offaxis guiding - has anyone actually used one of these

[h0ughy (David)]

http://www.innovationsforesight.com/

would be used in the 2.3m dome with the 14" meade, dslr etc?

[Alchemy (Clive)]

No but the main consideration would be back focus flexibility including field flatteners etc, I think it would work well with those GSO RC scopes as they have plenty of space out the back to the imaging equipment. Don't know how much room you've got there on the 14.

[gbeal]

No from me too, but a natty principle. I agree with Clive, back focus is one of the issues. Certainly for the bog-standard ~55mm I doubt it will work, but any of the R/C scopes I have seen seem to have back focus to burn.
Did you glean a price?
Gary

[h0ughy (David)]

Originally Posted by gbeal

No from me too, but a natty principle. I agree with Clive, back focus is one of the issues. Certainly for the bog-standard ~55mm I doubt it will work, but any of the R/C scopes I have seen seem to have back focus to burn.
Did you glean a price?
Gary

cough - about a grand $US

[gbeal]

Cough indeed, but mere pocket money to you Huff.
I'd be headed in the direction of the MOAG to be honest.
In my own case though, the Starlight Xpress OAG is a wonderful piece.
Gary

[bert (Brett)]

a grand?

Buy a Monster moag. The monster moag is very thin, and I'll bet precise parts will make an adapter to fit direct to the back of the scope, depending on what focuser you are using.

I have the use of mmoag and I own a moag. Unless you are using sbig AOL, the moag is a waste of space. The mmoag could be quite handy for your setup, especially with a rotator.

The idea of using ir to guide on..... you will lose a lot of sensitivity.

Brett

[gregbradley]

MMOAG is also about $1000 + Adapters so it will end up more. But it is a pretty good piece of equipment but it has a lousy focuser setup.
With my ST402 you can't use the helical focuser anyway as it is too far out.
But even if you could how does one use a helical focuser on a rectangular guide camera? What, spin the whole camera around including all the cables? eehhh no.

That means you slide the guide cam in the eyepiece holder until its in focus and mark the spot. Pretty crude.

Apart from that it works well although you have to make sure you camera adapters aren't tilted from hitting the prism holder. It is adjustable so no big deal but it did take me a few days to work out where the tilt was coming from in images when I first used it.

Also it can be tricky to setup. If the MMOAG is even a small amount out of focus the guide stars completely disappear making it hard to work out where focus is. They only appear when you are quite close.
This made it frustrating for me when I first got it. So much so I assumed it did not work properly and did not use it for many months.

This unit though looks heavy and looks like it may cause flexure as that heavy camera and filter wheel sitting right above it - that's a lot of pressure.

Also reflecting the light to the camera is saying the guide camera is more important than the imaging camera and the imaging camera can get the weakended reflected light along with all the artifacts, possible distortions and dust on the mirrors whilst the guide camera gets beautiful views.

I wonder though if it may be more compact in those situations where backfocus is limited so its not all bad.

I laugh at their 2 x 1 minute exposures added to simulate a 38 minute exposure. Do they think we're stupid?? Why not simply post a 38 minute exposure or was that all eggy ?

Greg.

[JohnH]

Originally Posted by h0ughy

http://www.innovationsforesight.com/

would be used in the 2.3m dome with the 14" meade, dslr etc?

Hmm, there is prior art...

http://www.iceinspace.com.au/forum/s...=beam+splitter

[gaston (Gaston)]

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.

[Marke (Mark)]

Should have a look at one of these if after an OAG I can highly recommend it : http://www.iceinspace.com.au/forum/s...ad.php?t=77439
There is a bigger one again OA6 for very large sensors and cheaper
than MMOAG

[g__day (Matthew)]

Seems like a great idea, two question spring to mind:

1. Are all typical guide cameras sensitive to the pass through wavelengths your gear provides? For example I use a Meade DSI II Pro mono camera - would that work fine to guide with your gear?

2. How does one achieve fine focus on the guide camera? Is it a lot easier as you're right on the main light path - so will using a Bhatinov mask give a better illuminated image on axis rather than off axis - and how do you fine tune your ONAG camera's position once your main camera is correctly focused?

Smart idea!

Manual is very helpful: http://www.innovationsforesight.com/...anual_V2_7.pdf

[gaston (Gaston)]

We have several customers using the DSI II pro monochrome for guiding without any problem. The DSI does not have any IR blocking filter.

Focusing is a three step process. First you need to select the right extension tube(s), if any, to reach focus with imager and guider (see the DBF table in the ONAG manual) for your set up.

Now you can focus your imager as usual, then the ONAG included guider focuser (compression ring) is used for focusing the guider (about 9mm travel). You can use your Bhatinov mask for that exactly like for your imager, I do the same with my set up.
The ONAG manual and website FAQs give more information on that matter with some images, I suggest you have a look.

One thing I learned from experience and customer feedback is the importance of the right tracking software setting. The ONAG means you are guiding with the same focal length than imaging, for instance at prime focus.
The guider FOV is smaller than using a guide scope, therefore if you have experience with a guide scope you will need some time to learn the right tracking software setting (see the manual for advices). However this is not different than using an OAG for that matter, excepted that the integrated X/Y stage gives you access to your scope FOV. You can search for a guide star on, and off axis as well.

