ZWO ASI120 Colour v Mono mini sensitivity comparison

Imme (Jon) · #1 24-08-2018, 09:08 AM

I'm making the move to an OAG from a ZWO guidescope.
The guidescope has worked flawlessly for me......I'm changing purely to try and improve my pictures as I'm told this is what I need to do (yet to be proven but I'll give it a try!)

I have been using an asi120 colour (USB3) with the guidescope but probably require something a little more sensitive for the OAG so I started looking at a mono camera. The obvious (cheapest) choice was to go down the track of the asi120 mono mini.

You know what though......NOWEHERE on the web could I find actual evidence of how much more sensitive a mono camera was compared to a colour one. Best I could find is numerous posts on what people thought, but no actual evidence. Ridiculous hey!

So here is the evidence.......all pics taken of the same patch of sky within a 5 minute period under 98% moon. PHD2 screen contrast slider was all the way to the right (darkest)

Pic 1 ASI120 Colour (USB3) through a ZWO guidescope (Gain 95, Exp - 3 sec)
Pic 2 ASI120 Colour through a ZWO guidescope (Gain 95, Exp - 3 sec). Binning 2x2
Pic 3 ASI120 Mono mini (USB2) through a ZWO guidescope (Gain 95, Exp - 3 sec)
Pic 4 ASI120 Mono mini (USB2) through a ZWO guidescope (Gain 95, Exp - 1.5 sec). Binning 2x2. Note this was a 1.5 second pic as background sky brightness due to the moon gave me a white screen.

I'm feeling confident the extra sensitivity of the mono, especially when binned, will work for the OAG.

Telescope is a WO FLT132 F7, OAG will be the ZWO model (yet to arrive).....anyone comment on their experience with this camera/OAG setup?

Symon (Simon) · #2 24-08-2018, 09:14 AM

Thank you for showing this as I was actually trying to look up similar information
Many Thanks
Simon

lazjen (Chris) · #3 24-08-2018, 09:16 AM

I can't comment on the actual camera, but I used to use a ST-i colour for my oag before switching to a mono ASI174 - it worked ok, but of course the mono works a heap better.

I'd be willing to bet that even your colour ASI120 would do an ok job for the OAG in most situations.

As for the OAG improving things - it will depend. What problems are you having now?

Imme (Jon) · #4 24-08-2018, 09:40 AM

Quote:

Originally Posted by lazjen

I can't comment on the actual camera, but I used to use a ST-i colour for my oag before switching to a mono ASI174 - it worked ok, but of course the mono works a heap better.

I'd be willing to bet that even your colour ASI120 would do an ok job for the OAG in most situations.

As for the OAG improving things - it will depend. What problems are you having now?

To be honest, I'm not really having problems, I can guide for 10 minutes and still sit under a 1 pixel variation on PHD2.......but I have a single 'issue' in my head that keeps coming back to plague me.

The issue - I run a guidescope that is approx. 1/4 of my focal length. Pixel size on my guidecam is about equal to the image cam......so in my head that says that every guide error I get is multiplied by a factor of 4 on my image cam. Yep, I understand about sub pixel guiding and all the rest but at the end of the day there will always be that factor of 4 thing there.

A simple way to remove that factor of 4 thing is to use an OAG.
Will it make a difference? Possibly....investigating that 'possibly' thing is worthwhile to me.
Also, the secondary benefit I'll get is saving approx. 600g of weight on a setup that is probably a little heavy already.

If it all goes to crap and causes me issues I can easily swap back to a guidescope so I'm not too fussed. I'm setting up a second scope in the ob's so needed another guidecam anyway.

Icearcher (Chris) · #5 24-08-2018, 09:59 AM

Hi Jon

Thats fantastic information.

In hindsight, if you were buying one for the guide scope again, would you choose mono over OSC? Have you ever had any issues guiding with the OSC version? (im about to by my first guide camera)

Regards
Chris

Imme (Jon) · #6 24-08-2018, 10:10 AM

Quote:

Originally Posted by Icearcher

Hi Jon

Thats fantastic information.

