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Hello, I've decided to go remote with a 5.5" refractor. I'm deciding upon guiding with an OAG or a guide scope. Focal length of the refractor is 938mm. The scope will be in excellent very dark skies. But I still worry the OAG, under certain circumstances, will not be able to find a suitable guide star. I know 938mm focal is not very large, but I'm still wondering if I should use a guide scope. The guide camera would probably be an ASI174mm mini. Any thoughts? Jerry |
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Not in a hundred years would an OAG with the 174 fail to find a guide star Especially in very dark skies The OAG is the one to get; make sure it's one with a large prism, like the Celestron, or ZWO OAG-L; the Askar OAG would also work. Make sure you also have a good mount. |
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Not in a hundred years would an OAG with the 174 fail to find a guide star That's pretty much what I want to hear  Thanks messierman3000. Yes, the mount is the 10Micron GM1000 HPS.
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5.61
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Not in a hundred years would an OAG with the 174 fail to find a guide star Agreed! Even with my CDK 14 I have never failed to find multiple guidestars and that is a much longer FL... |
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| Agree with the others. 174 chip on oag will have no problems at your focal length. If the site has good seeing an oag will likely get you tighter stars as well. |
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Well then, Here's to a new adventure in remote astrophotography. It should be a fascinating learning experience and a real challenge as well. |
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You already received the same feedback I was going to give, but let me double down so you have zero doubts. I own this scope and I ONLY use an OAG with a ZWO 290 guide camera, EFW and ASI6200MM Pro. I have been using it under Bortle 8/9 skies for the last 2.5 years and I have NEVER had an issue finding a guide star to guide on with this combination. Under dark skies and with the 174 you will have absolutely zero issues. I am also thinking about sending this to a remote site and if I do I will use the OAG without hesitation. My guide exposures are 4 seconds. Using an OAG is nice from the guiding point of view (better results) but the other great benefit in my opinion is that it frees up the top of the scope to piggyback other stuff on it. A computer, power distribution, a small refractor or 135mm lens for widefield shots..... |
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I have that scope now on a Losmandy G11G. Now under Bortle 4/5 skies, but may end up moving to 5/6 skies. Is an OAG still the way to go under those skies? Is an OAG harder/slower to set up than a guide scope? I travel now to image and don’t have much time each night. Thanks |
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Tom: Using an OAG is easier. The you just focus your main camera first. Then focus your guidecamera. Then never touch it again. When you focus your imaging camera you are also focusing your guidecamera so after that initial focusing setup is a breeze. You wont need to refocus every night like with a separate guidescope, and you dont need to worry about focus changing as it cools... like with a guidescope. As long as you maintain focus with your main imaging camera...your guidecamera will also maintain focus. |
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Another viewpoint. OAG not necessary in this case. I have used a 60mm guidescope on this exact scope and a 130mm TMB to excellent effect. Zero issues with flexure or anything else, never had to refocus it. I use a 178mm and get a large field of view with lots of guidestars. Also the SVX140T is about 968mm effective focal length using the Stellavue SFFX-2 flattener, fyi. -Pete |
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Pete Bouras: I do think a guidescope could work ok, however the OP indicated that this setup was going to be used for remote imaging. In that case if you have a guidescope, you should have a focuser on it in case focus was to shift over time. This adds a level of complexity for gear, maintenance and expense. With an OAG focusing the main camera also keeps the guidecamera focused. |
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Chris White- Overcast Observatory: Hi Chris - one obervation. The OAG chosen should, I think, be fairly robust to prevent slipping of the OAG over time which could create a huge problem. I have had this happen to me on occasion. The fix is simple in a backyard, less so if remote. Also for the OP- ensure your filters are the same thickness. Otherwise the OAG will need to be refocused if you change to a filter of different thickness. Example: the Chroma Lo Glow filter is 1mm thick, every other Chroma filter I am aware of is 3mm thick. I find I have to reposition the OAG if I use my Lo Glow filter. |
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Arun H:Chris White- Overcast Observatory: Valid point but I would suggest to put OAG in front of filter wheel and add one separate filter for guiding cam (I've read that IR filter could help with seeing, but never tested it). This way your guiding is much more predictable and not depending on filters used for main camera. What would be benefit to put OAG after filter wheel? |
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| The filter wheel does in fact go behind the oag. But the focuser is ahead of both. Changing to a different filter changes the optical distance to the main cam, but not the oag. |
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Arun H: Ah, of course you are right. Never had this issue (with different filters) so I haven't thought it through. Thanks for explaining and as mentioned: very valid point (without but this time) |
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Chris White- Overcast Observatory:Pete Bouras: *** No reason the focus should shift over time on a guidescope, same could happen on an OAG? Never had to refocus guidescope on my rig, would be over kill to put auto focuser on guidescope, imo. To each his own. I see no clear advantage of an oag over a guidescope set up for an SVX140. |
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I would also point out that an OAG is going to be more reliable than a guide scope especially at this focal length (close to a meter). The longer the FL the less useable and reliable guide scopes are due to differential flexure. An good solid OAG is just going to be more robust and give you fewer problems. OAG vs Guide scope: 1) OAG does not require refocus 2) OAG does not suffer from possible differential flex 3) OAG operates at same FL as the imager so has better sampling 4) OAG does not require other separate items such as dew heat, mount rings, etc. I know there are folks that will disagree (aren't there always ) but I would not use a guide scope for any FL greater than around 500 mm or so.
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+1 to what Bill said and for the same reasons. I've played around with both. OAGs, at least for me, have been much less bother. If/when I go remote, an OAG is what I'll go with. |
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Just to pile on a bit, I have a Stellarvue SVX 140T-Raptor, an ASI 6200mm and the 174mm for guiding with the ZWO OAG-L. I have never had a problem finding guide stars in my Bortle 6/7 skies. I also have a Celestron C11 EdgeHD, on which I use the same imaging train, and likewise I've never had any trouble finding (lots) of guide stars. I do check the focus now and then on the OAG, but really it doesn't need to be touched very often at all. |
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Thanks all. No idea why I am having such trouble with my SV50 guide scope. Probably should not have let it fall off my scope onto the concrete the first time. Or let it get knocked off onto the said concrete the second time. Ouch! This will only be used if/when the 2600MM Duo guide cam cannot see through the 3nm SHO filters. So maybe not every nights shooting only LRGB. Edit: Viewed on YT and it seems like a good, easy, robust setup. Will order the ZWO OAG-L and ZWO 174mini. May send the 2600MM Dou back and exchange for the basic 2600 MM and put the the $500 difference towards the OAG/140. |
