed over the primary cell and bolted in place from below with a T-handled allen key. It's best to slide the cell partly over the edge of the bench to insert the bolts.

 

center section attatch.jpg

 

The baffle tube is then installed as detailed in an earlier post.

 

IMG_6911.JPG

 

 


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#75 Peter Ceravolo

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Posted 26 August 2023 - 06:13 PM

The OTA's main section is kinda long, and hard to get at the bolts without really getting into it...

 

IMG_4140.JPG

 

I stared at the partially assembled OTA for a long time thinking of how I could collimated the primary in the shop and avoid the fuss in the observatory.

How do you adjust the tilt so it's square with the tube? Thinking of how aligned the baffle tube by placing my eye at the primary's center of curvature, it occurred me that I could do the same sort of thing with the primary, lining up it's axis with the tube's axis. How do you easily define the OTA's axis?

 

This is where I, again, assumed that the obvious precision machining was more than just for show. To define the tubes axis I stretched string across the front of the main tube section and at the back plate. The precision machined holes made it easy.

 

IMG_4142.JPG

 

IMG_4154.JPG

 

I then positioned my self at a distance in front of the tube so that the two cross hairs lined up. The reflection of the front crosshair in the mirror should be on top of each other, of course they weren't.

 

IMG_9519.JPG

 

 


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