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ZWO ASI120mm mini vs ASI290mm mini for OAG - sensitivity

astrophotography beginner equipment
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#1 salehest6990

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Posted 11 June 2019 - 12:00 AM

In another thread regarding OAG performance, it was stated that ASI120mm mini is not sensitive enough for OAG and ASI290mm mini is a better option because of its higher sensitivity. However, after looking at the specs, The only difference that I see (except for a small difference in sensor size), is the increased resolution. With the increased resolution, pixel size is decreased and my understanding is that it would actually make it less sensitive. Am I missing something? Thanks!



#2 Ken Sturrock

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Posted 11 June 2019 - 12:19 AM

You are correct. "Sensitivity" is kind of a bogus description. More specifically, you can look at sensor efficiency and pixel size. Gain and the resulting noise can also factor in because some reputedly "sensitive" guide cameras also have their gain cranked up and, therefore, also exhibit a lot of noise. Both efficiency and pixel size contribute to "sensitivity" but in different ways and, in the case of pixel size, a trade-off with resolution.

 

I admit that I don't own a 120, but do own (and like) a 290. If it were me and the cost didn't matter, I'd choose based on the image scale. If the guider setup has a shorter focal length, I'd lean towards the 290 because it will give you better resolution which would, typically, allow a more accurate centroid measurement. The difference in efficiency between the cameras doesn't appear to be huge. There may be other differences between the cameras, however, of which I am ignorant.

 

The modern ZWO purpose-built guiders are nice little cameras. Nice packaging, good performance, competitive price and reliable multi-platform drivers.

 

Best of luck with the decision.


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#3 salehest6990

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Posted 11 June 2019 - 12:37 AM

Thanks Ken. My focal length is 480mm so with 120mm mini, I would get an image scale of 1.6 and with 290mm mini, I would get an image scale of 1.25. My imaging camera is the 1600mm Pro which yields an image scale of ~1.6 too. I guess the 120mm mini should be fine with OAG; worst case I would return it and get the 290mm mini in case it gave me too much trouble.



#4 Ken Sturrock

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Posted 11 June 2019 - 12:59 AM

It sounds like you'd be fine with either one.

 

For what it's worth, I found that my 80mm f/6 (native 480mm focal length) OAG guided just fine at 3.18 AS/pixel (SBIG ST-2k and SBIG ST-i). I also guided my now-departed SV-70t (336mm focal length due to 0.8x reducer) with an ASI-290 at 1.8 AS/pixel and typically guide my SVQ (580mm focal length) at 3.0 AS/pixel with a Lodestar X2. I prefer the look of the stars in the higher resolution cameras, but they all seem to guide fine.


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#5 Stelios

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Posted 11 June 2019 - 02:34 AM

If you will never guide any longer focal length than 480mm you might be OK. 

 

But it's *not* an image scale issue. It's a "can I find a star to guide on" issue.

 

An OAG gets a small window into the image, unlike a guidescope. There are far fewer stars to choose from than with a guidescope. You need to be able to see fainter stars so you are not left with nothing to guide on.

 

Think of it this way--if it was possible to use a $149 camera for the OAG, why would people spend $300 (ASI290), $500 (ASI174) or $650+ (Lodestar X2, Ultrastar?) Why wouldn't someone with unbridled success with an ASI120 have enthusiastically posted of how smart he was?


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#6 happylimpet

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Posted 11 June 2019 - 03:31 AM

The 290 has much lower read noise and much much lower fixed pattern noise; probably better QE also.

 

The combined effect of all f these things is to make the camera a heck of a lot more sensitive - ie a field of view which might have 1 guidable star with the 120 seems to have more like 5 with the 290. When I was using the 120 I would fairly frequently have problems findings a guidestar. I never have with the 290.

 

PS yes, youre right that the smaller pixel scale works against it, but the pluses outweigh this!  Note I bin with both of them.


Edited by happylimpet, 11 June 2019 - 03:32 AM.


#7 ChrisWhite

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Posted 11 June 2019 - 05:49 AM

Theory and specs aside I have used both sensors to guide with oag, and there is no question that the 290 is a better camera for this purpose. Call it sensitivity or whatever, you get more stars to choose from with the 290 in the same fov as the 120. (Ignoring the larger sensor)

#8 Ken Sturrock

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Posted 11 June 2019 - 10:30 AM

I hear this a lot, that the 120 isn't as good as the 290. I don't doubt it. It does cost more (as Stelios notes), but I wonder why it does so much better?

