Which zwo guide cam for my scope

[Craig a]

Hello everybody, in the market to buy a new guide cam to replace my Orion starshoot, 

my setup is 130pds f5 also the zwo OAG 

So it’s either the 120mm mini or 290mm mini 

pixel size on the 120 is the same as my main imaging cam the 533mc pro

pixel size on the 290 is smaller but the camera is pretty much twice the price 

would the 120 be good enough for my setup? I’m also buying the helical focuser to go on the zwo OAG 

would I get enough stars with the 120 with its small sensor? Or would I be best just going straight for the 290 and be done I’m with it as it has a bigger sensor 

Edited August 5, 2021 by Craig a

[newbie alert]

I'm using a qhy 290 oag guiding my sct 

Can't fault it so far

[vlaiv]

If price difference is significant for you - go with cheaper - 290 might be a bit better - but certainly not twice better - not even 10% better in my view.

[Craig a]

Thanks vlaiv, I’m just abit worried about the smaller sensor on the 120 and getting enough stars in it to pick from, scope is 650mm fl, but I always managed to find stars using my 1000mm f4 newt with the Orion starshoot that is quite an old camera now 

[michael8554]

The 120MM-mini sensor is 4.8mm x 3.6mm.

The 290MM-mini sensor is 5.6mm x 3.2mm.

So the 290 sensor is slightly wider, less high, so not significantly bigger.

Michael

[fifeskies]

the smaller pixels on the 290 only really matter if you have a long focal length scope that you want to guide

as Michael points out the 2 guidecameras have nearly the same field of view , the 290 just giving finer resolution which will matter for a long focal length scope

you can check guidecam suitability here

https://astronomy.tools/calculators/guidescope_suitability

 

it shows the 120mm as a good match for the 130p

 

Edited August 6, 2021 by fifeskies

[Craig a]

Thanks everyone for your input, I will go ahead and get the 120😀

[tooth_dr]

16 minutes ago, Craig a said:

Thanks everyone for your input, I will go ahead and get the 120😀

I switched from a 120MM to a 290MM due to lack of stars in OAG.  I would disagree with vlaiv. I found the 290MM to be 10x (or more) better compared to the 120MM. I also think the 120MM is flaky in terms of usb connectivity. 

[tooth_dr]

It’s the sensitivity not the size of the sensor. The 290MM is just better despite having smaller pixels. 

[vlaiv]

7 minutes ago, tooth_dr said:

I switched from a 120MM to a 290MM due to lack of stars in OAG.  I would disagree with vlaiv. I found the 290MM to be 10x (or more) better compared to the 120MM. I also think the 120MM is flaky in terms of usb connectivity. 

I'm rather surprised with that.

With difference in read noise being due to quoting highest read noise for ASI120 and lowest read noise for ASI290

Highest read noise for ASI290 is 3.25e so slightly better than 4e of ASI120, but read noise is really not that important for guiding where usual SNR is 50 or more per exposure.

QE is the same. Not sure why would ASI120 be x10 worse than ASI290?

I must say that I did not compare the two, but I did guide with QHY5LIIc which has same sensor as ASI120 color and I guide now with ASI185 and I did not notice any improvement in guiding due to camera alone.

[tooth_dr]

Just now, vlaiv said:

I'm rather surprised with that.

With difference in read noise being due to quoting highest read noise for ASI120 and lowest read noise for ASI290

Highest read noise for ASI290 is 3.25e so slightly better than 4e of ASI120, but read noise is really not that important for guiding where usual SNR is 50 or more per exposure.

QE is the same. Not sure why would ASI120 be x10 worse than ASI290?

I must say that I did not compare the two, but I did guide with QHY5LIIc which has same sensor as ASI120 color and I guide now with ASI185 and I did not notice any improvement in guiding due to camera alone.

Hi Vlaiv

No theoretical analysis on my part - I found 1-2 stars with the 120MM with gain at 100%, switched to a 290MM getting 20+ stars to choose from, even with gain turned down, at times to 50-75%.  Both work fine as a guidecam with a short focal length guidescope, but as an OAG, the 290MM is just better as it works.

[vlaiv]

8 minutes ago, tooth_dr said:

Hi Vlaiv

No theoretical analysis on my part - I found 1-2 stars with the 120MM with gain at 100%, switched to a 290MM getting 20+ stars to choose from, even with gain turned down, at times to 50-75%.  Both work fine as a guidecam with a short focal length guidescope, but as an OAG, the 290MM is just better as it works.

First hand experience is really all that matters, but I do have a question, if you don't mind.

Did you use both cameras on the same patch of the sky?

I've found (with same camera) that if I point the scope at part of the sky along galactic plane - I get many guide stars even with OAG, but if I point to the part of the sky that is away from plane of our galaxy - it is far more sparser and I often have only 2-3 guide stars in field of view.

Maybe above discrepancy between cameras might have something to do with that?

[tooth_dr]

Just now, vlaiv said:

First hand experience is really all that matters, but I do have a question, if you don't mind.

