302 replies to this topic

[jragsdale]

Interesting! Is this the Pro version? Does cone error etc. get compensated?

 

-drl

Yes, since you're only moving the RA axis, not Dec, cone error shouldn't be an issue. But they do have a separate program called ConeSharp that you can use to easily remove Cone Error as well: https://www.sharpcap.co.uk/conesharp

[Peter Ceravolo]

This sounds similar to the method that SharpCap uses which is very handy, fast and accurate to under 1 arc-minute. A guide scope is attached to the scope, you set the scope pointing polar north, and it takes a quick photo and plate solves it, then you rotate the RA axis about 90 degrees and it takes another photo and compares the 2 to where they should be. Then it picks a bright star in your live view with an arrow and a bullseye, then you simply adjust the mount to get the star to the bullseye. For my small EQ mount the whole process takes about 90 seconds.

Thx Jordan.

I use SharpCap Pro for occultations, but I admit to not using the software tool to its full potential. I've been imaging since before the introduction of CCD's, so I am accustomed to doing everything manually. Back in the very early '90's there weren't many software tools to make the task easier.

When my wife Debra was learning to acquire, not "just" process images, I had her do everything, focus, point etc, manually first so she would understand what's going on with the equipment. When the tools failed for some reason she would better understand why and could go back and do it manually if necessary.

I know, I know... I'm an old fart... :-)

[photoracer18]

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

You mean Mark Dearing. He and I have sold each other things many times over the years.

[MassiveRedTelescope]

You're welcome, KGB!

[Peter Ceravolo]

My wife Debra and I are giving a talk on what it's like to do imaging with the B&C 16" tonight 9pm Eastern on the Astro Imaging Channel.

 

https://www.youtube....h?v=XemMNYK4MvU

 

Anyone who's followed this thread won't learn much new from my intro, but Debra's will be of interest on the joys and frustrations of imaging with an f/18 system.

 

[macdonjh]

Peter Ceravolo, excellent, I hope I'm awake.  I've spent two all-nighters in a row at my observatory.  As with so many things astronomy, I was frustrated 75% of the time.  I didn't make any progress on Friday night despite being up until almost 4:00am.  Last night started the same way, but I had a break through just after midnight.  I'm pre-processing my data now.  Fingers crossed.  I left my main camera unbinned, so my image scale is a manly 0.15"/ pixel.  Still need a Barlow to get to 0.11... 

[Peter Ceravolo]

Who says ya can't photograph a comet with an old scope at f/18?

My wife Debra was at it again, using a Canon R6 camera on the 16" Boller and Chivens Cassegrain:

 

 

It was a real PIA to process because of the image scale.

You can see a less mangled image on Debra's Astrobin page along with acquisition details, or look her up on Faceplant:

https://www.astrobin.../debraceravolo/

[tim53]

Looks like one could measure the width of the nucleus by that "shadow" in the center of the tail.

[Kasmos]

Peter, you're probably aware of it, but for those that may be interested and never looked them up,

I recently came across the Boller & Chivens website that has lots of history and cool photos.

 

https://bollerandchivens.com/

 

 

I also recently noticed this page from an article on Torque mounts

from the May 1968 issue of Sky & Telescope.

[Kitfox]

I appreciate that link, Kasmos.  Brought back a lot of memories.  Peter, you are a lucky man to have unfettered access to that 16" instrument.  I had four years of such access to the 24" example at The University of North Carolina's Morehead Observatory back in the 80s, and one amazing summer with the 90" at Kitt Peak.  Many a 4 by 5 plate and 90 minute exposure were joyfully processed in those aromatic chemical baths (who can't remember those smells, even if only experienced a single time?).  

 

These scopes are so well figured, they are both still in use with professional digital sensors.  The poor 24" in Chapel Hill, North Carolina is still used for teaching purposes, even under the Bortle Infinity skies of the Research Triangle.  I will always have a hankering for the simplicity of two well figured mirrors, and optics designed for science.  But have to stick to my two Tinsley's to fulfil this need, vicariously follow this thread, and dream!

 

Oh, and the mounts?  We talk of settle time and weight capacities with our hobbyist-grade mounts...these mounts laugh at sledge hammer strikes!

Edited by Kitfox, 30 October 2024 - 09:49 AM.

[JohnH]

Is it possible to make a coma corrector for scopes of these type?

[Peter Ceravolo]

John,

The Boller and Chivens 16" classical Cassegrain with the f/3 parabolic primary and matched hyperbolic secondary operating at f/18 does not need a coma corrector.