 

I know that the 290 sensor is larger. Yet, it's not amazingly larger on the sky:

 

astronomy_tools_fov.png

 

I know that it's slightly more efficient. I know that it's a bit quieter. It must be, as happylimpet says, a sum of the parts issue...

 

Oh well. I'll vouch for it. I've been quite happy with mine.

 

 

 



#9 ChrisWhite

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Posted 11 June 2019 - 11:46 AM

The 290 also has adjustable gain. It's been a few years since I used the 120 sensor but indont think it does.

I had meridian flips where the 120 sensor couldn't find a guide star while the 290 on the same target had a dozen to choose from.

#10 Lead_Weight

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Posted 11 June 2019 - 12:03 PM

I have the 290 and 174. I tried the 290 with my OAG, and it worked moderately. But it is intended to be used with a guide scope. The main issue I had was that the sensor is so small, it could only see 1-2 or often no guide stars due to the physical size of the sensor. (On an Edge 11) so I moved to the 174 because while the resolution wasn’t high, the sensor is physically larger....4x larger than the 290 sensor, so now I can pick up multiple guide stars as the FOV is larger. So, that aspect might be worth considering.
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#11 ChrisWhite

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Posted 11 June 2019 - 12:26 PM

I have the 290 and 174. I tried the 290 with my OAG, and it worked moderately. But it is intended to be used with a guide scope. The main issue I had was that the sensor is so small, it could only see 1-2 or often no guide stars due to the physical size of the sensor. (On an Edge 11) so I moved to the 174 because while the resolution wasn’t high, the sensor is physically larger....4x larger than the 290 sensor, so now I can pick up multiple guide stars as the FOV is larger. So, that aspect might be worth considering.


Yeah I can see that. For really long focal lengths it might not be as good. I've never imaged past 920mm fl.

#12 Ken Sturrock

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Posted 11 June 2019 - 01:54 PM

Interestingly, the ZWO OAG prism fits the ASI-290 sensor perfectly. A larger camera, like a 174 or a Lodestar, has a lot of its sensor area wasted on that OAG. A larger prism works well on the 174. Like Chris, I've only used my ASI-290 on shorter instruments where the FOV is fine and the small pixels are useful. My SV-70t, for example, was smaller than some folks separate guider scopes.


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#13 cfosterstars

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Posted 11 June 2019 - 02:49 PM

I am just sort of getting started with OAGs. I have two QHY-M and one celestron OAGs for my three OTA. I have one ASI290MM-min, one ASI174MM-mini and one QHY 5-III 290 so i dont have an ASI120 to compare. However, so far I have the most experience with the ASI290MM-mini on the celestron OAG for my Meade 10" ACF at 2159mm F/L. In very limited testing, it has worked fine. Tonight, I will doing my first real test with that set up. The other two OAGs are not ready yet and I am using my ASI174MM-mini on my guidescope for now. Its a VERY nice camera and it should be for the price. I have another thread going on my learning for these guiders and will keep posting data as I go along. At least with my limited experience, even at long F/L the ASI290MM-mini looks quite workable and nicely sensitive. The main issue with ASI174MM-mini, is that the QHY-M prism would not fill the sensor and it looked like a porthole - very ugly and I did not like it. It would probably have worked just fine though. Just did not really test it out.


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#14 robedude

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Posted 11 June 2019 - 03:03 PM

I have the 290 and 174. I tried the 290 with my OAG, and it worked moderately. But it is intended to be used with a guide scope. The main issue I had was that the sensor is so small, it could only see 1-2 or often no guide stars due to the physical size of the sensor. (On an Edge 11) so I moved to the 174 because while the resolution wasn’t high, the sensor is physically larger....4x larger than the 290 sensor, so now I can pick up multiple guide stars as the FOV is larger. So, that aspect might be worth considering.

Lead_Weight, what OAG are you using? I also have an EdgeHD1100 and am plagued by issues with finding a suitable guidestar using my ZWO OAG & ASI120mm mini. I'm wondering if moving to a 174 with the bigger sensor size might help me in this aspect.

 

Interestingly, the ZWO OAG prism fits the ASI-290 sensor perfectly. A larger camera, like a 174 or a Lodestar, has a lot of its sensor area wasted on that OAG. A larger prism works well on the 174. Like Chris, I've only used my ASI-290 on shorter instruments where the FOV is fine and the small pixels are useful. My SV-70t, for example, was smaller than some folks separate guider scopes.

Yes the narrow prism poses a challenge on my C11, often times I get set up for imaging and there is not a single suitable guide star in my field of view on the asi120mm mini - I wonder how much advantage a larger sensor would give me before it gets cut off by the diminishing light cone.



