What would the up/downsides be of a newtonian telescope without secondary mirror?

[pipnina]

Naturally, a secondary mirror isn't required for light rays to reach focus, but for visual observing even a 12" scope wouldn't get any light because the human would be in the way if it wasn't there. But for imaging with scopes on the larger end of the scale (maybe 12" or 14" upwards) could it be optically superior to leave the secondary mirror out of the equation and attach camera equipment where the secondary would normally sit?

I'm not particularly well-read in this area but my first thought would be that it would allow larger reflecting telescopes to achieve a wider field of view, since the secondary is a big choke point for light rays, at the expense of more difficulty focusing? Is it also possible that for large telescopes, the size of a focuser and camera could be more minimal than a secondary mirror resulting in less obstructed area?

It's pretty interesting to me, I wonder if any telescopes like that are in use?

Cheers.

[alanjgreen]

Thats how "fastar" works with Celestron SCTs. The secondary is removed and the camera is fitted where the secondary was.

Seems to me that it would only be a matter of time before you drop something onto the Primary Mirror and wished you had not bothered.

Main thing is that the "focal length" of the scope takes into account some horizontal light path from the secondary to the eyepiece, if you remove that from the equation then you need to extent the position of the camera out beyond where the secondary is to reach the "focal point".

[Cornelius Varley]

Off-axis reflector.

[SilverAstro]

Herschelian telescope. For largish Fratio the mirror can be tilted to bring the focal point out to the side of the tube. William Herschel did this with a 48" f/10 mirror.

Schiefspieglers as well.

[vlaiv]

Actually it is feasible, but care must be taken to account for following:

- must use narrow body camera (so smaller aperture scopes like 130mm or 150mm would see degraded performance compared to secondary mirror because camera body is larger than that)

- probably helical focuser should be used in front of camera - small format - no electronic focusing.

- Tube should probably be longer, well at least longer for half diameter + regular back focus distance. Focal position is not at secondary but further away. Plus side to this is that stray light will have less impact on image quality (no scatter behind secondary mirror).

- Can't use filter wheel - some sort of small format filter slider will need to do - smaller or equal to camera body size (or otherwise will cause larger obstruction). So no electronic filter switching.

- Plus side to this setup (Apart from better stray light control) is greater light grasp - no additional 90-something % secondary mirror to cut down light throughput, better overall strehl ratio of scope - secondary is not ideally flat. Probably less dewing issues. No vignetting caused by undersized secondary. Apart from that image definition will stay the same.

[Louis D]

Sure, it's been done before.  The 200 inch Hale telescope had an observing cage at prime focus where photographic plates could be placed.

[vlaiv]

8 minutes ago, Louis D said:

Sure, it's been done before.  The 200 inch Hale telescope had an observing cage at prime focus where photographic plates could be placed.

Oh, I want a go on that ride!

[Peter Drew]

I went a long way down the path of making a 18" F3.6 reflector without a secondary for use as a video telescope as the cameras are smaller than the required secondary, the whole lot was mounted on a stretched 12" LX200 fork mount. Somehow the thought of not being able to use it visually, potential problems with heat from the camera and finally that the light loss these days with special coatings is probably less than that of the secondary obstruction, I've put it firmly on the backburner for now.  

[pipnina]

1 hour ago, Peter Drew said:

I went a long way down the path of making a 18" F3.6 reflector without a secondary for use as a video telescope as the cameras are smaller than the required secondary, the whole lot was mounted on a stretched 12" LX200 fork mount. Somehow the thought of not being able to use it visually, potential problems with heat from the camera and finally that the light loss these days with special coatings is probably less than that of the secondary obstruction, I've put it firmly on the backburner for now.  

Hmm, I can understand not being thrilled with a photography-only scope, and not caring greatly about small % of light loss. How would heat affect it? Active cooling causing air currents? Warming the metal spider veins and causing the camera to move?

[Peter Drew]

I have two Watec 120n integrating cameras. Both create heat that might disturb the image, one of them is cooled. No practical experience as yet with a no secondary system, just taking note of what others have said in the past.