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Newtonian Camera?

ATM astrophotography EAA imaging
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#1 hukhound

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Posted 20 July 2019 - 12:54 PM

Greetings,

Has anyone ever built what I'll call a Newtonian camera?  Since my first posting 5 days ago, I've read a lot and learned more about coma correctors and sizing of secondaries.  Again, my desire is to cover my APS-C sensor with the 100% illumination zone while using a small secondary mirror.

 

The idea of a "Newtonian camera" came to mind.  This "camera" would be the entire OTA, purely a camera or EAA scope, no visual use possible.  I suspect this is done in large observatory instruments.  I remember seeing pictures of observatory reflectors with all of the instruments located where the secondary mirror would be. 

 

So the idea would be to eliminate the secondary mirror all together and replace it with an image sensor + coma corrector tube assembly, aligned on the axis of primary mirror.  I'm thinking this tube assembly would take the shape of a cone, with the sensor near the tip of the cone, and the coma corrector optics located in the wide portion of the cone, where the primary mirror rays are entering.  In theory the diameter of this sensor tube assembly could be very small, possibly smaller than the typical secondary mirror.

 

Anyway, not a simple DIY project for sure, but just wondered if such a "camera" had merits or had been built. 

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  • Newtonian Camera.jpg

Edited by hukhound, 20 July 2019 - 12:57 PM.

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#2 Steve Dodds

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Posted 20 July 2019 - 01:03 PM

Lots of people have done that, lots of people have done that with film cameras.


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#3 iwannabswiss

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Posted 20 July 2019 - 01:28 PM

jgraham did a similar DIY project not too long ago.  He converted one from film to digital, a write up of what he did is here.


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#4 KLWalsh

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Posted 20 July 2019 - 09:26 PM

Seems like a good idea. You eliminate some loss of light from the reflection and you avoid any loss of sharpness from a less than perfect secondary.
But at the same time you lose (or at least, make more complicated) the ability to fine-tune the focus and the ability to add filters for H-alpha and or other interesting wavelengths and to reject light pollution.
And if you want to upgrade the camera your options may be limited.
But, everything has tradeoffs and compromises.
Good luck.
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#5 Billyboy78

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Posted 20 July 2019 - 10:22 PM

I ran that idea past a learned colleague a number of years ago and was told the heat generated by the camera would distort the image. I never pursued it.



#6 TxStars

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Posted 21 July 2019 - 12:14 AM

Since you are using  a regular newt mirror you can make it as a truss tube.

This will help reduce how much the heat from the camera distorts the image.

You could also go with liquid cooling as a way to pull the heat away from the scope.



#7 billdan

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Posted 21 July 2019 - 10:11 AM

For focusing, you could get away with a single stalk instead of a conventional spider and use a sled focuser outside of the Tube that moves the camera up/down..

Find the balance point with the coma corrector screwed onto the camera and attach the stalk at this point.

 

You would probably want to use a colour camera as changing filters would get annoying if you only had a mono camera.

 

The warm air being  blown out from the TEC cooling fan may be an issue, but I think it will be OK, my QHY12 camera only blows mildly warm air out when its temp' is set to -15C, its never hot.

 

You would also have to use a separate guide scope as an OAG is not practical.


Edited by billdan, 21 July 2019 - 10:17 AM.


#8 gregj888

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Posted 21 July 2019 - 10:43 AM

For fine focus and collimation you could fix the camera and move the primary , assuming it wasn't too large.  Could be easily motorized.

 

Using a water cooled camera would minimize the heat plume with a slightly larger diffraction pattern.



#9 hukhound

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Posted 21 July 2019 - 12:06 PM

Yes, ok a Schmidt camera...The link to jgraham's project brought it all back to me.  I had a vague recollection that some type of scope-camera setup had been commercialized, but couldn't remember. You'll have to forgive me, I'm just now returning to this hobby after a 38 year intermission. Now I do recall back then the Schmidt camera was a commonly discussed tool in astrophotography. I see the RASA design is a variation on this.



#10 mconnelley

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Posted 21 July 2019 - 06:51 PM

Hello:

 

   Many cameras allow liquid cooling for the TEC.  That way the heat from the camera can be brought out of the telescope tube before it get dumped into the air.  Problem solved.

Cheers

Mike



#11 Oberon

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Posted 22 July 2019 - 05:00 AM

Yes it can be built, but does it merit being built?

As a corrected Schmidt Camera, yes. As a large professional Prime Focus Camera, maybe. There are plenty of examples so it must have been in the past. But as an amateur sized Newtonian variant? Sadly no. The compromises involved are all greater than the alternative of using a classic secondary and keeping all wiring, cooling, focusing mechanisms and the like outside of the beam. 


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#12 Jon Isaacs

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Posted 22 July 2019 - 06:23 AM

Yes it can be built, but does it merit being built?

As a corrected Schmidt Camera, yes. As a large professional Prime Focus Camera, maybe. There are plenty of examples so it must have been in the past. But as an amateur sized Newtonian variant? Sadly no. The compromises involved are all greater than the alternative of using a classic secondary and keeping all wiring, cooling, focusing mechanisms and the like outside of the beam. 

So the idea would be to eliminate the secondary mirror all together and replace it with an image sensor + coma corrector tube assembly, aligned on the axis of primary mirror.  I'm thinking this tube assembly would take the shape of a cone, with the sensor near the tip of the cone, and the coma corrector optics located in the wide portion of the cone, where the primary mirror rays are entering.  In theory the diameter of this sensor tube assembly could be very small, possibly smaller than the typical secondary mirror.

