Old 10-03-2008, 09:41 AM
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Random thoughts....An on-axis guider

I was experimanting with LPR and IR/UV cut filters recently and it occured to me that these filters reject photons that are not good for the image but could be used for guiding. I was thinking about a a beam splitter - but for guiding it would look like a flip mirror system with two light paths the imager would recieve the reflected image (front surface mirror so no abberation) and the mirror would pass the IR/UV to the guidecam.

This avoids the issues with guidescopes and OAGs....of course the mirror would have to be very high quality (but then diagonals are these days) and there would be some loss of signal.

So far I have only found one person who has attempted this:


It would seem the problem is that no mirror with the required characteristics (pass uv/ir reflect the rest) is currently made.
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Old 10-03-2008, 08:36 PM
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Interesting. A speculations about another way this could be done would be to build the splitter/guider into the secondary mirror.
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Old 10-03-2008, 10:16 PM
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On of the potential problems that I see is that these filters usually work by having a thin dielectric film that reflects the undesired IR wavelengths and these wavelengths are dependent on the film thickness. (In comparision, the UV cut is by absorbtion, not reflection.)

If the filter is tilted, the thickness of the film which is hit by the light changes and so there is a colour shift - you can easily see this by holding up a visible light filter interference filter to your eye and tilting it - the colour changes. It's also how most room-temperature solar H-alpha filter achieve their 'tune'. Probably, the IR filter at an angle would let through the IR and only the very long wavelength Ir would be reflected.

For optimum results, you would need to choose a filter with a shorter cutoff (for the normal orientation) to compensate. Alternativly, if there was enough long IR to operate the guider, you may need another correctly orientated IR filter to block the shorter IR.
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Old 11-03-2008, 07:20 PM
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Beamsplitter guiding

Without worrying about UV etc a microscope glass slide at 45% will reflect about 8% of the incoming light, this can be used for guiding.
Another option is to use a "proper" beamsplitter ( Surplus Shed is a good starting point) these are usually about 25mm wide and 37mm long and come in various percentage reflection/ transmission ie 70/30 or 50/50 etc.

John H, I'm a little surprised to see the astigmatism see in the web link; beamsplitters are widely used on all sorts of sophisticated optical equipment, so I would have thought the imaging would be very good.

Although I have modified a Meade OAG ( a good starting point as the prism holder is easily removed) and incorporated a beamsplitter ( for guiding my spectroscope) my imaging performance requirements are not as critical as photographic ( I just need to maintain starlight on the slit). Silvio Mete uses a similar system for his cylinderical lens spectroscope.
Setting up and alignment of the beamsplitter is a pain! I had to include 4 X-Y adjusting screws to get mine aligned.
When I can get some observing time I'll take some photos through/ reflected from the beamsplitter and see if I can replicate the astigmatism.
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Old 13-03-2008, 07:52 AM
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beamsplitter no astigmatism!???

I set up the OAG with the beamsplitter on the ED80 and used my old focault tester and a Ronchi screen.
No visible astigmatism....
My beam splitter is only 0.7mm thick. I can only suggest if you were using a very thick beamsplitter and maybe it was being "bent" in the holding device you might see a problem, but I think for general use it works well.
If you have a good OAG in the first place, there's really no need for a beamsplitter. I only use mine due to the difficulty of positioning ( and keeping) a star image on a 20 micron wide slit.
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Old 21-03-2008, 10:53 PM
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You can also get a beam splitter from myastroshop
Hope this is of some use to people.
Regards, Allan
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Old 14-04-2008, 02:19 PM
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To all

I too have been recently considering an on-axis guiding system for my relatively fast newtonians.

The use of a "hot mirror" to transmit the visible and reflect the IR to a guider is what is described on the referenced Kazuyuki web site--a prototype. Turns out that this has been done commercially, see http://www.rcopticalsystems.com/dbs.html While they seem to have abandoned the imager/guider principle for dual imagers, it shows it can be done. But at $2500 USA, about 10X too much. In any case, the RCOptical hot mirror fails to properly handle the Ha wavelength (transmits half, reflects half). And transmitted light will always be subject to aberration that could degrade the image unless corrected.

A better choice, perhaps alluded to on the Kazuyuki site, is to use a "cold mirror" to reflect the visible light to an imager and transmit the IR for guiding. The wavefront error imposed by the beam splitter will surely be much less than the coma imposed by trying to guide off axis at F 4.5 or 6, as in my case. Most commercial cold mirrors suffer the problem of splitting the Ha light at 656 nm, but at least one does not, according to their specs--http://www.toweroptical.com/hot-and-cold-mirrors.cfm These mirrors are said to be only 1/2 wave in flatness, that may or may not be a problem for deep sky astrophotography. But they meet the other specifications!
Regards, Rick
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