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Converting an Celestron 8” f/1.5 Schmidt Camera from Film to Digital

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#1 jgraham

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Posted 11 November 2018 - 06:06 PM

 I previously converted a 5.5” f/1.65 Schmidt Camera from film to digital as described here…

 

https://www.cloudyni...ra#entry8601810

 

The conversion of the 8” was similar. The only significant difference is that after I removed the original film holder support I left the spider attached to the internal support ring and fit the camera adapter with the spider vanes in place.

This shows the original film holder. It is an ingenious little device that presses a single piece of 35mm film into the 3D shape of the focal plane; a segment of a sphere with the same radius of curvature as the primary.

 

Film Holder (11-3-2018)-1.jpg

 

The film holder is held in place with a magnet that is built into a kinematic mount…

 

Film Holder (11-3-2018)-2.jpg

 

The focus position is extremely critical and this arrangement ensures that the film holder goes back into precisely the same location every time. The camera was originally focused at the factory as was not adjustable. They even designed a second holder that was shifted slightly to accommodate filters mounted in front of the film. To prevent the thermal expansion of the steel tube from causing problems with the focus the spider is joined to the mirror by 3 Invar rods, forming an assembly that slips inside the body of the camera

 

Internal Structure (11-3-2018)-1.jpg

 

The spider assembly slips off of the end of the Invar rods by removing the mounting nuts on the end of each rod.

 

The spider vanes are press-fit into the support and captured by threaded lugs on each end. The holder can be separated from the vanes by removing one set of lugs and using the others to push the vanes out of the holder. These will only push the vanes about half way out, to push them the rest of the way I backed the lugs out and used the tip of a blade screwdriver as a spacer and then ran the lugs back in. A couple of passes was enough to do the job.

 


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#2 jgraham

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Posted 11 November 2018 - 06:07 PM

To replace the camera support I found a PVC union that was exactly the same O.D. as the original support and whose I.D. was just a tad larger than 1.25”. (If I recall right this is a nominal 1.5” PVC union.)  I cut slits for the spider vanes with a coping saw and widened them with a hack saw. Small half-cylinder recesses were milled into the inside of the union using a cordless drill and 3/16” drill bit held sideways along each of the slits. As the slots approached the right depth I widened the slits a bit more with a sharp knife so that the vanes slipped snuggly into place. With care, everything press-fit together quite nicely.

 

Camera Spider (11-3-2018)-1.jpg

 

A quick coat of flat-black paint finished it off…

 

Camera Spider (11-3-2018)-2.jpg

 

 


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#3 jgraham

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Posted 11 November 2018 - 06:08 PM

 The camera then slipped into the adapter…

 

Camera Spider (11-3-2018)-3.jpg

 

…making a snug friction-fit. The camera that I chose was the Starlight Xpress Superstar-M guide camera. This monochrome camera is about the size of a 1.25” eyepiece and easily fits within the original cross section of the film holder. In fact, the final cross section is slightly smaller. The small type ½” sensor is also a good match for the curved focal plane of the camera. I was tempted to try something a bit larger like the IMX294 color chip, but I suspect that the much larger type 4/3” chip would not work well without some sort of field flattener. Also, I originally used the color version of the Superstar, but the images were a tad soft, so I swapped it out for the monochrome version.

Once the camera was installed the support ring was then attached to the Invar rods and the assembly was slipped back inside the tube…

 

Camera Holder (11-3-2018)-1.jpg

 

The USB cable was attached so that it was aligned with one of the spider vanes and routed along one of the Invar rods using Velcro wire ties. The cable was then routed out through the film access door and attached to the outside of the camera with Velcro pads.

 


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#4 jgraham

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Posted 11 November 2018 - 06:09 PM

The camera was finished with the addition of a Celestron Losmandy style rail and mounted on my Atlas…

 

Atlas SC8 (11-3-2018)-2.jpg

 

The thin corrector is quite a dew magnet, so a heated dew shield is a must…

 

Atlas SC8 (11-3-2018)-4.jpg

 

The focus was set using a Bahtinov mask while carefully shifting the camera within the adapter. This is a painstaking process that takes a very light touch, a lot of patience, and a little luck. If it turns out that I need to occasionally reset the focus I have an idea for a tool that will help make that process easier.

