24 replies to this topic

[jgraham]

From what I have read, Celestron introduced the 5-1/2” f/1.65 Schmidt Camera is 1971 and these occasionally show up on the used market. I have been curious about these for quite a while and I finally picked one up off of the Cloudy Nights classifieds. The example that I have is serial number 14 and is in excellent condition, though it did not include the original film holder. That’s no loss for me as I had no intention of trying to shoot film with it, but I was curious whether I could replace the film holder with a small digital camera either installed in place of the film holder, or off to the side using a pick-off mirror. The later would have been tough given the very fast f/ratio; the former was tough given the small size of the scope. The overall approach was settled when I found a Starlight Xpress Superstar-M monochrome guide camera. This is an end-on camera that is only 1-1/4” in diameter and about the size of an eyepiece making it smaller than the film holder and a perfect match for the 5-1/2” camera if I could figure out a way to mount it. After staring at it for quite a while and weighing several options I came across a PVC union that was a good fit for the Starlight camera and about the same diameter as the original mount for the film holder, so I decided to see if I could move the spider legs from the film holder over to the PVC union, and then mount the Starlight camera inside the union, and finally mount the assembly inside the Schmidt camera.

 

I modified the PVC union by removing a small step-down ring inside the piece, and cutting slits for the spider legs with a coping saw and then widened to fit the spider legs 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. With care, everything press-fit together quite nicely.

 

 

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

 

 

 

[jgraham]

The Starlight camera also press-fit snuggly into the holder, making it possible to focus the system by adjusting the position of the camera in the holder...

 

 

The assembly mounted inside the Schmidt Camera…

 

 

 

[jgraham]

For initial testing I glued soft foam rubber strips to a Celestron Losmandy D rail, secured the camera with lengths of double-sided Velcro, and mounted the camera on an Atlas.

 

 

This worked okay, but I need to come up with a better way to mount the rail on the camera.

Edited by jgraham, 20 May 2018 - 03:13 PM.

[jgraham]

The final touch was an Astrozap flexible dew shield…

 

 

 

[jgraham]

M3 - Celestron 5.5” f/1.65 Schmidt Camera First Light
Camera: Celestron 5.5” f/1.65 Schmidt Camera, Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 16x30sec saved as TIFF
Darks: 16x30sec saved as TIFF
Flats: 32x3sec, LED tracing tablet
Average Light Pollution: Red zone, poor transparency
Lensed Sky Quality Meter: Variable 18.3 mag/arc-sec^2, thin haze with passing clouds
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap  Pro, Deep Sky Stacker, Nebulosity, Photoshop

 

 

The first night out was pretty rough with high humidity and poor transparency, but it was adequate to rough-in the focus and to take some quick test shots. The camera is a little tilted and I had an odd gradient on the left and right edges that I cropped off, but the overall performance was wonderful! I’m not sure how much imaging I’m going to do with this camera, but I will be using it for real-time camera-assisted observing.

Edited by jgraham, 20 May 2018 - 03:35 PM.

[J A VOLK]

I take it you realize the original film holders were curved quite a bit to a accommodate the optical configuration. For this reason EAA on these Schmidt Cameras is quite limited unless someone comes up with a flattener. As your camera has a very small sensor it could serve the purpose you propose if the subject is well centered.

[jgraham]

Yeppers, but as you pointed out the sensor in this camera is quite small, so the effect of field curvature is pratically nil. I see a bit in the corners, but its not nearly as bad as the coma that I have seen with other scope/camera combinations. The bigger problem is a slight tilt in my assembly, not surprising given that this was done all by hand. Still, the effect is quite small and well within the tolerance of the inteded purpose. I spent several hours last night under incredibly bad transparency having a great time galaxy-hoping across the Virgo cluster. It will be fun seeing what this camera can do when it is actually clear!

 

Fun stuff!

[siriusandthepup]

How much actual sky field do you get with that little chip?  ...asks the owner of a Celestron 5.5” f/1.65 Schmidt Camera sadly sitting unused in the closet.

[jgraham]

I took 5 test images last night and I'll check the actual field when I get a chance, but I made a quick check by slewing a star from top to bottom and it gave a field that was about 1 x 1.5 degrees, maybe a bit wider, very nice for observing and basic imaging. I was very happy with how clean the field was.

[jgraham]

Example real-time screen shots can be found here...

 

https://www.cloudyni...ages/?p=8590259

 

If you look closely at the scroll bars you will see that these are nearly full-frame views of what the source images look like.

[opticsguy]

How much actual sky field do you get with that little chip?  ...asks the owner of a Celestron 5.5” f/1.65 Schmidt Camera sadly sitting unused in the closet.

Yup said I, also the owner of same scope . . . . . sadly sitting unused in the basement . . . . .

[Stargazer3236]

How did you determine what the distance from the sensor to the focal point was? How do you know you have good focus?

