I have been reading up on ON-Axis Guiders and I am slightly confused about how they work.
I understand that the ONAG directs the visible light to the imager and the near infared light (NIR) to the guider for guiding.
What I don't understand is if the visible light is all directed to the imager, then how does the guide camera "see" any light for the purposes of guiding / focussing? In the screenshots that I have seen, the guide camera clearly see's the guide star as a visible star.
Does there need to be some config / setup to make the NIR light usable for guiding or is it just that guiders are more sensitive to NIR??
Not sure if I have explained my question well, but hopefully it makes sense..
Yes most digital camera's are sensitive to IR light, to prove this to yourself, grab a point-n-shoot or DSLR(with liveview on) then aim a TV remote at the camera and press a button.
You will see the remote's infrared LED flashing on the camera screen.
IR LEDs operate at around 780nm so the camera's infrared blocking filter is cutting that quite a lot, but enough gets thru for the cam to detect it. Most astro cams as far as I am aware don't have an IR cut filter.
Yes most digital camera's are sensitive to IR light, to prove this to yourself, grab a point-n-shoot or DSLR(with liveview on) then aim a TV remote at the camera and press a button.
You will see the remote's infrared LED flashing on the camera screen.
IR LEDs operate at around 780nm so the camera's infrared blocking filter is cutting that quite a lot, but enough gets thru for the cam to detect it. Most astro cams as far as I am aware don't have an IR cut filter.
Hi Simon, Thanks for the reply. I tested the tv remote with the DSLR and you are correct - the camera does "see" the IR when the remote is pressed.
If this is the case, then for most astrophoto's the IR is in fact contributing to the image captured by the camera.
Presumbly this would mean that when using an ONAG, the final image will be different from that without an ONAG because any IR would not make it to the imaging camera.
My understanding from reading the ONAG information is that the ONAG relies on the light from specific stars which radiate light in IR. Does this mean that these stars will appear dimmer in images captured with an ONAG?
Would there be any other differences when using an ONAG - i.e. brightness of other objects like galaxies, nebula's etc??? Are any of these differences considered a pro or con of an ONAG?
Thanks,
Mark
Last edited by Mark_Heli; 20-06-2014 at 02:15 PM.
Reason: Typo
This was discussed previously on another ONAG thread.
Bear in mind that a IR controller test (I thought the wave lengths were closer to 940nm) doesn't infer that your camera will record that much IR in astrophotography...it's like comparing a searchlight to a candle....
The beam splitter in the ONAG will act similar to an IR cut filter. All stars emit some light right across the spectrum (Planck Curve) so the brightness (for guiding) may vary but still be usable.
These were with my TEC140/PMX setup. I initially found guiding to be more difficult than I expected in the IR. I think this was due to several factors. 1. being a "beginner." 2. perhaps not enough light due to small aperture. 3. perhaps due to the way APOs deal with IR. 4. mount issues with my former MX.
Since getting my TEC180 guiding is a snap due to more energy in the IR reaching the guider. But, I am also not chasing any mount issues! Also, since I'm using an APO I must use an IR blocking filter (built into luminance or LP filter anyway) so losing the IR has made no difference to me as best I can tell.
One great advantage of the ONAG is that one has a HUGE field of stars to pick for a guide star and none of the problems faced by OAG solutions. The one drawback to the ONAG (other than BF) is the fairly crude guider focus arrangement. However, I have recently completely solved the issue after installed a custom made focuser made by Gary Jarrette. No flex, and perfectly reliable and EASY focus with no guide camera rotation. I have replaced the Borg helical focuser with this one as it is far superior. (see photo below. the focuser has c mount threads that go to t-threaded extension tubes)
Another most interesting development is the ONAG "Sharplock" focusing possibility. As soon as I have a TEC focuser solution I will be trying this as it sounds great. It is based on the change in shape of the guide star in IR as focus changes slightly. This shape change can be used to create an automated feedback to the electronic focuser. Theoretically it is possible to go for a whole night without leaving the target to focus. At the moment the software is a bit limiting such that you must use MaxIM or one other unknown package called PRISM. Hopefully TSX will shortly be included though I have no idea if such a solution is even being considered.
In conclusion I think it is a very good and well made system! If you have the BF (SCT or refractor) I personally think it may be better than OAG.
Thanks for the detailed response - much appreciated. In your experience with the ONAG, have you found that there are any limitations in the objects you can image using the ONAG because of the lack of NIR going to the imaging camera?