These high amplification classical Cassegrain systems, 6x secondary in this case, have little to zero coma. In fact the Ritchey-Chretien and classical Cass optical designs for this Cass system would differ only slightly. In any RC system the primary mirror trends closer and closer to the parabolic form as the secondary amplification increases.

The B&C 16's dominant aberration is field curvature, 260mm radius concave to the sky. This may sound bad, and look bad in the spot diagrams, but with such a slow beam the slight defocus is mostly imperceptible in practice. The big challenge with this system's full Cassegrain field of view is collimation. For example with the old 16803 CCDs each 9 micron pixel sees 1/4 arc seconds of sky at the Cass focus, a wide high resolution system. One can be spot on at field center for very high power visual but the corners of the big CCD will be a bit wonky if the collimation is not truly spot on. Large format (and I do mean format - not field) high res systems are a harsh mistress... :-)

 

 

[JohnH]

Thanks for doing the math Peter.

[deSitter]

John,

The Boller and Chivens 16" classical Cassegrain with the f/3 parabolic primary and matched hyperbolic secondary operating at f/18 does not need a coma corrector.

These high amplification classical Cassegrain systems, 6x secondary in this case, have little to zero coma. In fact the Ritchey-Chretien and classical Cass optical designs for this Cass system would differ only slightly. In any RC system the primary mirror trends closer and closer to the parabolic form as the secondary amplification increases.

The B&C 16's dominant aberration is field curvature, 260mm radius concave to the sky. This may sound bad, and look bad in the spot diagrams, but with such a slow beam the slight defocus is mostly imperceptible in practice. The big challenge with this system's full Cassegrain field of view is collimation. For example with the old 16803 CCDs each 9 micron pixel sees 1/4 arc seconds of sky at the Cass focus, a wide high resolution system. One can be spot on at field center for very high power visual but the corners of the big CCD will be a bit wonky if the collimation is not truly spot on. Large format (and I do mean format - not field) high res systems are a harsh mistress... :-)

 

curved fields.JPG

 

flat fields.JPG

I assume the spacing is also super-critical.

 

-drl

[Peter Ceravolo]

I assume the spacing is also super-critical.

 

-drl

Actually no. I ray traced the system and found that moving the secondary to make the focal plane is coincident with the back of the OTA has negligible effect on image quality.I recall the same cannot be said of a much faster RC system, move the focus too much and the correction deteriorates.

But, because of the high amplification secondary, it doesn't take much of a change in separation to throw the focus a significant distance.

This "wicked focus shift" can be a pain in a CCD imaging set up going from summer temps to winter temps given the CTE of an aluminum tube. I've complained about this in an earlier post.

[Kitfox]

The 24” at UNC Chapel Hill showed very small, round stars even at the corners of 4” by 5” plates.  …I need to reach out and see how difficult it would be to get reacquainted with that scope…

[Bob4BVM]

The 24” at UNC Chapel Hill showed very small, round stars even at the corners of 4” by 5” plates.  …I need to reach out and see how difficult it would be to get reacquainted with that scope…

Me too.

I miss those long ago nights running the 24" B&C at U of O's Pine Mtn Obs.  I need to get on the volunteer staff again now that i'm retired !

The public was usually gone by midnight and we had some amazing views thru that baby in the wee hours.

The core of M42 at high power is still burned into my memory. I have never seen any photo or "image" which comes anywhere close to what we saw that night

 

CS
Bob

Edited by Bob4BVM, 19 November 2024 - 01:44 PM.

[JohnH]

The 24” at UNC Chapel Hill showed very small, round stars even at the corners of 4” by 5” plates. …I need to reach out and see how difficult it would be to get reacquainted with that scope…

Do you have any information about that scope? Like what design it is what the focal ratio of the primary is and what the final focal ratio is? If the answer is 5.6 and f11 I may know where a duplicate of it is at least optically

[RichA]

Who says ya can't photograph a comet with an old scope at f/18?

My wife Debra was at it again, using a Canon R6 camera on the 16" Boller and Chivens Cassegrain:

 

Comet T-A_small.jpg

 

It was a real PIA to process because of the image scale.

You can see a less mangled image on Debra's Astrobin page along with acquisition details, or look her up on Faceplant:

https://www.astrobin.../debraceravolo/

Ahhhhh.  After being bombarded by small scale images of tiny galaxies on wide but non-essential expanses of skies, it's so nice to see something like this.  Great detail.

[JohnH]

I assume the spacing is also super-critical.