[jgraham]

I had a rough estimate based on photographs of the film holder, so it had to be just a bit in front of the spider. I confirm this my holding small white cards inside the tube and focusing distant objects on the cards. I roughed in the focus using Arcturus, and then fine tuned it using a Bahtinov mask. It is just a slip focus, so it takes a very light touch and a bit of luck. It wouldn't take much to install a fine threaded push rod in the back of the cell to provide a fine focus in at least one direction. If you captured the rod on the camera you could have fine focus in both directions. However, since the spider is connected to the primary via 3 Invar rods, once the focus it set it should stay put very well.

 

You'd think a little adapter like this would make a fun 3D printing project.

[GlennLeDrew]

Excellent work, John! I knew the small sensor size would reduce field curvature concerns, but I'm surprised by the outer field 'tightness' nevertheless. It's eminently useable as is, most certainly for nebulae.

 

If you ever decide to try a field flattener, a plano-concave lens having its curved surface radius of curvature about 1/3 the instrument's curved field radius will do. Place this lens's flat surface almost in contact with the sensor's window. If coated, so much the better, so as to reduce annoying reflection ghosts. The lens should have the largest diameter which will permit squeezing into place.

[Ed Jones]

Nice shot John!  Here's a shot of my Schmidt conversion to roll film.  I mounted it om the forks for my refractor.  The film holder is mounted but not focused yet and I have a slight out of balance to fix.  I also need to make a tangent arm drive for AP.

Attached Thumbnails

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[jgraham]

Glenn; Thanks! It will be interesting to see if I can find an off-the-shelf lens that will work. This has turned out so well I plan on using it regularly so it's worth investing a bit more effort in cleaning it up. 

 

Ed; Ahah! I was wondering how you were getting along with the 8". It'll be neat seeing what you can do with it.

 

Neat stuff.

[starcanoe]

The images would be so much better if the darn set up wasn't so undermounted....its a wonder all that wobbling didn't ruin the images

 

All kidding aside....cool as beans !

 

Do a bit of refinement regarding focusing, getting things perpendicular, and maybe a crude field flattener and you'll have one bang up imaging system...not to say as is that it isn't pretty darn good already !

 

Love it.

[jgraham]

Heh, heh, I think that the rail weighs as much as the camera! Speaking of which, the camera is now firmly bolted to the rail. Next up... a dust cap. The corrector looks awfully fragile nearly flush with the front of the tube. It is weird seeing a corrector without a secondary sitting in the center.

Edited by jgraham, 22 May 2018 - 07:17 PM.

[jgraham]

This will give you some idea of the field of view. This is a stack of 32x0.002s images taken of the moon this evening. I had to stop the aperture down to about 2" to keep from saturating the source images. This is a full frame, flipped horizontal.

 

 

[siriusandthepup]

That is sooooo cool!

 

It really is sharp!

 

And that's a nice field of view too.

Edited by siriusandthepup, 23 May 2018 - 12:33 AM.

[jgraham]

Ahah! Looking at what I could find on the Web about this camera I assume that the focus was set at the factory and was designed to be fixed, that is why the spider is held separate from the steel tube on 3 Invar rods. My simple mod is a slip focus which is a real bear to set, so the last time I had it hot I spent quite a bit of time tweaking it as best I could using a Bahtinov mask. Once I was happy with the focus, I reach in with a hot melt glue gun and carefully applied a bead between the camera and the mount. Needless to say I was a tad nervous. The weather has been clear the last couple of nights, but I was set up for planetary imaging so the SC5 was placed in storage. The weather looks iffy for planetary imaging tonight, so I am setting up for remote observing with the SC5. It is warm and humid this evening as compared to cool and dry when I set the focus. The moment of truth... the focus held perfectly! This makes this system extremely easy to set up! We'll see how it performs across the summer and into next winter, but so far I am very encouraged.

 

Fun stuff!

[jgraham]

Camera: Celestron 5.5” f/1.65 Schmidt Camera, Starlight Xpress Superstar-M
Filter: None
Guide scope: None
Exposure: 32x30sec saved as PNG
Darks: 16x30sec saved as TIFF
Flats: Synthetic
Average Light Pollution: Red zone, poor transparency, haze, waxing gibbous moon
Lensed Sky Quality Meter: 18.1 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
Software: Sharpcap Pro, Deep Sky Stacker, Photoshop

 

Yep, the focus held pretty well...

 

 

This was a survey image taken while using my Schmidt camera for remote observing. This is a nice way to scout ahead for targets for follow-up imaging and it was also a lot of fun observing deepsky objects on a hazy night with a brilliant moon in the sky.

 

[Stargazer3236]

Curious. I was experimenting with Sigma clipping in SharpCap 3.1. What is a good sigma clip to use while imaging?

[jgraham]

Good question. I am still learning my way around Sharpcap Pro, last night I just left the stacking method on default and saved the source images for later processing. I tinkered with its sigma clipping option during an earlier observing run and it seemed to work okay. Sharpcap has grown quite a bit since I last used it.

[cometchaserde]

Hello,

in the German astroforum a user described his modification of a 5.5" Schmidtcamera -> Link

The result looks quite fine.

 

Stefan