What OAG Guide Cam resolution needed

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Christian Großmann avatar
Hi folks,

I have two scopes that I plan to use in the future. Which one I choose will depend on the target. My main scope will be the Explore Scientific MN-152 Comet Hunter for nebulae and wider fields. The second one is a TS Photon 10" f/5 that I plan to use on galaxies etc.

I am kind of that "set and forget" guy and equipped both scopes with a Raspberry Pi and some basic tools like automatic focusers etc. All the technical things are mounted directly on the scopes. That way, I could set up the software on the Pi to fit the needs of the scope it is mounted to and I only had to plug in 1 power cable and I'm ready to go.

On the Comet Hunter, I use a 60mm guide scope. On the TS Photon, I plan to use a ZWO Off-Axis-Guider which is already installed. The OAG is new and I wasn't able to use it, yet. I also have two guide cams. A ZWO 120MM Mini and an Altair GPCam3 178M. The resolution of the GPCam is three times the resolution of the ZWO 120MM. I was using mainly the GPCam before for my guiding and it seemed to work better than guiding with the ZWO. But until now, I only had one scope and could use the preferred camera easily. With two setups, I would put every guide cam to its own scope and leave it on. But I need help with that decision.

The question is, what camera would theoretically fit which scope better? I may buy another guide cam later but recently I invested in a lot of stuff and therefore want to live with the things I have. Because everything works fine on the ES Comet Hunter, I tend to leave the Altair cam on this scope (it is currently on there). But if I think about it, the ZWO cam with the lower resolution may be a better fit for that scope (because of the shorter focal length it must handle). I guess, that the Altair cam is more sensitive (at least with binning) and therefore may be the better option for the OAG and the 1270mm focal length scope. Am I right or am I missing something here?

What do you think?

Thanks for the help…

Christian
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Marc Agostini avatar
Hi Christian,

I have the same two guidecams and I am using the 120MM for my OAG (Imaging at 400mm f/5) and the 178M for my 32/120 guidescope, unbinned. The reason I did it that way is because my main imaging camera is a QHY183, which also has 2,4micron pixels, and the guidecam sampling ratio doesn't need to be as low as your main camera, it can easily be twice or three times as high for better sensitivity. Another consideration: The GPCam 3 178M has a pretty big sensor for a planetary/guidecam, and am not sure if the image circle of an OAG is big enough to illuminate the whole chip.

 If you use your 178M unbinned on the guidescope on the other hand, you get advantages concerning guiding resolution and usable field. This way you will get near perfect sampling ratios for both of your setups. Especially if you continue to use your ASI 183 as the imaging camera on the Comet Hunter, the 178M is the better fit in my opinion.

Clear skies,

Marc
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Christian Großmann avatar
Hi Marc,

thanks for your reply. The lager sensor of the GPCam may indeed be problematic on the OAG. I guess that also the small pixels paired with the long focal length of the Photon may cause oversampling. That will surely not improve the guiding as well.

With your post I recognized that I forgot to mention, that the main cameras (ZWO ASI183M Mono and ES (ToupTek) ATR3 CMOS 16000 KPA OSC are the only thing that will be swapped constantly between the scopes. I assume, that the Mono cam will be most of the time on the Comet Hunter, because the targets within that focal range may benefit more from narrow band images.

CS

Christian
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