Guide scope easier than OAG ?

I am fairly new to AP and until last week I didn't know what OAG is. I use a guide scope and I got curious so I watched a few youtube videos and people seem to agree that a guide scope is easier to use than OAG but I don't understand why.

A guide scope needs to be focused, aligned with with your main scope or lens, connected to computer wether it's pc or asiair, it needs its own dew heater.

OAG doesn't need any of this except for the computer connection so I am wondering. Why do people say guide scope is easier ?

I find it easier in the setting-up and operation and the small ones don't even need a dew heater (just an heat shield). As far as focusing goes it isn't that critical and really isn't a concern (it might take months before I have to refocus and then it is easier than doing it on an OAG), Alignment isn't that critical either and once done you can forget about it. Allows a lot more freedom than having to add complexity to you main imaging train, especially if it is already short-charged by spacing requirements. This said, YMMV…

Honestly, i’m liking my celestron oag. I just leave everything behind it attached and swap it over to my other scopes. All the cabling and backfocus stays the same.

The original smaller zwo one I had was a bit of a nightmare.

I love my OAG. Invest in one with a helical focuser if you decide to try it. I also have a filter drawer between the OAG and camera, and I have to tweak the focus between some filters.

I used both, But guider scope is easier for me:
More and clearer stars–> Helps PHD2 to get Multi stars average.
Easier if you want to rotate the main camera for framing
Focus and align once in 3 months or more, OAG focus is awful.
Maybe others like OAG, But for me OAG is in rest in the box.

Ali Alhawas:
I used both, But guider scope is easier for me:
More and clearer stars--> Helps PHD2 to get Multi stars average.
Easier if you want to rotate the main camera for framing
Focus and align once in 3 months or more, OAG focus is awful.
Maybe others like OAG, But for me OAG is in rest in the box.

Yeah, forgot about that... I bought an OAG as I thought I might use it. I haven't. That was 11 years ago.

For me a guide scope is far better (once it is correctly focused…). You have much more stars in the field, and you do note depend on the filters used. An OAG through a Ha or S II filter will be difficult to use !
I use a common guide scope with an ASI 120 mini via ASIAIR and the guiding starts immediately. The guiding system comes from PHD, but the computer does all the annoying job…

Philippe

While it's not best practice, your guide scope doesn't need to be aligned with your main scope. Anyway, there are a few reasons why a guide scope is considered easier. One is that when you're imaging at long focal lengths it's easier to find a guide star. When I use a 30mm guide scope with my 120mm mini, on any given night I probably had 20 stars in the FOV for Phd2 to choose from. Another reason is getting things all in focused is easier with a guide scope. In general, with a guide scope you slide your camera in/out a bit and somewhere there's a spot where it's in focus. With an OAG, that isn't necessarily the case. You need to be able to get focus in two cameras at the same time. I need a minimum of 23-25mm of space between my OAG and my main camera. With my SCT it's been a real pain in the butt trying to get the right adapters and spacers to have the right backfocus for both my OAG and main camera while still having clearance with my EAF.

For now I am using a Samynag 135mm lens and ZWO guide scope and camera. It's working well, I don't have any issues, or if I do I just don't know about them

. The ZWO guide scope has a focuser which makes it easy, I don't have to fiddle with sliding the camera so yeah, guide scope IS quit easy right now.

I was just wondering if OAG would be a good investment to make with the holiday sales for when I get a big telescope but I guess I could just get a big guide scope too.

Thanks everyone for the answers, it's all very informative.

I have the ZWO OAG-L and use all Chroma filters of same thickness.

- Focus on the guiding camera is not the same between the broadband and narrowband filters. I have to refocus between them.
- Any frame rotation when changing target mean that I have to recalibrate the guiding.
- This OAG is leaking light from 3 different places. You should see how much tape I have on it.
- You can't guide on the comet that your main camera is looking at.

I'm thinking of trying a guidescope to avoid this.

Quinn Groessl:
While it's not best practice, your guide scope doesn't need to be aligned with your main scope.

Due to the effect of atmospheric refraction, the apparent motion of stars will be different depending on location in the sky. For this reason, it is a good idea to make sure your guide scope is at least roughly aligned with your main scope. Obviously not an issue with an OAG. One other issue that an OAG solves is differential flexure, where the movement of the main OTA is slightly different than the guide scope due to gravity, loose connections, etc. All of these tend to be greater considerations for longer focal length/finer arcsec/px imaging.