#15 Stelios

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Posted 11 June 2019 - 07:41 PM

Lead_Weight, what OAG are you using? I also have an EdgeHD1100 and am plagued by issues with finding a suitable guidestar using my ZWO OAG & ASI120mm mini. I'm wondering if moving to a 174 with the bigger sensor size might help me in this aspect.

 

Yes the narrow prism poses a challenge on my C11, often times I get set up for imaging and there is not a single suitable guide star in my field of view on the asi120mm mini - I wonder how much advantage a larger sensor would give me before it gets cut off by the diminishing light cone.

If you use a ZWO OAG, the field of view will be the same with the 290 and the 174. 

 

What you will gain vis-a-vis the 120 is "sensitivity"--translated to mean more stars visible for the same area.

 

If you can fit in a Celestron OAG, then with the 174 you will be in hog heaven. You'll be able to sell the surplus stars to other OAG users lol.gif lol.gif lol.gif


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#16 cfosterstars

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Posted 11 June 2019 - 07:49 PM

If you use a ZWO OAG, the field of view will be the same with the 290 and the 174. 

 

What you will gain vis-a-vis the 120 is "sensitivity"--translated to mean more stars visible for the same area.

 

If you can fit in a Celestron OAG, then with the 174 you will be in hog heaven. You'll be able to sell the surplus stars to other OAG users lol.gif lol.gif lol.gif

Stelios,

 

I have a celestron OAG and an ASI174MM-mini, but have not tried the combo. How much vignetting do you get with the larger prism on the celestron? My QHY-M with an 8mm prism looked like a porthole. I got the celestron with the 174 in mind. Have you tried one?

 

Chris



#17 Lead_Weight

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Posted 11 June 2019 - 09:02 PM

Lead_Weight, what OAG are you using? I also have an EdgeHD1100 and am plagued by issues with finding a suitable guidestar using my ZWO OAG & ASI120mm mini. I'm wondering if moving to a 174 with the bigger sensor size might help me in this aspect.

 

Yes the narrow prism poses a challenge on my C11, often times I get set up for imaging and there is not a single suitable guide star in my field of view on the asi120mm mini - I wonder how much advantage a larger sensor would give me before it gets cut off by the diminishing light cone.

I have the same OAG. I also have a Moonlite focuser, so at times, I've had to rotate the focuser to find a guide star. I'm tempted to pick up a rotator motor for the Moonlite so I don't have to do this by hand. But because it's manual now, I essentially have to babysit it through meridian flips and such. But compared to the 290, the 174 has much larger pixels, and has no issues picking out stars. There were times with the 290 I could see an extremely faint star, but the guiding software would often lose it after guiding some. The 174 has fixed those issues.


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#18 Lead_Weight

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Posted 11 June 2019 - 09:09 PM

If you use a ZWO OAG, the field of view will be the same with the 290 and the 174. 

 

What you will gain vis-a-vis the 120 is "sensitivity"--translated to mean more stars visible for the same area.

 

If you can fit in a Celestron OAG, then with the 174 you will be in hog heaven. You'll be able to sell the surplus stars to other OAG users lol.gif lol.gif lol.gif

The field of view isn't the same. But with the 174 there is vignetting, unless I can move the camera back some, which I've not attempted yet since it means moving the main camera back too. The other possibility as you suggest would be to get a system with a larger prism, but that also means changing the spacing to the main camera since the Celestron OAG is huge (about twice the thickness of the ZWO OAG).

 

Here's what the difference in the 290/174 looks like. The large rectangle is the Edge11 FOV, and the really small offset rectangle is the 290, and the larger offset rectangle around it is the 174. It's very easy to see in an image like this, how many more stars fit into the field of view of the 174. I count at least 10 or so, and inside the 290 area there's only around 4-5. What isn't shown here is the apparent magnitude of those stars. So out of the ones you can see in the chart, only 2-3 will be visible enough to guide on.

 

edge11.jpg


Edited by Lead_Weight, 11 June 2019 - 09:12 PM.

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#19 RossW

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Posted 11 June 2019 - 09:46 PM

Lead_Weight, what OAG are you using? I also have an EdgeHD1100 and am plagued by issues with finding a suitable guidestar using my ZWO OAG & ASI120mm mini. I'm wondering if moving to a 174 with the bigger sensor size might help me in this aspect.

