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Celestron C5 750 f/5.9

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#1 BQ Octantis

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Posted 22 July 2017 - 03:20 AM

After owning a Celestron C5 750mm for over 20 years, I suddenly found myself living somewhere with a sky worth shooting. The cantilevered foot for a camera tripod that came on the spotter always had in too much vibration on my EQ-2 mount, resulting in barbell stars when shooting deep sky objects. With a little searching, I found a set of 5.5" rings (ostensibly made for a Newtonian) with a Vixen base on eBay. I happened to get an Orion SkyView Pro tripod to carry my new SkyWatcher 180 (with a Vixen base)—overkill for the weight of the newly-ringed C5, but stable as a rock!

 

With this setup, I finally had a good enough platform to shoot some deep sky objects. The Lagoon Nebula happened to be in an ideal viewing position, so I shot it. The result was fairly impressive:

 

Lagoon Nebula

 

The edge coma is fairly obvious, which surprises me given that the scope was sold as a telephoto for a 35mm camera (the APS-C crops off an edge that would be more severe). Is there a field flattener that would work on the C5?

 

BQ


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#2 bobzeq25

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Posted 22 July 2017 - 08:19 AM

 

After owning a Celestron C5 750mm for over 20 years, I suddenly found myself living somewhere with a sky worth shooting. The cantilevered foot for a camera tripod that came on the spotter always had in too much vibration on my EQ-2 mount, resulting in barbell stars when shooting deep sky objects. With a little searching, I found a set of 5.5" rings (ostensibly made for a Newtonian) with a Vixen base on eBay. I happened to get an Orion SkyView Pro tripod to carry my new SkyWatcher 180 (with a Vixen base)—overkill for the weight of the newly-ringed C5, but stable as a rock!

 

With this setup, I finally had a good enough platform to shoot some deep sky objects. The Lagoon Nebula happened to be in an ideal viewing position, so I shot it. The result was fairly impressive:

 

 

 

The edge coma is fairly obvious, which surprises me given that the scope was sold as a telephoto for a 35mm camera (the APS-C crops off an edge that would be more severe). Is there a field flattener that would work on the C5?

 

BQ

 

Had one for years (the standard astro model, which maybe was slower?).  Loved it.  A grab and go SCT.  Sold it when my eyes were no longer good enough, well before I discovered astrophotography.

 

The coma is an unavoidable aspect of the inexpensive spherical mirrors used in the SCT.  The optical design of the SCT makes for a decent inexpensive long focal length scope, but it is fundamentally flawed, which is why there are no "high end" SCTs.  If someone wants to make a high end long focal length scope, they use an optical design with more complex mirrors like a Ritchey-Chretien or a Corrected Dall Kirkham.  Physics.

 

Enjoy it for what it is.  Which is pretty nice.


Edited by bobzeq25, 22 July 2017 - 08:23 AM.


#3 Walter E.

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Posted 22 July 2017 - 08:21 AM

I used to own one of those many years ago. The central sharpness was excellent but the outer 1/2 fov was filled with partial donuts probably from spherical abs. I never tried it for astro photos but your results are surprisingly good. It would be interesting to see what would happen with a flattener introduced into the light path.

 

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#4 Eddgie

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Posted 22 July 2017 - 08:38 AM

 

After owning a Celestron C5 750mm for over 20 years, I suddenly found myself living somewhere with a sky worth shooting. The cantilevered foot for a camera tripod that came on the spotter always had in too much vibration on my EQ-2 mount, resulting in barbell stars when shooting deep sky objects. With a little searching, I found a set of 5.5" rings (ostensibly made for a Newtonian) with a Vixen base on eBay. I happened to get an Orion SkyView Pro tripod to carry my new SkyWatcher 180 (with a Vixen base)—overkill for the weight of the newly-ringed C5, but stable as a rock!

 

With this setup, I finally had a good enough platform to shoot some deep sky objects. The Lagoon Nebula happened to be in an ideal viewing position, so I shot it. The result was fairly impressive:

 

 

 

The edge coma is fairly obvious, which surprises me given that the scope was sold as a telephoto for a 35mm camera (the APS-C crops off an edge that would be more severe). Is there a field flattener that would work on the C5?

