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On- axis guiding?


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Has anyone got experience of on-axis guiding, especially the setup from Innovations Foresight? Not cheap but wonder how it compares with offaxis guiding when using bigger scopes/smaller fov where it is difficult to do multiple star guiding

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9 hours ago, kmaslin said:

Has anyone got experience of on-axis guiding, especially the setup from Innovations Foresight? Not cheap but wonder how it compares with offaxis guiding when using bigger scopes/smaller fov where it is difficult to do multiple star guiding

Hi, I 've used the ONAG gadget from IF on my SCT. It works fine, plenty of guide stars to choose from. But it takes alot of backspace, around 60 mm if I remember. Since the SCT I've moved on to a 8 inch refractor, and my field flattener has a backfocus of 90 mm, and I can't fit the ONAG+FW+camera in those 90 mm.

With the refractor I use Optecs Saggita 3 inch  OAG that has a decent 12 mm prism and solid build quality, and I have no problems with guide stars (ASI 174 mini camera).

Overall it is a nice solution specially if you image at long FL, where guidestars could be rare. 

Regarding the guiding in Near Infrared, from what I have read and experienced ,I don't see any important, earth shattering benefit over guiding using the full spectrum. Also the quantum efficiency of CCD/CMOS guide cameras is lower in the NIR. For example my ASI 174 mini has 20% QE at around 800 nm, that means my guide exposures need to be longer to get a decent SNR for the guidestar. Since I have a Mesu mount 5-10 sec exposures are just fine. 

If your mount requires shorter guide exposures because you need frequent corrections, I suggest using IF own guiding software (Skyguide), because it can deal with lower SNRs in the NIR, much better than centroid guiding algorithms like PHD2.

A last note, if you go this route make sure you have enough backfocus space and search for a guide camera with higher QE in the NIR. 

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Thanks Dan most informative. Still reviewing this as possible option as usually guiding is good for my SCT, but galaxy season can get hard work finding guide star. I use Starlight Xpress cameras as guide cameras as their QE etc is excellent compared to previous 120/178mm camera. Any more advice would welcome 😀 

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I didn't know these products existed! I knew that professional observatories use systems like this all the time (such as the large binocular telescope's adaptive optics system that splits the sodium line from the image into the AO wavefront sensor), so they might not get that pending patent approved unless the patent system is as broken as I suspect it is.

I had thought of an idea where using three dichroics and the right arrangement of angles for the cameras, three mono cams could be used to create "one" shot RGB or SHO, allowing a single telescope to collect colour images at full efficiency. Seemingly based on this the camera behind a filter would sadly see astigmatism, so perhaps it can't work out.

It would also be interesting if this kind of system can work well, if a full-frame sensor could be used at video speed in combination with one of those tip/tilt adaptive optics systems available to us. Since getting stars from within the field would improve their function and help us get tighter stars in our images.

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IF do 2 versions of the on axis unit, one for aps sized sensors and one for full frame approx 50mm diagonal. As for video can't see why it doesn't work the same. Also most cmos sensors have very good ir sensitivity and usually needs to be cut off to stop star bloating. Oh and here is a fun article about clever multi-split imaging 

https://link.springer.com/article/10.1007/s12567-021-00418-9

 

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  • 3 weeks later...

Hello,

I’ve been using the ONAG now for 5 years or so with my Edge HD11 at prime focus. Typically I get PHD2 rms figures of 0.5”, but that is a much due to the mount as the ONAG. 

Guiding works very well. In general you will not lack for guide stars. I would recommend the use of a CMOS camera as the near IR view of stars can make them quite faint and CCD cameras with their fixed gain can struggle. Having said that I used a Starlight Xpress Ultrastar for years, but then I have a mount that can image without needing corrections every few seconds so I just upped the exposure time. You can also bin.

I would recommend a CMOS camera as you can wack the gain up to max to help as well as bin the pixels. I have just change from the Ultrastar to a Player One Apollo Max: 9um pixels and a much larger field of view compared to the Ultrastar. 

The backfocus can be an issue, but (on an Edge HD at least) the distortions that come from being too far back work their way from the edge of the field in, so it depends on how far out your are and how big an imaging camera you are using. A full field camera might have issues, a 1 inch chip, not so much. 

It is one of my most favourite pieces of astro kit. Guiding the Edge HD with it has been easy and the ability to focus continuously in real time is absolutely great. Expensive, yes, but worth every penny as far as I’m concerned. 

Cheers, Ian.

 

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