 

 

I agree with Jonathan.  The size of the secondary in an imaging scope is of little concern,most imaging scopes have very large secondaries, much larger than a typical Newtonian.  Secondary size is important visually for viewing the planets.

 

Trying to mount the camera, coma corrector etc in the OTA creates numerous issues, they can be solved with a great deal of effort but when all is said and done, it is unlikely the camera will be smaller than the secondary that would illuminate it.

 

Jon


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#13 brebisson

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Posted 22 July 2019 - 08:20 AM

Hello,

 

I personally like the idea (but have never tried it)...

 

HOWEVER, I will (humoristically) have to comment on the fact that what makes a Newtonain a Newtonial is the secondary mirrors. Others had 'invented' telescopes similar to the one Newton build, but without the secondary. This secondary is Newton's real invention there... So, your scope would NOT be a newtonian anymore!

 

Cyrille



#14 gregj888

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Posted 22 July 2019 - 09:42 AM

Pine Mountain converted one of their Cassegrains to this configuration some years ago.  32" if I remember tight and cameras were bigger then.

 

You would want to scale the camera for the scope.   BTW, the scope/camera thing SETI is sponsoring looks to be this configuration, at least close.  

 

For a lot of applications it makes a lot of sense.   EAA grab and go-- 6" - 8" with a Flea of Skyris, even a Raspberry Pi camera.  12" plus, scaled to the camera could be good for a lot of imaging and survey work, though the filters are an issue.  OSC camera maybe, but you loose some of the advantages. 



#15 tim53

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Posted 22 July 2019 - 02:39 PM

Pine Mountain converted one of their Cassegrains to this configuration some years ago.  32" if I remember tight and cameras were bigger then.

 

You would want to scale the camera for the scope.   BTW, the scope/camera thing SETI is sponsoring looks to be this configuration, at least close.  

 

For a lot of applications it makes a lot of sense.   EAA grab and go-- 6" - 8" with a Flea of Skyris, even a Raspberry Pi camera.  12" plus, scaled to the camera could be good for a lot of imaging and survey work, though the filters are an issue.  OSC camera maybe, but you loose some of the advantages. 

OSCs have pretty small pixels these days, so this is less of a problem, so long as the pixels are small compared to the stars at prime focus of the system.  The other possibility would be something like a Foveon dector based camera, but these are not particularly low-light sensitive (they're actually lousy, but you'd be hard pressed to find any actual tests to show that), so you'd need to be using a pretty fast mirror to make it worthwhile.

 

-Tim



#16 darkapollo

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Posted 22 July 2019 - 02:42 PM

PiKon is this very concept.

 

https://pikonic.com/



#17 gregj888

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Posted 22 July 2019 - 09:51 PM

OSCs have pretty small pixels these days, so this is less of a problem, so long as the pixels are small compared to the stars at prime focus of the system.  The other possibility would be something like a Foveon dector based camera, but these are not particularly low-light sensitive (they're actually lousy, but you'd be hard pressed to find any actual tests to show that), so you'd need to be using a pretty fast mirror to make it worthwhile.

 

-Tim

Tim, one advantage would be the extra 5-10% uptick in photons by not having a secondary.  Loosing 50% for OSC negates that from a practical standpoint.

 

OSC even with smaller pixels may alter the shape or location slightly as the mask is likely offset from the stars.  Minor, sure... but the design is kind of splitting hairs.



#18 TxStars

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Posted 22 July 2019 - 11:01 PM

Perhaps a used GSO RC main mirror and a corrector in a truss tube would be a fun project..

A truss tube would make changing filters easier with a mono camera.

 

Not sure why GSO has not made this as an astrograph already?


Edited by TxStars, 22 July 2019 - 11:03 PM.


#19 hukhound

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Posted 22 July 2019 - 11:26 PM

Yes, I certainly agree there are numerous issues that would have to be worked out, and it seems that an open truss would help mitigate or make some of those easier to live with.  In fact, the sensor/cc tube assembly wouldn't have to be located inside the scope tube or truss, it could be projecting forward beyond the end of the structure that is holding it, making it easier to access.

 

Jon, I really appreciate your comments about the larger secondary size use in imaging.  As mentioned in my opening post, this newt-camera idea stemmed from my desire to cover my APS-C sensor with the 100% illumination zone while using a small secondary mirror.  It had occurred to me that putting the sensor directly in the path of the light cone had benefits, especially when trying to sort out back focus and proper position of the CC, all while using a small secondary, trying to get 100% illumination at the sensor, and trying to find a way to keep the focuser from protruding down into the light path.

 

I'm still not clear on this topic of the secondary size and the need for 100% illum zone on the sensor.  Most if not all of the detailed discussion I've found about secondary size is related to visual.  I've seen little to no details about the best sizing for the secondary in imaging.  Would sure appreciate any feedback on this topic, if you could post it to my separate posting here.  Thanks!


Edited by hukhound, 22 July 2019 - 11:27 PM.


#20 darkapollo

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Posted 23 July 2019 - 09:42 AM

Tim, one advantage would be the extra 5-10% uptick in photons by not having a secondary.  Loosing 50% for OSC negates that from a practical standpoint.

 

OSC even with smaller pixels may alter the shape or location slightly as the mask is likely offset from the stars.  Minor, sure... but the design is kind of splitting hairs.

You are just replacing the secondary with a camera so you will still be blocking light in some form.




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