For wide-field refractors you don't need an OAG. Conventional wisdom has long been that for long focal length OAG's were best because of the risk of flexure (the guidescope moving a bit during the night). But the problem with that is that many times you may not have many stars in your guide camera FOV. Whereas with multi-star guiding algorithms, it really makes a beneficial difference to have a good number of stars. For my long focal length (f/15 1850mm fl) imaging, I use a 50mm f5 guidescope and it has been just fine. I would not use a finder shoe for that guidescope though - that is tightly mounted in some WO uniguide rings and clamped on to the imaging OTA.

I only use an OAG, but even so some thoughts:

An OAG wastes a lot of time in imaging with multiple filters, as it typically requires a stop/start guide after filter change (even using offsets unless you have a really, really great focuser), and always during an auto-focus run. As an related issue if using a mount that badly needs guiding, this can allow a lot of drift during all the times without guiding.

People buy cheap guide cameras (ASI120MM for example) that work great for guide scopes. If you want to use an OAG get one with a large prism (e.g. OAG-L is decent) and wide guide camera (ASI174MM for example). The 174 is 464% of the 120's area, so over 4 times as many stars visible. I have NEVER not had a guide star (including with a C11 @ 2800mm).

OAG's, even a thin one, takes a lot of backfocus room; some OTA's may not have adequate due to having been designed for DSLR attachment. If you wanted, say, a GERD CTU for tilt adjustment in line, or a rotator, you may not have room if you use an OAG.

Never, ever use the ZWO tilt adapters (e.g. required with an OAG-L) to adjust tilt, they are garbage. They are required for mating up the threads, but as tilters they are awful.

An OAG is almost always going to guide more accurately than a guide scope, period. In some cases substantially better, in some it may make little difference. But it does come at a cost.

In my case, at 3910mm with a full frame main imaging camera, I use "On" Axis Guiding which I've found to be brilliant - always many stars available and really good live focus monitoring. Not sure that OAG would be as good in my case as it would have a much smaller available area for stars so as to not intrude upon the main camera's image plane. But in any case, using a guide scope at this focal length, would probably not work at all unless it was a fairly big and heavy scope with a suitable ratio (of the main scope) for its focal distance.

Eric Gagne:
I am fairly new to AP and until last week I didn't know what OAG is. I use a guide scope and I got curious so I watched a few youtube videos and people seem to agree that a guide scope is easier to use than OAG but I don't understand why.

A guide scope needs to be focused, aligned with with your main scope or lens, connected to computer wether it's pc or asiair, it needs its own dew heater.

OAG doesn't need any of this except for the computer connection so I am wondering. Why do people say guide scope is easier ?

I agree completely with your assessment. You didn't mention that it's lighter and, most importantly, more accurate. Flexures are inherently different between guidescope and primary OTA. For very short focal lengths it won't matter but if you are near to resolving your seeing you want and OAG.

Kevin

It will come back down to your setup (magnification and back focus requirements). With a helical focuser and a good guide scope, the OAG is great. It also eliminates issues like flexure. I never find that I have an issue finding stars even with my highest magnification scope (1,800mm). I just switch to a larger imaging sensor guide scope. However, where I have back focus challenges (e.g., Samyang 135mm lens), I need to use an external guide scope which also works great. However, even with a guide scope, helical focus makes it so much easier to "set and forget." Good luck and clear skies!

Miguel T.:
I have the ZWO OAG-L and use all Chroma filters of same thickness.

- Focus on the guiding camera is not the same between the broadband and narrowband filters. I have to refocus between them.
- Any frame rotation when changing target mean that I have to recalibrate the guiding.
- This OAG is leaking light from 3 different places. You should see how much tape I have on it.
- You can't guide on the comet that your main camera is looking at.

I'm thinking of trying a guidescope to avoid this.

If your filters are all the same thickness the focus should not change enough for it to be an issue for the OAG. PHD2 is extremely forgiving when it comes to star shapes and focus. Additionally if you have configured auto focus filter offsets, the focus is automatically adjusted when switching filters while guiding is active with no perceived impact. This allows for maximum efficiency with the need to only focus a few times a night.

If you use an electronic rotator, it can be connected to PHD2. After which PHD2 is capable of automatically adjusting it's calibration to compensate for rotation.