 

The ZWO OAG plus the 120mm on an EdgeDH 1100 is a killer combination: it will literally kill your ability to image many DSOs (I speak from experience). To quote the oft-spoken words of the current US president, "it's a disaster". For OAG guiding at 2.8m focal length you need a sensor with larger real estate than the 120. The 174 would be much better but not with the ZWO OAG due to the small prism; you'll see a tiny porthole FOV that uses only a small proportion of the 174's sensor. The majority of the sensor area of that relatively expensive guide camera will be wasted.

 

But if you want to stay with the ZWO OAG due to back-focus considerations there is another less expensive alternative that works well (for me at least): the ASI178mm. It has about twice the sensor area as the 120 and 290, so in general you'll have twice as many stars to choose from. There is still some vignetting but I've regularly guided on stars even outside the vignetting "porthole". The ASI178mm has an M42 female threaded connection and that will screw directly onto the OAG's M42 male thread. Focus is then achieved by adjusting the height of the camera holder on the prism stalk. 

 

Getting back on topic, I also have an ASI120mm plus ZWO OAG for guiding on my 740mm focal length refractor. So far I have always had ample stars to guide on, and given the 290 has basically the same sensor area it would see the same number of stars as the 120. Slightly lower QE on the 290, plus it has smaller pixels than the 120, so SNR per pixel should I think be slightly less than the 120, so like Ken I am surprised that others have found the 290 to provide a better outcome (unless the colour version of the 120 was being used?). But experience counts, so if you can afford the 2.5x price difference the 290 may be the way to go. 

 

I'm now experimenting to see if I can replace my ZWO OAG with the much fatter (body-wise and prism-wise) Celestron OAG. It's going to be a very tight fit along with an external focuser and ZWO filter wheel, given just 146mm of back focus.

 

Cheers,

 

Ross

 

 


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#20 robedude

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Posted 11 June 2019 - 11:01 PM

If you use a ZWO OAG, the field of view will be the same with the 290 and the 174. 

 

What you will gain vis-a-vis the 120 is "sensitivity"--translated to mean more stars visible for the same area.

 

If you can fit in a Celestron OAG, then with the 174 you will be in hog heaven. You'll be able to sell the surplus stars to other OAG users lol.gif lol.gif lol.gif

Lead_Weight, what OAG are you using? I also have an EdgeHD1100 and am plagued by issues with finding a suitable guidestar using my ZWO OAG & ASI120mm mini. I'm wondering if moving to a 174 with the bigger sensor size might help me in this aspect.

 

The ZWO OAG plus the 120mm on an EdgeDH 1100 is a killer combination: it will literally kill your ability to image many DSOs (I speak from experience). To quote the oft-spoken words of the current US president, "it's a disaster". For OAG guiding at 2.8m focal length you need a sensor with larger real estate than the 120. The 174 would be much better but not with the ZWO OAG due to the small prism; you'll see a tiny porthole FOV that uses only a small proportion of the 174's sensor. The majority of the sensor area of that relatively expensive guide camera will be wasted.

 

But if you want to stay with the ZWO OAG due to back-focus considerations there is another less expensive alternative that works well (for me at least): the ASI178mm. It has about twice the sensor area as the 120 and 290, so in general you'll have twice as many stars to choose from. There is still some vignetting but I've regularly guided on stars even outside the vignetting "porthole". The ASI178mm has an M42 female threaded connection and that will screw directly onto the OAG's M42 male thread. Focus is then achieved by adjusting the height of the camera holder on the prism stalk. 

 

Getting back on topic, I also have an ASI120mm plus ZWO OAG for guiding on my 740mm focal length refractor. So far I have always had ample stars to guide on, and given the 290 has basically the same sensor area it would see the same number of stars as the 120. Slightly lower QE on the 290, plus it has smaller pixels than the 120, so SNR per pixel should I think be slightly less than the 120, so like Ken I am surprised that others have found the 290 to provide a better outcome (unless the colour version of the 120 was being used?). But experience counts, so if you can afford the 2.5x price difference the 290 may be the way to go. 

 

I'm now experimenting to see if I can replace my ZWO OAG with the much fatter (body-wise and prism-wise) Celestron OAG. It's going to be a very tight fit along with an external focuser and ZWO filter wheel, given just 146mm of back focus.

 

Cheers,

 

Ross

Seems like the celestron OAG is the way to go lol.gif . I too also have the ZWO filter wheel but no external focuser. I'm thinking using the celestron OAG should not work since I have 55mm of space to fill after the SCT T-Adapter & the ZWO camera and filter wheel+thread takes up 28.5mm, leaving me with 26.5mm of space but the body of the celestron OAG is already 29mm before any adapters. I need to investigate and see if I can make this work since the ZWO OAG ain't workin' at this focal length.


Edited by robedude, 11 June 2019 - 11:15 PM.


