onag-rev

Guiding is one of those necessary evils for high quality astro imaging. One camera company, SBIG, offers cameras with a built-in guide chip, but some of their cameras and all others require some kind of external guiding. The two main external guiding options up till now have been 'guide-scope guiding' and 'off-axis guider guiding'. The ONAG (on-axis guider) from Innovations Foresight provides a new third option with unique features that combines the best qualities of both guide scopes and off-axis guiders. I was looking for a better guiding solution and was very interested to see if this new guiding product would work out.

Background: For those of us with longer focal length scopes (over 2000 mm), guiding is a difficult proposition. In order to get nice round stars (and not oval or 'trailed' stars) and maximum detail in our images, extreme guiding accuracy is required. Even a good mount has some tracking error and a guide deviation of just a few pixels during a 5 minute exposure will show a distorted shape in the brighter stars. On top of this, it can be difficult in some areas of the sky to even find a bright enough guide star to use. So selecting the optimum guide strategy is a vital decision for astro imagers and is almost always some kind of compromise.

Here is a review of the various guider options, based on my own education in that famous school of hard knocks:

Guide scopes: For this option, a guide camera is connected to a separate telescope that rides the mount along with the primary scope. The main imager is attached to the primary scope and it is assumed that guide corrections supplied by the guide scope/guide camera will accurately keep the main imager aimed at the target. The tremendous advantage to this setup is the wide field of view that you can get with the dedicated guide camera on the guide scope. This makes it easy, in fact, almost foolproof, to find a suitable guide star. The tremendous disadvantage is the near certainty of differential flexure, a term with describes independent movement of the guider and main scope relative to each other. Although shorter focal length/wide field imaging rigs seem to work OK with guide scopes, longer focal length systems do not seem to produce a good yield of high quality images using this technique. I fought for a long time with my 2500mm scope using increasingly maniacal bondings between the guide scope, guide camera, focuser, and imager to lock every single item into perfect rigidity, but my yield of 5 minute exposures was variable at best, and 10 minute narrowband exposures were seldom acceptable. In all fairness, some, but not many, have claimed to have overcome all these issues and get great images using guide scopes, and my sincere congratulations go to those who can pull this off!
Off Axis Guiders: In this option, a 90 degree-angled pick-off mirror or prism is inserted at the edge of the field near the prime focus of your imaging scope, to relay starlight to a guide camera. Because the guide camera is attached directly to the imaging scope you have a good chance to avoid the differential flexure issue. However, I have found that the field of view of that tiny mirror makes it pretty difficult to find a guide star near many objects. And unless your scope has a large corrected image circle, the quality of the star images way out there at the edge of the field may be quite poor. Finally, when I attempt to move the pick off mirror a LITTLE farther into the beam, to get a halfway decent view of the guide stars, the darn thing starts casting a shadow or reflections on my image. The best results from this option seem to come from those who use the highest quality off-axis guiders, and invest in telescopes that have image circles of 50 mm or more. In addition, either manual rotation or the purchase of a camera rotator will probably be needed in placing a suitable guide star on the guide chip.
The ONAG: Based on my testing, this new option provides the advantages of the guide scope (easy to find a guide star) with the freedom from differential flexure of the off-axis guider. In addition, since you are not guiding on a star at the extreme edge of the field of view, this guide strategy will work with smaller corrected image circle and let you image with an affordable scope, such as the SCT that I started imaging with.

So exactly what is the ONAG, and how does it work?

Here is a diagram that shows the various parts of the ONAG

The ONAG is attached to your scope as shown above (toward the left). Your imaging camera is attached to the side port (top in the diagram above) and your guide camera is attached at the back (the right). A high-quality, optically flat multicoated dichroic beam splitter, or "cold mirror" is installed at a 45 degree angle inside the body of the ONAG. It reflects the visible light, from 370nm to 750nm, toward the imaging camera. The remaining near-IR portion of the light passes through the mirror and travels toward your guide camera.

It should be apparent that the guider has a great view of the star field presented to your scope, and the large amount of Infrared energy in starlight (and the great sensitivity of CCD cameras to IR) means that you get a great selection of stars to use for guiding.

What is in the package?

The ONAG comes with T-Thread adapters and some extension rings as show here:

The scope port allows the ONAG to be attached to any scope using a standard T-threaded female connection (M42 x 0.75). The ONAG comes standard with a 2" tube adapter, and a low profile SCT female adapter. I found that this connection was nice and rigid to prevent drooping of the fairly heavy imaging equipment that I was using.