 



#5 jgraham

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Posted 11 November 2018 - 06:10 PM

This is The First Light Image taken with the modified camera

 

M2 – Globular Cluster in Aquarius – Celestron 8” f/1.5 Schmidt Camera First-Light
Telescope: Celestron 8” f/1.5 Schmidt Camera
Camera: Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 32x15sec saved as TIFF
Darks: 32x15sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, fair transparency
Lensed Sky Quality Meter: 18.6 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

M2 (11-7-2018)-1j.jpg

 

There is a bit of a tilt issue with the camera, but for my first real night out I am happy with the result. This is a good example of how well this system performs as a patrol camera. I had only a few minutes before M2 reach the meridian, but it was plenty of time to grab 32x15s source images. For the rest of the night I settled on 64x30s as my standard set unless the target saturated, in which case I took 64 shorter subs.

 


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#6 jgraham

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Posted 11 November 2018 - 06:11 PM

M42/43 – Celestron 8” f/1.5 Schmidt Camera First-Light
Telescope: Celestron 8” f/1.5 Schmidt Camera
Camera: Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: (128 + 128)x4sec saved as TIFF
Darks: 32x4sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, fair transparency, low altitude (25deg)
Lensed Sky Quality Meter: 18.5 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

M42 (11-7-2018)-4j.jpg

 

Note that the exposure that I used here was only 4 seconds, at 5 seconds the core was saturating. I originally took two sets of 128 subs and reframing it a bit between the two sets to frame the outer loop. Just for yucks I combined the two sets into a single stack of 256 subs. Still, it only took a few minutes to capture the entire set.  This system is so fast and easy to use I ended up imaging 12 different targets even though I only ran about half the night due to exhaustion and thickening haze.

 

In summary, once I found the right camera and figured out how to mount it, the conversion was fairly quick’n simple. The conversion of the 8” only took about 4 hours from start to finish plus two nights of fiddling to get the camera sorted out (switching from the color to the monochrome camera) and getting the mirror aligned. It turned out to be whacko out of alignment the first night that I had it out, but that is another story. There is still a bit of a tilt to the focal plane, a problem that I also ran into with the 5.5”. However, it is a minor issue given that I plan on using these primarily as patrol cameras and for EAA.

 

Enjoy!


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#7 TOMDEY

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Posted 11 November 2018 - 07:04 PM

Nice! I think there is a chance that a plano concave singlet might flatten the field to go wider. But what you have there is already nice!  Tom



#8 Ed Jones

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Posted 12 November 2018 - 11:48 AM

John,

   Did you make your dew shield or buy it?   I guess I'll need one too.  I too acquired an 8 inch Celestron Schmidt but I put a roll film holder I made on it.  I've focused it with another telescope and now need to get the mount ready.



#9 jgraham

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Posted 12 November 2018 - 04:02 PM

The dew shield is an AstroZap flexible dew shield for the C8. Parts for the C8 should fit the Schmidt camera. In fact, this one has all of the holes and bolts to mount it on the original C8 forks. I am curious to see how your film images work out. The few examples that I can find on the Web are very impressive. Alas, my film gear is long gone, though I can still smell the chemicals. :)

Edited by jgraham, 12 November 2018 - 04:03 PM.

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#10 davidc135

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Posted 12 November 2018 - 04:53 PM

Congrats on your successful project. Also looking forward to Ed's results.  David



#11 Ed Jones

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Posted 12 November 2018 - 04:54 PM

Well I'll probably need to get away from Cincinnati, my sky isn't so good.  Here's my film holder pic.  I made it because the Celestron film cartridge holder was too big and I didn't have one anyway, this one is a lot less obstructive.  The small wire is a clamp that pushes a curved plate against the film and the hole is for the film winder.  I have to wind the film backwards in another cassette so it faces the mirror.  It's been a while since I did film developing, I remember the smell toovomit.gifbut I still have my film stuff.  With a cassette I can send it out for developing, especially color. 

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  • film holder.jpg

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#12 Ed Jones

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Posted 12 November 2018 - 05:23 PM

I mounted it on a alt-az fork and will use a equatorial platform.  To set focus I printed a lot of Air Force targets on a dot matrix printer and mounted it where the film goes.  It wasn't helpful at all but the printer dots were very useful; amazing resolution.  I used another scope to view the image.  This is the third set of alt bearings I had to make for it to get things right.

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  • IMG_09491s.jpg

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#13 jgraham

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Posted 12 November 2018 - 08:07 PM

Coolness! It'll be a lot of fun seeing what you can do with it. I do all of my imaging from my backyard in Kettering (no Milky Way for me) so film is not an option, though I still have my large format roll film and sheet film cameras. :)

The monochrome images from this camera are quick'n easy to process and does a great job cutting through my light pollution.