In hindsight, if you were buying one for the guide scope again, would you choose mono over OSC? Have you ever had any issues guiding with the OSC version? (im about to by my first guide camera)

Regards
Chris

Hey Chris,
I have never had an issue with the OSC and guiding. Never failed to find a guidestar wherever I pointed it.....but obviously the mono is more sensitive so rather than just finding a guidestar, you will find multiple. As indicated in another post in this thread 99.9% of the time I have no limit to how long I can guide and stay within a 1 pixel tolerance.
The benefit and reason I went OSC is it gave me the option to do planetary/moon stuff in colour if I wanted to.
Based on where I was in the hobby when I got the OSC yes, I would buy the same again as it served the purpose I needed it for and served the secondary planetary purpose.
However now, as I get 'deeper' in the hobby I want to try and get the best I can from the time I spend under the stars so have upgraded this camera.

Icearcher (Chris) · #7 24-08-2018, 10:20 AM

Thanks Jon

Thats reassuring, Im at the same stage and with the same ideas of using it for guiding and planetary work.

Also, in this hobby, they are not really an expensive item compared to what else we spend our dollars on.

Marke (Mark) · #8 24-08-2018, 11:04 AM

I tried out my 120 with my OAG briefly but found it wasn't sensitive enough for me so went back to my Lodestar which gave a lot more guide stars.

Imme (Jon) · #9 24-08-2018, 11:08 AM

Quote:

Originally Posted by Marke

I tried out my 120 with my OAG briefly but found it wasn't sensitive enough for me so went back to my Lodestar which gave a lot more guide stars.

Was that on the triplet or RC10?

How light polluted are your skies?

Marke (Mark) · #10 24-08-2018, 11:33 AM

Quote:

Originally Posted by Imme

Was that on the triplet or RC10?

How light polluted are your skies?

With both worse with triplet and my skies are just average suburban . Was quite a difference in sensitivity and my Lodestar is 6yrs old . At first I thought it was the region I was looking at till I swapped them over never went back to the 120 after that . It may be ok with a guide scope but struggles with an OAG.

Imme (Jon) · #11 24-08-2018, 11:36 AM

Quote:

Originally Posted by Marke

With both worse with triplet and my skies are just average suburban . Was quite a difference in sensitivity and my Lodestar is 6yrs old . At first I thought it was the region I was looking at till I swapped them over never went back to the 120 after that . It may be ok with a guide scope but struggles with an OAG.

Interesting. Guess I'll see how it goes for me next week. I'm pretty dark out this way so hopefully I'll get by.

Marke (Mark) · #12 24-08-2018, 11:39 AM

Quote:

Originally Posted by Imme

Interesting. Guess I'll see how it goes for me next week. I'm pretty dark out this way so hopefully I'll get by.

You may be fine depends on a lot of variables and as I said would be ok with guide scope but an OAG was a very small fov to pick stars from and I found the 120 just to frustrating for my use .

Imme (Jon) · #13 29-08-2018, 09:21 AM

Hi guys,

Just an update on this one.

I installed the asi120 mini mono and ZWO OAG last night. Set to 1x1 binning I had a significant number of stars to chose from to guide under a 97% moon using 2 second exposures and 95% gain. Even moving the scope to 'naked' areas of sky still produced a few good stars to guide from.

My view - the OAG and asi120 mini mono combo works great together. Based on what I saw in testing last night I should be able to point anywhere and have at least a few good guidestars in my field of view.

cometcatcher (Kevin) · #14 29-08-2018, 11:25 PM

Back about 20 years ago when we used to do EAA, my mate and I bought two frame integration video cameras. I bought the mono, he bought the colour. Same result as yours, the mono flogged the colour. In fact I still use the mono video camera now for guiding!