The imager port is used to attach the imaging camera and related accessories, such as a filter wheel, to the ONAG using a standard male T-thread. A low profile T-threaded ring is provided to secure the camera at the desired position.

A guider port is used to attach the guider camera to the ONAG using a male T-thread. A low profile T-threaded ring is provided to secure the camera at the desired position. The guider port contains a guider focuser drawtube, which provides up to 9 mm of travel to adjust the guider focus. The focuser uses a heavy duty compressing ring to insure a 360 degree grip on the drawtube. The drawtube is 1¼" in diameter and can be removed, so this allows the use of any standard 1¼" accessories.

An X/Y stage allows easy and quick search for a suitable guide star. It is attached to the ONAG body on one end, while it supports the guider focuser on the other end. The stage slides in both directions (X, Y axis) using two stainless steel shafts; each axis can be secured with four nylon screws to lock it into position.

Three T-threaded extension tubes are also supplied. These are 8 mm, 16 mm and 24 mm long. They can be used in any of the ONAG ports With the proper combination they allow a wide range of cameras (including DSLR) and guider to reach focus simultaneously.

Build quality: The build quality is excellent. The body is made up of aluminum and stainless steel and surfaces in the optical path are anodized with optical grade black, which provides prevents IR reflections that show up with less grades of anodizing.

Setting up the ONAG:

As with all imaging devices, some planning has to go into how the parts will connect together. I used the ONAG on my Starizona Hyperion telescope with a Apogee Alta U8300 camera. The filter wheel was a 7 position model from Finger Lakes imaging, with 50 mm filters. For guiding, I used my trusty SBIG ST402 camera. Since my scope's focuser was configured with AP 2.7 inch threads, I used the Innovations Foresight AP adapter on the scope port. The imager port was configured with the SCT thread (two inch, 24 tpi) adapter to connect to the FLI filter wheel. The filter wheel was directly attached to the Apogee camera. The guide port connection to the ST-402 camera was easy since like many guide cameras, it uses the T-thread for connections. After a little trial and error, I found that the 16mm extension brought the guide camera to the same focus as the imaging camera. The Hyperion has an excellent Feathertouch focuser, but the big focuser body uses up a lot of the back focus available to the scope. With the ONAG, I found that the focuser was racked in pretty far, but I had enough travel to use FocusMax for automated focusing with no problems.

Since I mostly use fully-automated imaging with CCD Autopilot software, I fixed the XY stage to be somewhat off axis and left it there during all imaging. There were many guide stars available in any given field of view, so I just let the software locate a star to use for guiding. Here is an image of my system with the ONAG in place:

Performance:
I ran several tests to look for any unusual effects from the ONAG as well as to evaluate its main purpose as a guider.
Image effects: Can you really bounce the image 45 degrees off a 'cold mirror' without noticeable optical degradation? That was my question, so I ran several tests with and without the ONAG in the imaging train. In these tests, I could find no aberrations in the images with the ONAG, even when I blew up the images to 400% in Maxim/DL. I also use a program called CCD Inspector to evaluate images, and the ones that I gathered with the ONAG provided the same FWHM readings that I was used to seeing. I saw no signs of vignetting with my imaging system using the U8300 camera (22.5 mm diagonal chip). What I did see was that the images were reversed since they were being bounced off a mirror. This is easily fixed with imaging software, however.

Guiding: I ran a guiding test on M13 where I took successive 1 minute exposures for over an hour, resulting in 60 plus images, while continuously guiding the whole time. The guide graph looked great but a careful examination of the images would tell me whether I was getting any differential flexure between the perfect guiding and the actual imaging camera. I made a movie of the frames so I could watch for movement. What I saw was very gratifying - the image stayed in the same place and although there were small random displacements due to seeing effects, there was no directional drift as I have seen from differential flexure in the past. Images from the first and last part of the exposure series were superimposed and showed that the stars were in the same place. I don't plan to run hour-long subexposures but it is nice to know that I can go as long as I want without any differential flexure drift.
Imaging tests: I gathered images as I normally would, using Maxim/DL, FocusMax, and CCD Autopilot. I found that the correct guide setting for CCD Autopilot was the 'self-guided' setting, which is the same one that is used for cameras with internal guide chips. Since I use TheSky v6 for running the mount and interfacing with CCD Autopilot, I planned to map out a custom Field of View reticle to help plan image rotation to select the best guide stars for images. However, in practice I found that no rotation was necessary to get a good guide star in almost all objects that I selected, so I never quite got around to completing this task.

Results:
Here are several images that I collected, stacked, and processed using Maxim and Photoshop: