I'm seeing strange star shapes when using an STi on my STF-8300M and telescope Takahashi FSQ-106N. Is it the internal optics of the OAG that can cause this? This is the best focus I can get with the STi while the STF-8300 is in focus as well. Guiding goes well despite the strange star shapes. Best regards, Geert Vandenbulcke
I'd suspect that the OAG-8300 is the cause. Inspect it carefully to see if any of the optics have become dislodged.
There is not much one can disassemble, the little lens closest to the prism is OK, the other lens that I can see in front of the guider cam as well, I don't know if there is anything else inside appart from another mirror to reflect the light to the guider camera? Geert
If everything looks okay then a quick check you can do: Take the OAG off the telescope and remove the camera Hold the OAG in front of you about a foot from your face, with the mirror facing you and pointing the camera port to the side. Look straight into the mirror from about a foot away Wave your hand in front of the camera port, about two feet from the port You should be able to see your hand in the mirror I tried to take a picture to show what I mean. The OAG is in a shipping bag so it's a little fuzzy but you can see my fingers waving in the mirror.
I did that check, looks OK to me. I have now mounted the camera with OAG on my TOA-150b with flattener to check the STi image on that setup on the next opportunity.
I put the camera, OAG and STi on my TOA-150 refractor, STi images look much better in that configuration, maybe I should add a IR-UV block filter? So it seems that the reducer/relay lens of the OAG does not work that well with my FSQ-106?
No, I have the second version of the FSQ-106 series, FSQ-106N with fluorite optics and retractable dewshield. In that version the use of a reducer is not possible, that only came with the FSQ-106ED versions. Geert
Maybe try the UV/IR block. Fast refractors are highly optimized for visual performance. These days the optimization is done using fancy ray tracing software (often Zemax). It uses an optimization routine that tries very hard to optimize the parameters you give it, at the great expense of anything that is not constrained. A friend of mine once specified a lens design using three wavelengths and so it only worked right exactly at those three wavelengths. When he built the lens the images were absolutely horrible! All he had to do was interpolate the points in-between and the lens would have worked. So if the designer of the FSQ didn't specify the performance (or in my friend's case the glass refractive properties) in UV and IR it might actually be quite horrible.
The FSQ-106 series are designed as astrographs with an +80mm image circle, not visual telescopes, so I expected a better performance with the OAG-8300. However the combination of the telescope optics with these in the OAG seem to have an influence on the image given by the guider camera with that specific telescope. Anyway, I bought a UV/IR block filter for the STi guider and will check results on next opportunity. Even with these bizarre star shapes, the guiding goes well, so maybe I should not bother further or switch to a separate guide scope instead of the OAG-8300.
There are focal reducer optics in the OAG. The FSQ is already F/5. That gets us to F/3.5. That's pretty darned fast. Also reducers tend to accentuate any aberrations in the system.
Am wondering if the pick off shape and its insertion into the light cone is why the odd assymetry. If it's guiding properly, it's doing its job.
It's very possible. In fast optics the light cone at the edge of the field is often highly asymmetrical before it hits the focal plane, like this: You can see how that might produce odd results if you put an off-axis aperture in front of the focal plane. I think Colin is right. Off-axis guider images often look crappy in many types of instruments. As long as there's a bright core to guide on, you should be fine. And with the wide FOV you should have lots of bright stars to guide with.
OK, thank you all. I tried to put a UV-IR block filter on the STi but because of the thickness of the filter housing, I can't focus the STi anymore in that case. So I will continue to use as it is.