In my experience one of the downsides of OAG can be lack of guide stars. With later versions of PHD2 which come with the multi-star guiding feature I find that more stars significantly improve guiding performance at longer focal lengths, all else being equal. Not an issue on some targets. Can be an issue on others.

On small and moderately sized refractors I'd always recommend a guide scope because you don't have to worry about all these considerations. Keep it simple as they say. The decision-making is more complicated as the focal length increases.

Miguel T.:
I have the ZWO OAG-L and use all Chroma filters of same thickness.

- Focus on the guiding camera is not the same between the broadband and narrowband filters. I have to refocus between them.
- Any frame rotation when changing target mean that I have to recalibrate the guiding.
- This OAG is leaking light from 3 different places. You should see how much tape I have on it.
- You can't guide on the comet that your main camera is looking at.

I'm thinking of trying a guidescope to avoid this.

If your filters are all the same thickness the focus should not change enough for it to be an issue for the OAG. PHD2 is extremely forgiving when it comes to star shapes and focus. Additionally if you have configured auto focus filter offsets, the focus is automatically adjusted when switching filters while guiding is active with no perceived impact. This allows for maximum efficiency with the need to only focus a few times a night.

If you use an electronic rotator, it can be connected to PHD2. After which PHD2 is capable of automatically adjusting it's calibration to compensate for rotation.

In my experience one of the downsides of OAG can be lack of guide stars. With later versions of PHD2 which come with the multi-star guiding feature I find that more stars significantly improve guiding performance at longer focal lengths, all else being equal. Not an issue on some targets. Can be an issue on others.

On small and moderately sized refractors I'd always recommend a guide scope because you don't have to worry about all these considerations. Keep it simple as they say. The decision-making is more complicated as the focal length increases.

Your OAG should be after your filter wheel unless you have an on-camera guide sensor. This eliminates any worry of filter thickness and focus or brightness if using narrowband. Also, for me, I recalibrate PHD2 every time I move to a new target so I don’t need to worry about my rotational changes with my rotator.

Norman Tajudin:
Your OAG should be after your filter wheel unless you have an on-camera guide sensor. This eliminates any worry of filter thickness and focus or brightness if using narrowband. Also, for me, I recalibrate PHD2 every time I move to a new target so I don’t need to worry about my rotational changes with my rotator.

That's the right place of course but it doesn't eliminate filter thickness issues, since when you refocus the main OTA it is changing the focus on the OAG. Thus you need reasonably parfocal filters when using an OAG. Not perfect, but close enough the stars stay recognizable as you move between BB and NB (or whatever). This refocus for different filters is also, for most focusers, going to kill guiding, so even if you use focus offsets (i.e. no full auto-focus run), the lateral shift in the star position from moving the focuser will screw up guiding, and you really need to stop, switch filters, and restart. Thus a 5 second filter change becomes maybe 20-30 seconds with settle time.

Running 2 espirit 100s both with OAGs. I refuse to switch to a guide scope.

Once you put an OAG on your imaging train and focus it, you should never have to touch it again unless you bump it. You can swap your imaging train to any scope and the OAG will again be in focus.

OAGs require parfocal filters, so if you're piecing together a filter set with different thicknesses, an OAG is not for you.

Of course, there is no risk of flexure.

Some folks here are mentioning that filter changes add time to your sequence due to settling, I'd argue you need to adjust your settings for better effectiveness. My swaps are around 5 seconds, maybe sometimes 10 seconds at most. Also, you don't need to constantly change filters every sub, I usually run about 10 per filter then switch.

Using an OAG keeps your sampling the same as the main camera, thus allowing you to guide accurately on any focal length. The argument of course is longer focal lengths have less stars, but I've only see that be an issue once.

I cannot imagine running a guide scope ever again. My rejection rate is less than 1%, my guiding even on the worst nights is still well inside my sampling. Even on my narrowband 20 minute subs, I've never had an issue. They perform flawlessly.

Brian Puhl:
Some folks here are mentioning that filter changes add time to your sequence due to settling, I'd argue you need to adjust your settings for better effectiveness. My swaps are around 5 seconds, maybe sometimes 10 seconds at most. Also, you don't need to constantly change filters every sub, I usually run about 10 per filter then switch.

The issue is quite dependent on the focuser. I have a nitecrawler on two OTA's. On those, it is possible to switch filters (and apply filter offset) without the guide star moving. For those, I find the most efficient thing is shoot something like R, R, G, G, B, B, <dither> and repeat. The dither requires a settle, but on these OTA's the filter change does not.