That custom guide camera holder is similar to what I use. I have a Televue 2X barlow holder installed in the MMOAG guidescope holder.
You can get eyepiece focus rings from Scope Stuff that you can fix in place when it is in focus so next time it slides in and stops at the spot where its in focus.
Although I manually focus my guide camera by sliding it in or out slightly. A bit of a pain but not too hard.
NIR guiding sounds interesting and the real time autofocus sounds appealling.
Thanks for the detailed response - much appreciated. In your experience with the ONAG, have you found that there are any limitations in the objects you can image using the ONAG because of the lack of NIR going to the imaging camera?
There is plenty of IR flux in almost all parts of the sky. If I compare an image through a blue filter and through a photometry I filter that samples in the near IR, the IR image will be brighter than the blue image. Except for the relatively rare very hot O, B or A class stars, all other stars have greater flux in the IR than in the visual. As the cooler stars are the most common then there are always stars visible in the IR for the guider.
This was discussed previously on another ONAG thread.
Hi Ken - Thanks, you are correct. I have found the other thread (below) which answers my question around the impact of the ONAG on the image being captured. Rally asked the question on pg2 of the thread which was answered by Gaston (who I presume is from from Innovations Foresight).
Foresight Innovations claim their ONAG will be less prone to seeing effects. Has anyone used one of these and can give a brief review?
Is it a good product and does it give better results than a regular OAG?
66mm of back focus and having the camera off to the side rather than at the end seem a minus (does it cause balance problems?).
Greg.
Greg, I recently purchased one, but as I am still setting up my new pier and mount, haven't yet gotten around to trying the ONAG in anger.
Just a small clarification regarding the camera position; the imaging camera actually screws into the "top" of the unit, with the guide camera attaching to the rear via a crude focusser. Obviously the whole unit can be rotated in the attachment to the scope, so yes, you could have the imaging setup to the side, but I imagine most would set it to be at the top, for the better balance.
My plan is for DSI II mono guide camera at the rear, and QHY 8 Pro OSC or QHY9 Hlrgb with filter wheel on top. Happy snap of initial trial attached.
Might do a review when I get it all together.
Mike
Thanks Mike Good point about being able to rotate the unit.
Greg.
Yep. Couple of points that came up after I posted; "side" actually depends on your point of view and RA position, so top side can easily change to L/RHS during a long set of exposures anyway!
Just some points on my Initial impression of the unit.
Pro: Beautifully made and comes complete with screw-on adaptors and spacers for T thread and SCT attachments, no compression clamps.
I find the NIR guiding concept technically elegant, as it gives you the full scope FOV to choose your guide star from. In a quick test, the DSI saw plenty in NIR. Obviously I had removed the IR cutoff filter from in front of the sensor. With an even larger sensor, (Lodestar?), there would be even more stars.
Con: I feel that they could have done without the fancy red contraption on the back that allows you to move the guide camera (and small sensor) over the entire FOV of the scope. It may have been considerably cheaper to make if that luxury had been omitted/optional extra, and perhaps a little more effort gone into improving the focusser mechanism, or lack thereof.
I may be wrong in the above, I guess time and use will tell.
Last edited by mldee; 22-06-2014 at 12:12 PM.
Reason: added screw-on connection info and clarity
For a while now these have been on my must buy list (but they fall just below my must travel to Europe priority) so most likely next year.
After a lot of reading the only Cons I see are:
1. The NIR guide camera is seeing light bounced off a single guide mirror (not a double) so any completely focused star will be oval not round. This is physics in action - however guide programs are very good at calculating the centroid of a bloated star - so this should not matter other than aesthetics.
2. The NIR guide camera attachment lacks even the simplest of threaded helix focusers, making locking in exact focus on your initial set up (presuming you aren't changing camera all the time) overly laborious. This means with a precision computer controlled focuser and a Bhatinov mask I can get my main camera focused within microns, but for the guide camera its push it in and out with clumsy fingers, over a minute distance trying to get it perfect. Too me this is a silly design flaw that Foresight Innovations could have addressed at very, very low cost a long while ago (by simply adding a threaded focusing tube and a lock nut)!
The Pros:
1. You have a lot more stars to focus on and the latest MaximDL v6 or the older version with an add on allows multi-star guiding (whereas say PHD only allows one guide star at present). Developers of multi-star guiding routines published data a few years ago that that showed multi-star guiding had far superior (my words) results in that you are chasing the seeing only half as much when you average the movements of 5 or 6 dispersed stars in your field of view versus guiding of a single star! I think of that as Guiding is twice as good!
