11 replies to this topic

[SMigol]

By the end of the year I will take ownership of an 8 inch Schmidt camera.  I wish to put it back into service.

 

The catch is that the magnetic film holder is missing.

 

I believe that if I can measure an existing one, a replacement can be 3d printed.  With the advancements of cell phones it may be possible to create the 3d model just from precise video.

 

Does anyone have a complete 8 inch f1.5 schmidt holder that would be able to help with inspection and measurement? 

 

I am in touch with Robert Reeves and once I have the camera in my hands I will be able to inspect it and determine what is missing.

 

Stephen Migol

[Todd N]

This guy converted his schmidt camera to digital. Maybe he still has the film holder and is willing to let it go.

 

https://www.cloudyni...ilm-to-digital/

[davidc135]

By the end of the year I will take ownership of an 8 inch Schmidt camera.  I wish to put it back into service.

 

The catch is that the magnetic film holder is missing.

 

I believe that if I can measure an existing one, a replacement can be 3d printed.  With the advancements of cell phones it may be possible to create the 3d model just from precise video.

 

Does anyone have a complete 8 inch f1.5 schmidt holder that would be able to help with inspection and measurement? 

 

I am in touch with Robert Reeves and once I have the camera in my hands I will be able to inspect it and determine what is missing.

 

Stephen Migol

The image surface's radius of curvature of a Schmidt camera is equal to its focal length ie 12 ins but it may be wise to measure the mirror's roc to check that it is exactly f1.5.

The depth of curve of the convex surface would be r²/2R with r being the semi diameter of the holder and R being RoC.

 

That's if you don't have an existing one to photograph.

 

David

Edited by davidc135, 02 November 2023 - 04:16 PM.

[Todd N]

It's also possible to employ a simple plano-convex lens against the film as a field flattener. But at this faster f-ratio there may be a problem with chromatism, IIRC. I think the formula is fl= (n-1/n) x fl. refractive index (n)  for n-BK7 is 1.5168. This comes out to approximately fl=155mm. *Correction:

 

R= (1-1/n)Rm

 

R= radius of curvature (focal length) of field flattener

n= Refractive index (BK7 1.5168)

Rm= roc/fl of main mirror

 

For 8" 300mm fl Schmidt camera R= 102mm

 

 

below Edmund has a PC 50mm round with a fl= 150mm. Close enough? They have coating options but I noticed in the case of MgF  it is stated for 0 degree incidence so, I don't know if that will diminish off axis performance. There is also a 75 mm diameter fl 150mm if you want to try larger format. Apologies ahead of times if this doesn't quite work out if you try this. One of these days I plan on making a lensless schmidt and will employ such a field flattener. 

 

50mm fl 100mm

https://www.edmundop...nvex-lens/2741/

75mm fl 100mm

Unfortunately, none around this focal length.

Edited by Todd N, 03 November 2023 - 04:51 PM.

[SMigol]

This guy converted his schmidt camera to digital. Maybe he still has the film holder and is willing to let it go.

 

https://www.cloudyni...ilm-to-digital/

I sent a note to that user earlier today, looks like the conversion was done back in 2021, they might have the old part.

 

Here's hoping!

 

The math provided by david above would allow a prediction of the curve needed - should make it much easier to model.  Then it's a matter of a design that allows shimming for precision.

 

I also like the idea of using a flattening lens, especially one that could handle the larger diameter.  The link is well appreciated!

Edited by SMigol, 02 November 2023 - 06:00 PM.

[Eric P]

I 3D printed circular medium format holders for my 8 inch camera.  Let me know if you are interested and I will send you the models.

Attached Thumbnails

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[SMigol]

These images are gorgeous!

 

Did you have issues with managing the curve of the film with the 3d print?

[Eric P]

Thanks Stephen, Here are the links to the TinkerCad in case anyone wants to download/improve the design or make it suit their particular camera.  There is a slot in the base plate to put large washers (I used thin neodymium magnets) secured inside with double sided tape. I printed the model vertically as it was much easier to remove without warping the film plane.  I used guitar string gauge with a 12" radius of curvature to check the print, and of course there was trial and error to get the thickness to match my metal holder. 

 

https://www.tinkerca...A37BvkhX-eric-p

 

I use an EK Tools 2.25 Paper Punch to punch out the circles of 120 film then sandwiched them between the baseplate and clamp using pieces of painters tape to hold it together.

[telesonic]

Yowza!

 

Nicely done Eric P.

 

I know they are rare as hens teeth, but if I came across a 5.5" Celestron Schmidt Camera, ooooh man. I'd be on it. Probably have to sell a few of my classic scopes and other stuff to pay for it though.

 

Not that I'm actively looking for one, they are just really cool... and I still dabble in film once in awhile.

[highfnum]

top part holds film is this what you are talking about?

 

 

 

[SMigol]

Yes, that is the part I am talking about.

 

The 8inch schmidt I'm getting in a couple of weeks is missing the holder, so I'll need to replace it (and re-focus for the holder).

[roger_ca]

Still want more parts? I have the film roller. Filter mask, for the film holder. I have both film holders. For filters and no filters. and what ever accessories for an 8 " celestron Schmidt. PLUS. The cold camera and optical plugs