I"m wondering the comparative sensitivity between the SC2 and the guide chip of the ST series cameras. Here's the thing, I have an AO8 in my imaging train. With the ST cameras (ST10 & ST 2k) running the AO8 @ .1 (10hz) I can guide off of 12th Mag star. If I went to 5hz, I can guide off of a 14 mag star. On the SC2, I can barely get a 10th mag star to display. So either the sensitivity is not the same or the fact that the FOV for the guider chip is so much farther away from the main chip (distance from image circle) and thus lower photons reaching the guider chip has a large factor in guiding. Another thing is the badly distorted shape of the guide star, again could this be the location of the FOV? It's not that I looked closely of the ST guide star shape, but I never really noticed distorted guide stars. On the SC@, I do have the pick off arm out as far as it will go. An FYI, my scope is an f/4.45 Below is two images. One, a screen shot of the shape of the guide star. The other is the FOV comparison location between the ST cameras and the SC2.
Comparing the sensors is not easy; almost apples and oranges. It also depends which vintage of ST-series camera you have. Some had the Texas Instruments TC-211 CCD, some had the bigger TC-237 guide sensor. QE about 40% peak at 650nm, but more like 20% at the start and end of the visual range. The 237 had an array of 658 × 496 of 7.4um pixels. The SC-2 has a larger CMOS Active Pixel Sensor that is 1280 x 1024, and higher, broader QE, peak about 55%, but it has smaller 4.5um pixels. So it is more sensitivity, but has a smaller full well capacity. The ST-series cameras have the guide sensor behind the filters, mirrored off to the side of the main sensor, so if you have a guide star that is not visible in the wavelength of the filter you are using, no guiding. The SC-2 will win every time in that scenario as it is ahead. Guiding in MaxIm uses the centroid of the star, so the shape doesn't matter very much. All off-axis guiders will have a coma-like shape as there are no correcting optics. For completeness, there were other off-axis guiders like the STX-Guider that have a 0.7x focal reducer in front of their guide chip, which was beneficial (more light packed into the pixels).
I never used filters on my ST cameras. I understand that the centroid of the star is used for guiding, but if the star is 4 pixels high, but is smeared across 12 to 15 pixels wide, then doesn't that have to be losing efficiency as far as being able to read a dim star? How would I go about getting the guider FOV closer the the main chip? Does DL have a longer pick off arm option? If not, could I get a drawing or a solid model of the pick off arm so that I can make my own?
If it is that smeared, then yes, it will not be as obvious. The SNR level could be adjusted lower (to like 10 instead of 50) and it may still be able to detected. However, I see your point. I don't understand what you are asking "getting the guider FOV closer the main chip". You have two parameters you can adjust - the focus and the position of the pick-off mirror. The idea is to get the pick-off mirror as close to the centre of the field without vignetting the main sensor, while still being able to reach focus. The guide sensor needs to be parfocal to the main camera, when using a filter of medium refractive index (RI). In other words, find the focus on the main camera with the lowest RI, and then highest RI, and split the difference, and use that to set the focus of the main sensor, then adjust the guide sensor. Many filters these days are parfocal, so the focus to the main sensor does not change much if at all. The alternative is to use something like the Innovations Foresight ONAG device: \https://www.innovationsforesight.com/products/on-axis-guider-onag/
If you look at the "Guider FOV" attachment in my first post, you will see how the SC2 FOV compares to the ST10 guider FOV. The bottom yellow box(s) around M64 is the main sensor chip(s). On the SC2, I have the pick off arm (mirror) extended all the way out. I would like to extend it further with a longer arm so that it is it would look more like the guider FOV from the ST10. My plan would be to 3d print a test arm and if that proves my idea, then I would make a permanent one out of aluminum. That's why I was asking for a drawing for the pick off arm.
Do you mean STC-7 (the modern CMOS APS camera) or ST-7 (the 20+ year old CCD) in that picture? There is no such thing as an ST-7C and that has me partly confused. Secondly, when you say extend it "out", you are trying to move the pick off mirror closer to the centre of the field? That would likely vignette the sensor.
Okay, That was a typo, I have the STC-7 with the SC2 and AO-8A. Right now this camera package is next to unusable. So yes, I'm trying to move the pick off mirror closer to the main sensor. If this means that I take a chance of vignette the main sensor, so be it. I would hope that with 20 years of technology I would not go backwards in obtaining guide stars.
I'm talking about getting a longer pick off arm to move the pick off mirror closer to the main sensor. Trying to get the SC2 FOV to match the ST10 guider FOV.
If you made a longer pick-off arm, the sensor might not be focusable. Also, are you binning the guider 2x2 ? That will give you a more compact guide star image.
Good Morning Colin. Yes I'm binning 2x2. As far as focusing the SC2, I had to make a custom spacer to go between the STC-7 and the SC2 to start with when I got the camera package because it wouldn't even come close to focusing with the supplied spacer. So making a new one or modifying the one I have is not a problem. The longer pick off arm is an experiment. If it doesn't work, I guess I'll just be doomed to use my ST10 or ST2k.