[g__day (Matthew)]

Gaston,

Thanks - my set up is Celestron C9.25 -> Meade motor focuser -> Lumicon OAG -> Canon 400D and Meade DSI II (using PHD or MaximDL to guide). I find lots of coma on half of the guide screen and lovely round stars on the other half.

An invention like your is quite interesting - I will watch with a lot of interest reviews of how well your product works in the field.

Very innovative!

Matthew

[gaston (Gaston)]

Matthew,

Your set up should work find with the ONAG.
Guide star distortions should not be much a concern for most tracking software, including PHD, or Maxim DL.

With the ONAG, when available, you have to select "self guiding" in the software setting since there is no reflection involved in the guider anymore, unlike OAG.

There is an independent ONAG review coming soon on http://www.cloudynights.com/

[Merlin66 (Ken)]

I can add a little info.
For the last few years I've been using a modified Vixen flip mirror - mirror removed and replaced with a beamsplitter plate, to guide my spectroscopes.
Depending on the quality of the beamsplitter you can get some image distortion ( and multiple images) in the guide port. This is not a major issue - PHD can always find the centroid and maintain excellent guiding.
I also use Al's reticule V3.2 overlay to define a "virtual" slit in the guide camera...dropping the target star on the "slit" guarantees that the star lies exactly on the entrance slit. Majic.
HTH
(Details of the mod in "Astronomical Spectroscopy for Amateurs", p 220)

[gaston (Gaston)]

Interesting.

Do you use a cold mirror as well, same concept than the ONAG?

Tis confirms my experience, a distorted guide star does not impact the ability of the tracking software, as long as the deformation remains constant over time and position.
To avoid multi reflections and possible ghost images on the imager side the BS most have wide band high quality multi layer coatings, including an AR coating on its back.

Imaging through any type of window with that amount of tilt (45 degrees) will compromise the picture quality (distortions and chromatic aberrations). This is why AO units, such SBIG AO8, AOL, are limited to few degrees of tilt/tip minimizing such effects.
Reflecting the light to the imager is a much better solution, today's high quality BS are almost perfect, so do star diagonals.

What is the cut-off wavelength of your BS?

[Merlin66 (Ken)]

ideally for spectroscopy we still want to access all the available wavelengths.
I have various beamsplitter set-ups:
A standard microscope slide - good, reflects 4% transmits 92% -unfortunately the lack of AR coating on the rear surface causes a 4% loss, and double guide images.
Edmund Optics 30/70 beamsplitter - very good
Edmund Optics pellicle plate (4/94) - very good (no secondary guide images)
Surplus Shed 70/30 beamsplitter - good
Surplus Shed 50/50 cemented prism beamsplitter - very good, but extra loss of light to the entrance slit.
IMHO a beamsplitter with rear surface AR coating and a 20/80 split would be close to optimum. Bright star images for guiding ( on fainter stars >10mag) and good transmission across a broad wavelength to the spectroscope.

[gaston (Gaston)]

Very interesting indeed.

If I understood correctly you split the all light in two paths. Transmission for your spectroscopy and reflection for guiding.
There is no splitting function of the wavelength (Visible, NIR for instance). It is not a dichroic BS, just a classical BS.
If so, with the IMHO 20/80, you use only 20% (reflection) of the incoming light (all wavelengths) for guiding, yet you can do so with a >10mag star.
Am I correct?

What is your guide star exposure time?

On the other hand the ONAG splits the light in function of the wavelength, acting as dichroic filter. It is another time of splitting.
>95% of the visible (<750nm) is reflected toward the imager, while >90% of NIR (>750nm) is transmitted toward the guider.
Unfiltered CCD/CMOS sensors are quite sensitive in NIR, since for most stars there is large NIR contribution and because we work with the same F number than the scope we can easily track with 11mag stars.

I do not have a good experience with pellicle standard BS, they do not handle well large temperature swings over time, nor dust. My equipment is always outdoor (under a dome). In summer the temperature can reach 40C, in winter -20C in common. Over time the membrane becomes lose and you can see some microscopic wrinkles.
Good dichroic pellicle BS are very expensive too.
Do you have some experience over a long time, large temperature range for those?
Maybe you do not need a very good star image for spectroscopy, the requirements are different than for imaging I would imagine.

[bert (Brett)]

Hi Gaston,

I'm interested in you ONAG and ao guiding. On a typical sbig tc237 guide chip, what percentage of the light is ir with an average star, and how does it affect guide star sensitivity and accordingly the guide integration times with the visible spectrum sent to the imager?

Brett

[Merlin66 (Ken)]

Basically yes.
With the 70/30 available options (there's currently no 80/20 on the market so it was my "wish list"), from memory about 3-4 sec guide integration for about 9mag. need to check my notes.
You may be correct about the pellicle - I have no long term experience...

Re the NIR v's stars....
The early stars (OBAFGK) emit predominatly from the UV into the green.
The later stars (MRN, carbon etc) peak from the red into the NIR.

As long as we can hold the maximum amount of starlight in a 20 micron slit, for as long as possible - spectroscopy is happy.