-drl

Astronomy Technology Today had an article with some analysis posted about various category types and the sensitivity. Turns out, the DK has fairly low sensitivity to this. The Scopes are noted for being quite accommodating with back focus distance as you can change the secondary position in order to bring stuff to focus

[macdonjh]

Actually no. I ray traced the system and found that moving the secondary to make the focal plane is coincident with the back of the OTA has negligible effect on image quality.I recall the same cannot be said of a much faster RC system, move the focus too much and the correction deteriorates.

But, because of the high amplification secondary, it doesn't take much of a change in separation to throw the focus a significant distance.

This "wicked focus shift" can be a pain in a CCD imaging set up going from summer temps to winter temps given the CTE of an aluminum tube. I've complained about this in an earlier post.

I believe the back focal distance for a classical Cassegrain varies with mirror separation multiplied by a factor of M² (secondary magnification squared):

[delta]BFL= M²S. 

 

For my f/15 classical Cassegrain (secondary M=5) a 1mm change in mirror separation moves the focal point 25mm.  As Peter Ceravolo says above, this really hit home when I set up my imaging train last winter with my focuser racked halfway out.  During the warmer spring evenings I nearly ran out of in-focus travel as my scope expanded (even with carbon fiber truss poles).  The focus point shifted approximately 19mm.  I ended up removing at least one, maybe two, spacers before it got really warm in the summer.

 

That is a very cool photo of the comet.  As others have said, very different presentation compared to typical photos. 

[jgraham]

I encountered this while refurbing a 6" f/20 classical Cassegrain. A small change in the secondary/primary separation made a big change in the back focus.

[Dick Parker]

Relative to the mirror spacing change and the larger change in back focus distance, FWIW I have a mitigation for that which may help someone.

 

When I built my 16 inch Classical Cassegrain I mounted my secondary spider on a ring that just slipped inside the tube. It was held in position by three threaded rods so the tube could grow and shrink with temperature and humidity and the spacing would only change by the much smaller change in elongation of the rods. I could have further improved this by using invar rods with zero coefficient of thermal expansion (CTE). The first picture shows the ring and two of the rods. The rods go back toward the primary mirror to a bearing block, with ball bearings, that can be seen just forward of the primary. The rods are immobilized in axial movement there. They continue beside the primary mirror through the backing plate. I have a similar mounting system for the primary except the rods are much shorter. The second picture shows three pairs of knobs around the perimeter of the back plate. Each pair has a larger knob and a smaller knob. The small knobs move the ring for the secondary and the larger knobs move the mirror cell for the primary. With this arrangement I can adjust the position, therefore the spacing, of both mirrors, put the back focus exactly where I want it, and collimate both mirrors from the eyepiece.

 

Dick Parker

Attached Thumbnails

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[deSitter]

Relative to the mirror spacing change and the larger change in back focus distance, FWIW I have a mitigation for that which may help someone.

 

When I built my 16 inch Classical Cassegrain I mounted my secondary spider on a ring that just slipped inside the tube. It was held in position by three threaded rods so the tube could grow and shrink with temperature and humidity and the spacing would only change by the much smaller change in elongation of the rods. I could have further improved this by using invar rods with zero coefficient of thermal expansion (CTE). The first picture shows the ring and two of the rods. The rods go back toward the primary mirror to a bearing block, with ball bearings, that can be seen just forward of the primary. The rods are immobilized in axial movement there. They continue beside the primary mirror through the backing plate. I have a similar mounting system for the primary except the rods are much shorter. The second picture shows three pairs of knobs around the perimeter of the back plate. Each pair has a larger knob and a smaller knob. The small knobs move the ring for the secondary and the larger knobs move the mirror cell for the primary. With this arrangement I can adjust the position, therefore the spacing, of both mirrors, put the back focus exactly where I want it, and collimate both mirrors from the eyepiece.

 

Dick Parker

That's a fantastic system! I only recently discovered that the Meade RCX scopes had similar motor control of the spacing.

 

-drl

[Garyth64]

The back focal distance for a Cassegrain varies with the mirror spacing according to this formula:

 

p' = RC x p

       RC - 2p

 

where, 

 

p' = the distance from the secondary to the system focus

RC = the radius of curvature of the secondary

p = the distance from the primary focus to the secondary

 

p'/p = amplification of the secondary

 

RC = 2 x p x p'

           p' - p

 

RC = 2 x p x amp

          amp - 1

 

p =     f + b

      amp + 1

 

where,

 

f = the focal length of the primary

b = the back focal distance

 

With these formulas, the parameters of a Cassegrain can be figured.  These are the same formulas that are used in the online calculator.

Edited by Garyth64, 16 December 2024 - 09:23 AM.