Edited by jgraham, 12 November 2018 - 08:10 PM.


#14 jgraham

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Posted 12 November 2018 - 08:37 PM

I'll upload a couple more of the first-light images as I get them processed. This was from late in the run (I'm processing them in the order that they are on my hard drive).

 

M35 & NGC 2158
Telescope: Celestron 8” f/1.5 Schmidt Camera, Atlas EQ-G
Camera: Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 64x15sec saved as TIFF
Darks: 32x15sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, fair transparency
Lensed Sky Quality Meter: 18.5 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

M35 (11-7-2018)-1j.jpg

 

 I had to use relatively short exposures (15s) to keep from over-exposing most of the stars in the field.

 


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#15 Ed Jones

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Posted 12 November 2018 - 09:36 PM

Well plan B would be to do what you've done if I don't like plan A. Your shots look great!



#16 jgraham

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Posted 13 November 2018 - 08:49 AM

Thanks! It is always good to have a Plan-B. :)

#17 jgraham

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Posted 13 November 2018 - 08:25 PM

NGC 457 – The Owl Cluster in Cassiopeia
Telescope: Celestron 8” f/1.5 Schmidt Camera
Camera: Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 64x10sec saved as TIFF
Darks: 32x10sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, fair transparency
Lensed Sky Quality Meter: 18.6 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

NGC457 (11-7-2018)-1j.jpg

 

This is another example where I had to use relatively short exposures (10s) to keep from over-exposing most of the stars in the field


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#18 hamishbarker

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Posted 14 November 2018 - 06:13 PM

Do you plan to use filters? All that speed could be helpful with keeping exposures reasonable length even with narrowband filters but perhaps focus differences could be an issue at f1.5?

I've been intrigued by the possibilities and calcs for schmidt correctors for a while, although it seems that vacuum pan method may be tricky at faster than about f2.5 due to glass tensile stress limits.

#19 Ed Jones

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Posted 14 November 2018 - 06:33 PM

John what happens when the stars are overexposed?  Is this typical of all digital cameras?



#20 jgraham

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Posted 14 November 2018 - 06:57 PM

It is common for bright stars to saturate, but it's usually not a problem. However, if they whacko overexpose then you can get excessive star bloat. With a modern camera with a nice large chip and lots of pixels its usually not a big problem, but this camera is so darned fast and the chip is small I gotta be a little careful. I'll post an example when I get a chance.

#21 jgraham

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Posted 14 November 2018 - 07:43 PM

As you guessed the problem with filters is the focus shift. The focus is so critical that the original film holder came in two styles; one with and one without filters. That's why I was interested in using a color camera with the 8". However, I wasn't happy with the pixel density with a type 1/2" chip. Someone with better skills than me might consider something along the lines of an IMX294 color chip with its larger type 4/3" chip. This could become an attractive option with a field flattener.



#22 jgraham

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Posted 14 November 2018 - 07:45 PM

NGC 869 & 884 – The Double Cluster in Perseus
Telescope: Celestron 8” f/1.5 Schmidt Camera, Atlas EQ-G
Camera: Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 64x5sec saved as TIFF
Darks: 32x5sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, fair transparency
Lensed Sky Quality Meter: 18.6 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

NGC869 & 884 (11-7-2018)-1j.jpg

 

Another example of short exposures, this time 5 seconds.

 

Next up are a couple of examples of galaxies and nebula (I hope).

 


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#23 wrnchhead

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Posted 15 November 2018 - 12:10 AM

I love the story of this scope, and the mod. And the images speak for themselves. 



#24 highfnum

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Posted 15 November 2018 - 02:35 AM

cool

 

i did one of these conversions a while. back 

https://www.cloudyni...e/17568-m45-sc/


Edited by highfnum, 15 November 2018 - 10:09 AM.

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#25 jgraham

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Posted 19 November 2018 - 05:08 PM

NGC 891 – Edge-on Spiral Galaxy in Andromeda – Celestron 8” f/1.5 Schmidt Camera First-Light
Telescope: Celestron 8” f/1.5 Schmidt Camera, Atlas EQ-G
Camera: Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 64x30sec saved as TIFF
Darks: 32x30sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, fair transparency
Lensed Sky Quality Meter: 18.7 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

NGC891 (11-7-2018)-1j.jpg

 

This is a fairly good example of the camera’s speed.

 


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