 

BQ

 

This design probably has about the same field curvature as you would find in a very small very fast Apo, so look for one like that.

The problem though is that the corrector is not going to reduce coma, but much of what you are dealing with here is the curvature and not the coma. When you defocus a comatic blur, you make it much larger than it would be in perfect focus.

 

There are forumlas on Telescope-Optics.net that you can use to calculate the field curvature but my guess is that it is going to be on the order of -175mm, so a flattener designed for a refractor of about 500mm or so will probably be close-ish.


Edited by Eddgie, 22 July 2017 - 09:16 AM.


#5 Phil Barker

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Posted 22 July 2017 - 04:14 PM

The central part is excellent a lot of the problem will be the curvature of field not just the coma.  Even the sct .63 reducer will probably work  but obviously not fully illuminate a standard digital slr.

 

My old f6.3 8 inch meade and a reducer resulted in excellent images albeit with an st7 400,000 pixels.  It was f-4.  The field was flat and sharp to the edge of the st7 chip. 



#6 Jon Isaacs

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Posted 23 July 2017 - 09:32 AM

The coma is an unavoidable aspect of the inexpensive spherical mirrors used in the SCT.

 

 

I believe the source of the coma is the fact that the corrector is not placed at the radius of curvature of the primary mirror.  The father of the SCT, the Schmidt camera, is coma free but with the corrector at twice the focal length of the primary mirror,  it's very long. It also means mounting the secondary with a spider and a port in the OTA for collimation. 

 

A C-8 that was about the length of an F/5 Newtonian would not have been a big seller and thus the compromise. 

 

Jon



#7 BQ Octantis

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Posted 24 July 2017 - 03:23 AM

Thanks for the great perspectives, folks! I found a decent empirical comparison of focal reducers on a Celestron C8 on Stargazer's Lounge:
 
https://stargazerslo...corrector-test/
 
Seeing as my f/5.9 has a massive curvature compared to the f/10, I don't think the results aren't directly applicable. I think my options are thus

  • Experiment with focal reducers for APOs and refractors for $150-300 apiece,
  • Buy a 6" Ritchey-Chrétien for $500, or
  • Simply enjoy the C5 f/5.9 for what it is.

Per bobzeq25, I'm leaning heavily toward #3…

 

BQ



#8 Cepheus Elf

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Posted 24 July 2017 - 06:53 AM

Very nice pic! I would go for option 3 and just crop the images!



#9 Cepheus Elf

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Posted 24 July 2017 - 09:25 AM

I haven’t actually heard of an 750 C5 before. How is this fast focal length achieved? Does it have a built in reducer or is it due to the properties of the secondary mirror?

 

Cheers,

Mick 



#10 celestronlover57

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Posted 24 July 2017 - 09:55 AM

It would be the secondary mirror.  Also it would have a larger diameter compared to the secondary in the f10 version of the C-5. 



#11 Geo.

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Posted 27 July 2017 - 10:17 AM

The central part is excellent a lot of the problem will be the curvature of field not just the coma.  Even the sct .63 reducer will probably work  but obviously not fully illuminate a standard digital slr.

 

My old f6.3 8 inch meade and a reducer resulted in excellent images albeit with an st7 400,000 pixels.  It was f-4.  The field was flat and sharp to the edge of the st7 chip. 

Surprising photo performance!. Short FL scopes can be difficult to collimate. The C5 750 won't come to focus with the f/6.3 or a f/5 reducer. But I'll have to try the f/8 refractor reducer. I've had a series of these that I've picked up for under $200 and sold when offered more than $300. Paid $150 for the current one.

 

As the OP claims, the tube is sized 5.5" as are most 4.5" Newts. This permits the use of cheap and easy to find 5.5" rings and old Newt mounts.



#12 BQ Octantis

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Posted 11 January 2018 - 03:37 AM

Here's the Tarantula Nebula through the C5/750.

 

Tarantula Nebula

 

Cheers.

 

BQ


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#13 Don W

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Posted 11 January 2018 - 06:36 PM

What camera are you using?



#14 BQ Octantis

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Posted 12 January 2018 - 12:40 AM

What camera are you using?

Camera is a stock Canon T3i. This was a 16-image stack; subs were 30 seconds each at ISO 1600.


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