2. COMA - avoided. I use an old Celestron Carbon Fibre SCT - great for holding focus but at the edge of a field - where an OAG must sit there is coma starting to be present on half the guide chip (stars look like tears on half of my guide cameras screen). An ONAG being in the centre of the field of view should have no COMA - so very round stars (apart from the single cold mirror design aspect - which should be neglible).
3. As mentioned above Sharplock - very smart idea for those using MaximDL!
I would love to see more user reviews on the ONAG - particularly as several members of IIS have one now!
With regard to #2 in your post you probably saw my earlier post about the Gary Jarrette custom focuser. It is far better than any helical focuser given zero flexure and very little back focus added to the ONAG. The helical focuser I have (the Borg 7315) positions the camera pretty far back whereas the Jarrette solution allows the camera to slide fully into the focusing mechanism. Yes, some BF is added but not very much.
Your proposal to just use a threaded screw/lock nut is actually possible with the supplied focuser on the ONAG. The big drawback is that travel is quite limited but more so that the camera rotates and the usb cable gets twisted. And if like me you like to have the chip with X=RA that is impossible. I've told Gaston about this Jarrette focuser and I hope he will point customers towards it as an almost ideal solution.
Just a small "correction" to #1. The IR light actually passes through the mirror. The distortion, as you say, isn't a problem for centroid calculation and is actually necessary for Sharplock to work. It does make for odd looking star profiles, however.
Multistar guiding with an ONAG, an AO unit and using Sharplock would have the potential of giving the sharpest stars and images around.
It may be something I would look to put on my CDK that needs help battling the seeing at home at times.
The point of lots of guide stars is a little overemphasised in my experience of using a MMOAG. Very occasionally I have to image off centre to catch a guide star but usually its not an issue at 3 metre focal length and an SBIG STi. The new Lodestar X2 may make that even less of an issue with its extra sensitivity.
Con: I feel that they could have done without the fancy red contraption on the back that allows you to move the guide camera (and small sensor) over the entire FOV of the scope. It may have been considerably cheaper to make if that luxury had been omitted/optional extra, and perhaps a little more effort gone into improving the focusser mechanism, or lack thereof.
I may be wrong in the above, I guess time and use will tell.
Mike, I'm sure that after using the ONAG you will change your opinion about the stage. Even with the Lodestar or ST-i chip size there are absolutely some targets where you need to move the stage to find stars to guide on. And, of course, this problem gets bigger the longer the focal length. What I really want is a motorized stage so that hunting stars can be automated or at least remotely controled.
Mike, I'm sure that after using the ONAG you will change your opinion about the stage. Even with the Lodestar or ST-i chip size there are absolutely some targets where you need to move the stage to find stars to guide on. And, of course, this problem gets bigger the longer the focal length. What I really want is a motorized stage so that hunting stars can be automated or at least remotely controled.
Peter
Peter, thanks for the input, I'm yet to actually see the additional effect of sliding the guide camera around on those nifty steel rails, nor have I done more than a quick fiddle with the "focusser", such as it is. I'm lucky to have a close friend whose son does CNC work, so will try and come up with a version of the rear focus design mentioned earlier, it looks quite do-able.
Being retired, I'm on a pretty tight budget, but always like to get tools that seem good value, even if a bit expensive, so hoping the ONAG will be a good accessory for my modest range of scopes. RC8 is my longest focal length. Almost ready to run some trials, starting with the QHY8 and Megrez 80 EDII to keep it simple.
Almost ready to run some trials, starting with the QHY8 and Megrez 80 EDII to keep it simple.
I'll be curious to see how this works with the Megrez 80 EDII. I had issues using it with my TEC140 as mentioned earlier. I didn't have a good focusing solution and and with my ST-i I had to take rather long exposures, even binned 2x2. And it might have been theSkyX camera add-on which was new to me. These issues have stopped now using my TEC180, but I've also learned a lot in the past year...mostly the hard way. Let us know how it goes. I think this was designed with larger apertures (and longer focal length) in mind which would deliver brighter images in the IR. Normally I doubt that an advantage would be seen (over using a guide scope) in such a short focal length setup for either OAG or an ONAG solution. But, as a test it will be interesting. I'll be very curious to see how you feel about getting good focus!
OAG always leaves guide scopes for dead. I use both. Sometimes I get great guiding with a guide scope, other times not so great. Apart from the adapter universe needed sometimes for various camera/scope setups with MMOAGs/OAGs it is a very accurate guiding tool.
