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Placing the sensor at prime focus for electronically-assisted observing

Martin Meredith

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On another thread it turns out several of us are thinking about -- or getting on with (not me  :smiley:) -- placing a camera at the prime focus of a reflector and doing away with the secondary. This might be an attractive option for the kind of small-sensor cameras typically used in this forum since their cross-section is (sometimes) also fairly small. 

These are what I see to be the pros and cons of the idea. I'd be very interested to hear others' views and on possible designs.


1. If the cross section is smaller than the secondary, we would expect an increase in contrast as well as a smallish increase in light-gathering.

2. One fewer reflective surface means fewer lost photons.

3. Only one collimation axis rather than two. Ought to be easier to collimate fast scopes, all other things being equal (which they won't be of course).

4. If camera is lighter than secondary then expect less sag and perhaps more repeatable collimation.

5. Possibility to decrease weight by removing the side-mounted focuser. Also, no weight hanging off the side means easier to balance scope. 


1. Mechanics of collimation may be non-standard

2. Longer tube

3. Collimation probably has to be done with camera in place i.e. difficult/impossible to use autocollimators

4. Depending on design, adding filters, focal reducers, coma correctors etc could be tricky.

5. Camera lead has to be pretty thin not to introduce artefacts

6. The scope becomes a dedicated astrograph (although with some designs it might be more or less easy to swap between eyepiece and camera use)


1. Camera more or less likely to get dewed up than secondary mirror?

These are just my uneducated musings -- I'm likely to have missed the main benefits or challenges  :smiley:



Edited by Martin Meredith
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That's all very interesting. But instead I will talk about the Watec camera..... :cheesy:

Only kidding... good luck with the project. Don't see the attraction myself. The secondary mirror does a very good job of bringing the focal plane out the way of incoming light, so you can put as much clobber there as you wish and its not casting a shadow. Regarding getting rid of the secondary, quoting Suitor: "as long as the obstruction is kept inside that fraction [20-25%], the image closely approximates that of an unobstructed telescope." And a camera might well weigh more than a secondary mirror. I know of someone who has the kind of setup you refer to. Hoping to take a trip to see his obs sometime, will be interesting how it works out in practise...

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Your design comes very close to the Faststar/Hyperster arrangement for SCT's. Except that you don't need the elaborate two-stage corrector system, if you use a parabolic mirror. Maybe just some coma corrector at f/3.6.

SCT's are focused with their moving primaries. The correct focus range is quite narrow for the fast f/3/6 optics. Are you planning to place the camera inside some helical fine focuser? Obviously, you will want to keep the obstruction as small as possible.

I don't know, if you plan to use aluminum or fiberglass bars as trusses. The lower thermal expansion coeff of fiberglass would allow for a narrower the focus adjustment range.


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Thanks for the suggestions/opinions. Yes, I'd seen the RASA and Hyperstar but in this case as you say Dom I don't need any corrector (and I'm happy with the negligible amount of coma I'm getting at present with the small sensor on the Lodestar).

If I can get away with adjusting the primary only it would be perfect, meaning I could use a fixed and very rigid camera support. However, this implies that focusing and collimation (of the single axis) become one operation. Hmmmm…sounds tricky but then instead of 6 or more adjustments + focus there are only 3. I imagine CCDInspector could help here? 

In any case, nothing is irreversible. For the Quattro it is simply a case of removing the spider and secondary and attaching the new structure perhaps using the same holes as held the spider.


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Regarding the issue of the cable creating artefacts...

My understanding is that the thickness of the cable is not particularly relevant.  A thicker cable will reduce the amount of light reaching the primary which may reduce contrast, but it's the edges that cause diffraction artefacts and the edge lengths of a thin cable and a thick cable are the same.  Just as with the spider I think you should be able to minimise the constructive effects of the diffraction by using a curving cable path at the expense of contrast loss due to a greater obstruction area.  I'm not 100% certain of this, but it's my understanding from what I've read.

Since people have been talking about cameras mounted in the place of the secondary I've thought it might be quite neat to either run the wires inside the spider arms, or to have them applied as copper tracks (say) onto the spider arms.  Probably tricky to do either though.


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The sensor will need to go at the focal plane of the mirror... Which is currently out the side of the tube, so you would need to mount it quite a way above where the secondary is currently located... So not easy to have a reversible mod. I agree if you were to custom make a fast big reflector it would be a great tool. Getting the tip and tilt of the sensor could be a right pain to achieve and I am not sure that you lose much by mounting it on the side... I can collimating all those degrees of freedom with a laser in a few minutes max!

I did see a post where someone had successfully done what you planned. So it isossible and the results were very good.



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The secondary of an 18"  f/3.6 is quite large. With the Lodestar you would have a minuscule central obstructions, which would have positive effect on contrast and fine detail.

Peter is right, the camera should be placed about 9" further away from the primary compared to the current location of the secondary. I assume that this is a truss Dob, in which case all that are needed are new longer truss poles. And yes, the camera will need some facility to enable tip and tilt adjustment.

I, personally, would put the camera into a Hutech/Borg 1.25" non-rotating helical focuser. That way, it would be easy to take the camera out for other uses, it could be replaced with a laser for the purpose of collimation and it would be easy to focus and to adjust focus, if necessitated by the thermal expansion/contraction of the trusses. For solar system work, the Lodestar could be replaced with a high resolution Superstar.

Sounds like a fun project. Not entirely trivial but doable with modest investment.


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Thanks James, Peter and Dom for further suggestions. While Peter D's project is for the 18" f/3.6 mine is for the 8" f/4 Quattro. So the mod I'm conceiving would be reversible if you think of the new camera holder as being similar to a light/dew shield, except that it would be rigid (anyone has a spare carbon fibre tube off-cut ? ;-) and firmly attached to the existing tube making use of the holes where the spider adjustment bolts currently live. Then the new 'spider' would contain a short tube to hold the Lodestar in my case, ideally with narrow cross-section bolts to hold it in position and make initial (once-only?) focus adjustments. These would line up wit h the spider vanes to reduce diffraction. 

Having looked more into CCDInspector I'm getting becoming convinced that it would be up to the job of focus+collimation using the primary adjustment bolts only. Shame they don't make a Mac version.


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Hi Martin - in the distant past I've built many scopes like below, including 26cm f/4 Dyson optics reflector with CCD cam at first focus, but that was before the market was swamped with ready-made gear to splash-cash-on !  Then there was no other way.   Mirror coatings could be relatively poor [compared with today] and omitting the secondary gave a photon boost.  To support the CCD camera [TI 211?] I used a non-rotating helical focuser from a photo close-up devise and it worked well.  I made my own 'spiders' too.

However today I'd adapt ready-made OTAs like my Lensless Schmidt camera.   Good luck if you follow this through. :police:


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