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Post Laureate

Reged: 10/19/10

Loc: Rapid CIty, SD
Re: The end of OAG? new [Re: Peter in Reno]
      #6153411 - 10/23/13 10:47 AM

Peter I agree. I need to give an OAG more time. Once again I bought the cheaper one and I think I'm learning why that's a bad idea.

I think an ONG would be neat but I worry about putting my camera sideways, my cables are already pretty maxed out.

Sounds like based on the last post there are issues with some of our favorite techniques such as dithering too

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Peter in Reno

Reged: 07/15/08

Loc: Reno, NV
Re: The end of OAG? new [Re: Raginar]
      #6153495 - 10/23/13 11:36 AM

Hi Chris,

ONAG (On Axis Guider) is a great device. It's really like guiding with a guide scope because it has very large FOV for guide camera. The major drawback is it has enormous minimum back focus of 66mm which will never work with my scope/equipment. I ordered it once but cancelled it due to huge back focus.

High quality OAG helps. Also high sensitivity guide camera for OAG is highly recommended but your signature shows you already own a Lodestar and that's good. I used to have Hutech OAG with Helical focuser at guide port. It was a really nice device. Astrodon MMOAG is also very good. Both Hutech and Astrodon OAG's are not exactly cheap. I think SX filter wheel combined with SX OAG would work well but it may not work well with non-SX cameras due to mis-parfocaling between guide and main cameras. SX FW/OAG is designed to work with SX cameras. SX cameras are well built (I owned SXVR-M25C and like it very much).

People who use OAG for the first time usually get confused about parfocaling guide camera with main camera. They sometimes make a mistake parfocaling under the dark sky for the first time. I did that myself. This forum suggested to me to parfocal during the day by aiming for a telephone pole or street sign pole far away. Once daytime parfocaling was done, all it needs is a tiny tweak under the dark sky.

What scope are you trying to use OAG? Is it 12" LX200 as shown in your signature? If you want to image at F/10, then ONAG would work. I am not sure how forgiving Meade/Celestron focal reducer is regarding to back focus (maybe 95mm - 120mm). You don't need to install the camera sideways, you can mount it on top. Also, rotating ONAG is not necessary to find guide stars. I highly recommend ONAG for equipment that allows long back focus. Lodestar is perfect for ONAG since it can see NIR pretty well. ONAG is also pricey. I never own ONAG but there are many satisfied ONAG owners and I have not read one single negative comment except for long back focus (that's me). Do a search for ONAG in this forum (DSLR and CCD).


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Photon Catcher

Reged: 07/20/07

Loc: California
Re: The end of OAG? new [Re: Peter in Reno]
      #6153820 - 10/23/13 02:33 PM

The back focus is a big issue many times for OAG. If you are using a focal reducer or field flattener it can be especially problematic. Many of the good OAG systems require a lot of back focus and there may not be enough distance between the flattener/reducer and the CCD chip to allow an OAG between them. Some new ones are thin, but may not work as well as the big ones that have big pick-off prisms. Some are also made with soft materials and are just not good enough for me to support the weight of a camera. I had a brand name THIN OAG that was made of some kind of pot metal or something so soft that the set screws would dig into the material when they were tight enough to hold things. That sucked!

This has been my biggest issue with the OAGs along with the lack of guide stars in areas of the sky outside the Milky Way where there are not so many stars next to a galaxy or whatever. In the Milky Way there are usually stars a plenty and not an issue.

But if you want a specific framing of an object, then many times the OAG will not see a good enough star to guide with at that angle. If you are not too particular about the framing, then there are always stars available.

Also, when you do the meridian flip and you do not have a rotator, then the star you were guiding on is no longer there, that can be a big issue if the OAG does not have a fairly large FOV so that a star will fall on it.

So OAG is good, ONAG is good too if it works for your setup. But there are complications and considerations when you decide to use one or the other.

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Vendor (Innovationsforesight)

Reged: 03/01/10

Loc: PA - USA
Re: The end of OAG? new [Re: blueman]
      #6393826 - 02/24/14 08:38 AM

My name is Gaston Baudat, CTO of Innovations Foresight.

I think there are some differences between OAG and ONAG features that should be explained in this thread, since the ONAG technology is relatively new.

With the ONAG you do not need a rotator and therefore you can reuse the flat frames as long as you do not change your imager optical train.

The ONAG allows on axis, as well as off axis guiding, the former is useful to guiding on the target (asteroid, comets, ...). The ONAG works with AO units.
The ONAG uses NIR which exhibits less seeing effects.

One important aspect of using an ONAG (or an OAG for that matter) is the fact that you work with your main scope, which usually has the largest aperture D.
From a star, therefore a guide star, standpoint the key figure of merit is D which defines how much starlight (energy) is gather by the scope. The numerical aperture F is relevant for extended objects, not for stars which remain points (Airy disks) at any magnification.

The energy received from a star increases with the square of D. For instance a 8" scope gathers 6.25x more than a classical 80mm guide scope, which is about 2 magnitudes. A 11" is 3 magnitudes more.
Depending of the star temperature the ONAG may loose from a half to one magnitude (using NIR), but at the end D is making all the difference.

see for further information.

The ONAG technology offers an unique new possibility, real time auto-focus while auto-guiding. This technique named SharpLock (SL) is available for beta testers.

See for further information

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Reged: 07/26/09

Loc: Netherlands
Re: The end of OAG? new [Re: blueman]
      #6394172 - 02/24/14 12:30 PM


The back focus is a big issue many times for OAG

I have a newton telescope, for imaging I use the ASA 3" Wyne corrector, it has a backfocus of only 58mm.
The canon DSLR takes 44mm, the camera adaptor 10mm, left over for a OAG, ONAG...4mm!
Even with a cooled CCD camera with external filterwheel the space for a OAG is only 18 mm.
furthermore the OAG must be large to fit the large 3" wyne corrector (65mm clear aperture).

'I want to have a CCD camera where you select any pixel(s) in the sensor for (AO) guiding'

Garret van der Veen

Edited by garret (02/24/14 12:52 PM)

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Vendor (Innovationsforesight)

Reged: 03/01/10

Loc: PA - USA
Re: The end of OAG? new [Re: garret]
      #6394825 - 02/24/14 07:15 PM

I agree that OAG and ONAG are not for every scope.
Yet SCT, RCT, and many refactors/reflectors have enough back focus, new scope designs usually consider and provide more back focus to allow for more equipment in the optical train, such as AO units.

Newtonian are challenging in term of back focus. Difficult to fit an OAG/ONAG, AO unit, filter wheel, camera, (or DSLR), and maybe a rotator.
My guess is that even with a guide scope and differential guiding the extra back focus required for an AO unit may be just too much.

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Post Laureate

Reged: 12/09/09

Loc: Merced CA
Re: The end of OAG? new [Re: garret]
      #6395230 - 02/24/14 11:24 PM

I'm having great sucess with the new guide scope setup from AP.
The rigidity of the setup has worked perfectly with my idk at 2160 fl.

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