#21 Stelios

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Posted 12 June 2019 - 01:54 AM

The vignetting with the 174 on the ZWO OAG is very severe. The "porthole" effect is quite pronounced (but the camera *will* find and guide with stars in at least part of the vignetted areas, i.e. at the rim of the porthole and even a bit beyond).

 

I don't have a Celestron OAG (I will eventually get one, but I have other priorities right now). So far I have experienced no issues with Meridian flips (I've only shot 6 targets with the Edge, only having started using it in late May due to weather: M51, M63, M101, M104, M27, M57) or with finding stars. I never had to rotate (I also have a Moonlite CHL2.5"--I don't think they make a rotator for it, the rotators are for the refractor focusers, and the Lightcrawler requires a 11" or larger SCT).

 

I don't babysit meridian flips. At my age I need my sleep. Maybe I'm lucky, maybe I have the right camera, maybe Bortle 3-4 skies help. 

 

The Celestron OAG will work with the Moonlite (someone made it work on another thread), but generally won't work without it. With the Moonlite you have a bit of a leeway--you don't really need to position the focuser at the exact middle (as recommended) as long as you're no more than 1-2mm off.


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#22 Gymbow

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Posted 12 June 2019 - 03:29 PM

Seems like the celestron OAG is the way to go lol.gif . I too also have the ZWO filter wheel but no external focuser. I'm thinking using the celestron OAG should not work since I have 55mm of space to fill after the SCT T-Adapter & the ZWO camera and filter wheel+thread takes up 28.5mm, leaving me with 26.5mm of space but the body of the celestron OAG is already 29mm before any adapters. I need to investigate and see if I can make this work since the ZWO OAG ain't workin' at this focal length.

The Celestron OAG is a big improvement over the ZWO OAG when using longer focal length scopes (I have both and use the ZWO on shorter FL scopes).  I also use the ASI174 mini, which works great with the COAG, but vignettes excessively with the ZOAG. 

 

I think you should have no trouble using the COAG with your setup, including the filter wheel.  Note, you don’t need to follow the ZWO 55mm recommendation with the COAG, and if you did, you’d complicate placement of your guider camera.  The key is to maintain the Celestron back focus requirement.  See link below. 

 

https://www.cloudyni...s-need-to-know/


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#23 cfosterstars

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Posted 12 June 2019 - 06:24 PM

The Celestron OAG is a big improvement over the ZWO OAG when using longer focal length scopes (I have both and use the ZWO on shorter FL scopes).  I also use the ASI174 mini, which works great with the COAG, but vignettes excessively with the ZOAG. 

 

I think you should have no trouble using the COAG with your setup, including the filter wheel.  Note, you don’t need to follow the ZWO 55mm recommendation with the COAG, and if you did, you’d complicate placement of your guider camera.  The key is to maintain the Celestron back focus requirement.  See link below. 

 

https://www.cloudyni...s-need-to-know/

Good to hear that the COAG will no have vignetting with the ASI174MM-mini. That's what I bought it for!



#24 Chapstick

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Posted 12 June 2019 - 10:07 PM

I was in similar position, I was looking at the 290 vs 174.  In the end I bought the 174MM-mini with celestron OAG for my Edge11@f10 - in every target I have multiple stars I can guide off, in fact I've not even had to rotate to search for stars yet.  I did have to order some adaptors for it as the included nose piece isnt just long enough for my liking.  I ended up going T28>T2 so that I could have a more 'solid' connection between guider and OAG.


Edited by Chapstick, 12 June 2019 - 10:10 PM.

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#25 Phishin_phool

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Posted 16 October 2020 - 06:26 PM

Currently I am uncertain if I will go to OAG guiding - I  guess it depends on how satisfied I am with guidescope guiding on the new set-up. Currently I have been using a 120mm mini with a 50mm guidescope and a WO 71mm f/5.9 with 0.8 FF/FR=332 @f/4.72 after reduction. I now also have a WO Z103 f/6.9 with 0.8 FF/FR=569mm fl @f/5.52 and the matching WO 50mm f/4 guidescope. I had planned on just grabbing another zwo 120mm mini but after reaidng this it seems there would be an advantage to a 290 if I ever planned to go OAG. If I stayed using a guidescope at the specified 569fl  f/5.2 would there be a drawback to the 290 mini?

(I still struggle to understand arcseconds per pixel etc and what is proper resolution for a given camera and imaging system- usually rely on astronomy tools to show me the line on a graph - looks like significantly undersampled )


Edited by Phishin_phool, 16 October 2020 - 06:28 PM.



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