Did you use both cameras on the same patch of the sky?

I've found (with same camera) that if I point the scope at part of the sky along galactic plane - I get many guide stars even with OAG, but if I point to the part of the sky that is away from plane of our galaxy - it is far more sparser and I often have only 2-3 guide stars in field of view.

Maybe above discrepancy between cameras might have something to do with that?

Yeah same part of the sky, I think it was M51, which is notoriously bad for guidestars.  I've been doing imaging long enough to compare apples with apples 😉 

 

[markse68]

Following with interest as I'm using 120mm usb2- only using single star guiding with 880mm scope and only once had to nudge scope a little to find a star (not a big deal really) Wouldn't a larger sensor size be more beneficial for OAG?

Mark

[tooth_dr]

Just now, markse68 said:

Following with interest as I'm using 120mm usb2- only using single star guiding with 880mm scope and only once had to nudge scope a little to find a star (not a big deal really) Wouldn't a larger sensor size be more beneficial for OAG?

Mark

There is so little difference I dont think it really makes much difference between these two, but if you go larger eg 174, then you will get vignetting from the small OAG prism (typically 8mm x 8mm).  I have no personal experience, and have found the 290MM works great.  Incidentally I still use the 120MM with a guidescope and it works perfectly as the demands are much lowers in terms of sensitivity.

[vlaiv]

8 minutes ago, markse68 said:

Following with interest as I'm using 120mm usb2- only using single star guiding with 880mm scope and only once had to nudge scope a little to find a star (not a big deal really) Wouldn't a larger sensor size be more beneficial for OAG?

Mark

Larger sensor with OAG won't help much. Even smaller sensors can be under utilized if distance of OAG is not optimized on faster systems.

ASI185 has 8.6mm diagonal (7.3mm*4.6mm) and it vignettes even at F/8:

 

[markse68]

14 minutes ago, tooth_dr said:

There is so little difference I dont think it really makes much difference between these two, but if you go larger eg 174, then you will get vignetting from the small OAG prism (typically 8mm x 8mm).  I have no personal experience, and have found the 290MM works great.  Incidentally I still use the 120MM with a guidescope and it works perfectly as the demands are much lowers in terms of sensitivity.

I think my prism is 1/2" so maybe a larger sensor could be utilised?

Mark

[tooth_dr]

7 minutes ago, markse68 said:

I think my prism is 1/2" so maybe a larger sensor could be utilised?

Mark

I'm not sure, sorry!  What OAG are you using, I've seen 10mm but not 12.5mm prisms.

[vlaiv]

3 minutes ago, markse68 said:

I think my prism is 1/2" so maybe a larger sensor could be utilised?

Mark

Larger prism surely helps.

Here is simple guideline - beam is not vignetted in single point (center of camera) - if you place OAG away at such distance that it matches F/speed of your scope.

Say you have 12mm prism and F/5 scope. At 60mm you'll have threshold - greater than that between OAG and focal plane and you'll have effective aperture stop (not only vignetting but reduced aperture). Closer than that and you won't have aperture stop but there will be some vignetting - just how much can be solved with a bit of trigonometry.

8mm prism will bring this distance down to 40mm for F/5 scope, while F/8 will have 64mm before aperture stop point (that is why it is more important to place OAG close for fast systems - slow ones have more "room").

[markse68]

1 hour ago, tooth_dr said:

I'm not sure, sorry!  What OAG are you using, I've seen 10mm but not 12.5mm prisms.

It's built into my camera. But I'm not sure how far the light cone reaches out so it would likely vignette the outer edge of a larger sensor anyway but could be useful to have extra width which is I think what Vlad said

Mark

Edited August 6, 2021 by markse68

[Craig a]

Ok my OAG is the zwo v2, I think that’s an 8mm prism, if I can find guide stars with my 1000mm newt and ancient Orion starshoot guide cam, surely the 120mm should have some stars to pick from through a 650mm newt

Edited August 6, 2021 by Craig a

[tooth_dr]

15 hours ago, Craig a said:

Ok my OAG is the zwo v2, I think that’s an 8mm prism, if I can find guide stars with my 1000mm newt and ancient Orion starshoot guide cam, surely the 120mm should have some stars to pick from through a 650mm newt

I don’t think you’ll have trouble finding stars, you just won’t have as many with the 120MM.  From personal experience with my current setup, the 120 wasn’t quite adequate for OAG, but the 290 gives completely hassle-free OAG guiding.  I’m using it with a 1200mm F4.7 Newtonian.  I just wanted to share actual practical experience rather than theorem. 

[tomato]

I use a 120 mini with a ZWO OAG v2 on an Esprit 150 FL 1050 mm. I have to date  never failed to have a guide star in the FOV, but there have been occasions where only 2 or 3 are visible. The USB connection  on the camera is a bit fragile, best to loop the cable back round and velcro to the camera body.

[michael8554]

Try this OAG Calculator to match camera size to prism size:

https://www.smallstarspot.com/oagcalc.html

Michael