G'day Greg,
Quote:
Originally Posted by gregbradley
I would question the assumption that guiding in the near infrared makes an improvement in guiding.
I use a MMOAG guider which is very effective. I read this claim on the ONAG website.
I bought a 1.25 inch infrared filter that is at the same wavelength as the ONAG. I installed on my guide camera (luckily it screwed onto an adapter I had).
I used it a few times. I noticed no difference in guiding errors or improved guiding results at all.
I have 2 infrared filters at slightly different wavelengths. Neither made any difference I could detect except make the guide stars a tad less bright (not massively though).
Anyway that was my experience. Sorry to be negative but its a claim not backed up by my testing at least.
Good polar alignment, good balance, a good T-point model that corrects for slow flexes, a good PEC are far more important in my opinion than any slight gain if indeed there is one from near IR guiding.
Even selection of the right type of guide star can make a huge difference. The first thing I do when I get larger than normal errors is to select another guide star. Often I see a massive drop in errors. Too bright a guide star that is large is not ideal.
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You needn't apologize for being negative. One of the primary motivations behind posting the ONAG review was to stimulate some discussion on guiding in the near-infrared, so I appreciate your input.
I also have a near-infrared pass filter here, the ProPlanet 742 which I purchased from Astronomik a few weeks ago. Don't get me wrong -- it is a great filter as I believe all of Gerd's filters are, but like the filter that Alistair mentioned in his post, a quick look at the full transmission profile reveals that it's nothing like the ONAG cold mirror.
Firstly, neither the filter Alistair linked nor the ProPlanet filter I just mentioned block all wavelengths below 750nm (the ProPlanet 742 obviously ending transmission at around 742nm). Alistair's filter transmits from around the 665nm mark.
Secondly, most NIR pass filters transmit small but significant amounts of visible light at various points along the spectrum.
Lastly, although the Earth's atmosphere prohibits UV wavelengths lower than 300nm, these NIR pass filters transmit the remaining UV light between 300nm and 400nm (around 70% at 375nm in the case of the ProPlanet 807).
This is highly significant because of the amount of UV (short wavelength) light coming from stellar sources and the relationship between Strehl Ratio and NIR/SWIR (long wavelength) light.
http://en.wikipedia.org/wiki/Strehl_ratio
Furthermore, the quantum efficiency of CCD sensors at these UV wavelengths is just as high if not slightly higher than in the NIR, which means that the UV light collected by your guide camera is going to cancel out any benefits you would expect to see from NIR alone.
I agree that all of the factors you've listed are very important for good guiding results, particularly polar alignment and properly balancing your mount to minimize backlash, etc.
I don't have a high-end mount and when I tested the ONAG I made a point of not spending 2 hours refining my polar alignment as I have done in the past, it was all very much a 10 minute affair and yet I achieved significantly better results over what I have previously. The PHD2 graph I attached to my article shows an RMS Error of 0.34, which I recorded after leaving PHD to guide for a few minutes. Ordinarily, after spending arguably an excessive amount of time refining my polar alignment, my RMS Error would be around 0.5 on a good night and anything as high as 0.7 to 0.8 on nights with average to poor seeing.
To conclude, your comment about selecting the right guide star hits the nail on the head -- a bright star is more than likely not a spectral class M object and is emitting high amounts of UV. UV light is more susceptible to the distorting effects of the atmosphere and will result in a significantly lower Strehl Ratio and thus significantly higher RMS during guiding. The ONAG simply builds on this by clipping out the UV and the visible.
Cheers,
Brett