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Onag
Let me help if I could.
I am the IF's CTO and following this thread's questions and comments I would like to answer/precise some aspects of the on-axis guider ONAG for clarification.
1) Back focus:
The ONAG back focus (BF) is 66mm (2.6"), which is about the same than the MOAG. The ONAG has been designed for medium to long focal, such as SCTs, it has been used successfully with various scopes like the Hyperion from Starizona, or Takahashi refractors.
The ONAG comes in standard with 3 T-thread extension tubes (8, 16, 32mm), as well as a 2" and SCT adapters to attached to scope visual backs.
SCT scopes are designed for a specific imager focal plane distance from their visual back to reach nominal performances (Focal, F number,...). For instance, according from Celestron, a C11 EdgeHD at prime focus requires a BF to the imager focal plane of 146mm (9.24"). Of course each scope as its own requirement for that matter. The ONAG BF will be part of this optical path, you may have to add T extension tube(s) associated with the imager BF to be close to the optimal BF. The ONAG BF is not likely to be the limiting factor in such case.
2) Dichroic mirror
The ONAG dichroic mirror, or beam splitter, BS reflects the light to the imager, like a star diagonal would do, while the near infrared (NIR) goes through the BS.
At 45 degrees of tilt there are unavoidable optical aberrations for the guider (geometric and chromatic) due to the thickness of the BS (3.3mm). Although this would not be acceptable for imaging, it has no significant impact for guiding since tracking software uses centroid algorithms. Only nitrocellulose pellicle BS would have a minimum of distortions in transmission, but they are very expensive and sensitive to vibration, microphonic effects, as well as temperature shifts.
The BS reflection is near perfect and since there is no refraction involved there is no optical aberrations either. Dust on its surface is less an issue than dust on the imager window since the later is much closer to the CCD ship. In any case the flat frame processing will solve such problems.
3) Test images
The IF's website presents results for a 38 minutes guiding session. Each sub frame was 1 minute exposure bin 1x1. The last image was made by stacking, without any alignment or registration correction (Maxim DL), the first and last sub-frames, 38 minutes apart. This allows to clearly see and quantify the tracking error, if any, using a 3D star profile or centroid for instance. The profile will exhibit a double peak in case of tracking issues. Stacking the 32 sub-frames all together will make this observation less accurate, and more subjective. In this current case it would not matter, since both images (first and last stacked) or (32 sub frame stacked) are the same, the error is too small to be seen.
4) Near infrared (NIR>750nm) for guiding
CDD/CMOS unfiltered sensors have their maximum sensitivity near deep red and NIR. The star spectrum is function of its color temperature, often given by its spectral class. More than 75% of the main sequence stars belong to the M class (<3700K). Let's assume to simplify that the sensor sensitivity is uniform across the range from 400nm to 900nm, which would be pessimistic for the NIR contribution. The visible range would be defined from 400nm to 750nm, the NIR from 750nm to 900nm. For a class M there is still about 70% of energy in the NIR in comparison with the visible range energy.
Since the ONAG uses all the scope aperture (F number) it will typically collect more energy than a pick-up prism used in most off axis guider OAG systems.
5) IR blocking
The ONAG requires a IR blocking filter for the imager. Color camera, DSLR, have a built in IR/UV filter. For monochrome cameras, with filter wheels, the LRGB filters do the job, if not an IR blocking filter should be added. Of course the guider must be free of any IR blocking filter, which is the case for most monochrome cameras.
6) Flexure
Like OAG the ONAG solves the differential flexure issue, while providing up to 1.3 arc-degrees (2m focal length) FOV to locate a guide star, This is done by using the integrated X/Y stage for on and of axis exploration of the FOV. This feature alone increase dramatically the likelihood of finding a guide suitable star, versus OAG.
The ONAG has been designed to be light and to minimize the mount extra load, it weights only 770g (1.7 lb).
I hope the above information will help to understand/clarify some of the ONAG aspects evacuated in this thread.
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