On another with an optec focuser, switching filters moves the guide star. It is necessary to stop/star guiding, otherwise the beginning of the next exposure is messed up as PHD2 chases the moved guide star back. On stop/start the restart time and settle is typically longer than the dither settle time. I find the best for it is R, R, dither, R, R, dither, etc. through a lot, then switch to B, B, dither, etc. This means the stop/start is only on filter changes.

There are two reasons you need to stop/start with less precise focusers, one is there is no way to just say "settle", but the other is that if the focuser moves a lot (e.g. the direction involves a large backlash compensation), PHD2 might actually lose the guide star. If it tries to re-acquire it can be slow, and might get the wrong star. When I tried doing this without stopping, I was finding every first exposure after filter change had a higher FWHM, which came from a short period of chasing the guide start to get it back.

Linwood Ferguson:
Brian Puhl:
Some folks here are mentioning that filter changes add time to your sequence due to settling, I'd argue you need to adjust your settings for better effectiveness. My swaps are around 5 seconds, maybe sometimes 10 seconds at most. Also, you don't need to constantly change filters every sub, I usually run about 10 per filter then switch.

The issue is quite dependent on the focuser. I have a nitecrawler on two OTA's. On those, it is possible to switch filters (and apply filter offset) without the guide star moving. For those, I find the most efficient thing is shoot something like R, R, G, G, B, B, <dither> and repeat. The dither requires a settle, but on these OTA's the filter change does not.

On another with an optec focuser, switching filters moves the guide star. It is necessary to stop/star guiding, otherwise the beginning of the next exposure is messed up as PHD2 chases the moved guide star back. On stop/start the restart time and settle is typically longer than the dither settle time. I find the best for it is R, R, dither, R, R, dither, etc. through a lot, then switch to B, B, dither, etc. This means the stop/start is only on filter changes.

There are two reasons you need to stop/start with less precise focusers, one is there is no way to just say "settle", but the other is that if the focuser moves a lot (e.g. the direction involves a large backlash compensation), PHD2 might actually lose the guide star. If it tries to re-acquire it can be slow, and might get the wrong star. When I tried doing this without stopping, I was finding every first exposure after filter change had a higher FWHM, which came from a short period of chasing the guide start to get it back.

Yepp, agree on all here. Backlash comp is usually what causes the loss in guide star. I run an Optec and an EAF. Haven't been able to justify the NCs cost right now as I'd rather have an obs first. NINA has the option to disable guiding during filter change, I'm sure you know this. With 1 second updates, good PEC, my settle time is low. It doesn't take long at all to recover. But again, I spend more time on a single filter to knock some of this settle time down.

OAG also offers potentially some weight reduction to your rig. I was using a WO z73 as a guider scope for my RC 10" and that was pushing my mount close to its limit. I now have an OAG and save a few KGs

In addition to what others have said, a guide scope is preferable for the popular new strainwave mounts like the ZWO AM5, which have very high periodic error that needs to be combated with very fast corrections. You need multi-star guiding to do 0.5s or shorter guide exposures, and that might not be possible with an OAG.

Linwood Ferguson:
Norman Tajudin:
Your OAG should be after your filter wheel unless you have an on-camera guide sensor. This eliminates any worry of filter thickness and focus or brightness if using narrowband. Also, for me, I recalibrate PHD2 every time I move to a new target so I don’t need to worry about my rotational changes with my rotator.

That's the right place of course but it doesn't eliminate filter thickness issues, since when you refocus the main OTA it is changing the focus on the OAG. Thus you need reasonably parfocal filters when using an OAG. Not perfect, but close enough the stars stay recognizable as you move between BB and NB (or whatever). This refocus for different filters is also, for most focusers, going to kill guiding, so even if you use focus offsets (i.e. no full auto-focus run), the lateral shift in the star position from moving the focuser will screw up guiding, and you really need to stop, switch filters, and restart. Thus a 5 second filter change becomes maybe 20-30 seconds with settle time.

Lynwood, you’re right on focus. Got sloppy with the quick response. 🤪 I use NINA with filter offsets. The offsets are small enough with my setup that they really don’t affect guider focus enough to create tracking issues. This allows me to take subs across my filters, only pausing to refocus when HFR changes arise.

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