OAG or guidescope

[Benklerk]

I'm in the process on building my fixed setup.

GSO 14 RC with a Moravian G4-16000 camera.

I'm using a Skywatcher 72ED on my portable setup as a guide scope with a ZWO 174mm mini guide camera.

I live in Bortle 3/4 skies and my seeing can get as low 1 arc sec avg is around 2", live around 1000m above sea level.

The mount i'm getting will be the new JTW friction drive mount.

I have attached the image scales using different setups.

If I don't get the OAG adapter I can get the 290mm mini guide camera instead as they cost the same.

So my question is, is going OAG worth it? The mount im getting will be very solid and I can do longish unguided. So when I do guide I can set guided pulses to 15 or 20 seconds or longer? 

[fwm891]

Quote

At that focal length an OAG would be the better option IMHO

 

[Benklerk]

37 minutes ago, fwm891 said:

 

I have read longer focal lengths better for OAG, but one down side is. Finding a star can become a problem. I stopped using my ZWO OAG on my 6" RC because it didn't find a guide star. I can guide 10min subs with no star trails using my guidescope.

[fwm891]

Larger format guide camera ?

[Jkulin]

OAG all the way, I use a Celestron OAG on my 10" and I don't usually have any problem with finding stars

[Benklerk]

Thanks for the replies, looks like OAG is the way.

Few things, the G4 16000 has its own OAG and I think it’s the only OAG that will fit it.
Without spending even more money I will be using my 174mm as it has a larger FOV. I’ll be aim to do 10 second guide subs, so that could be enough time to pick up faint stars. I can even bin it to improve pixel arc sec.

https://www.gxccd.com/art?id=436&lang=409

[michael8554]

OAG is the way to go but if you couldn't find guidestars with a 6inch RC then you will need a much more sensitive guidecam for the 14inch.

Larger format / fov ? Wouldn't  any camera cover a tiny oag mirror?   Big pixels and high sensitivity like a LodeStar 2 perhaps ?

Michael 

[Benklerk]

1 hour ago, michael8554 said:

OAG is the way to go but if you couldn't find guidestars with a 6inch RC then you will need a much more sensitive guidecam for the 14inch.

Larger format / fov ? Wouldn't  any camera cover a tiny oag mirror?   Big pixels and high sensitivity like a LodeStar 2 perhaps ?

Michael 

Hi Michael

The 174mm is about the same sentivity as the lode star but at 2/3 the cost. The Lodestar 2 is almost $1000 over here. The issue I had is, I could see the stars, but non of them we usable. They where comet looking shapes. Even a field flattener didn't help, and yes I made sure it was in focus. The other problem is the ZWO OAG mirror is half the size of the 174mm sensor.

[newbie alert]

6 hours ago, Benklerk said:

Hi Michael

The 174mm is about the same sentivity as the lode star but at 2/3 the cost. The Lodestar 2 is almost $1000 over here. The issue I had is, I could see the stars, but non of them we usable. They where comet looking shapes. Even a field flattener didn't help, and yes I made sure it was in focus. The other problem is the ZWO OAG mirror is half the size of the 174mm sensor.

Rugby ball shaped stars are quite common with oag but no probs with phd as it uses the centroid of the star to guide on..

In my head beyond 1000mm fl is a sure thing for a oag, even below that for some if they're chasing flexture..does a RC suffer with mirror flop like a sct?

Have you got  the mount with or without the encoders,  a friend's just ordered one of those  and is hoping to put a big RC on it in the future

 

Edited May 7, 2019 by newbie alert
Added info

[michael8554]

As Newbie said,  star shape isn't a problem, according to the PHD2 developers.

What did you mean, "none of the stars were usable" ?

I find using Star Mass or Min SNR in PHD2 leads to dropped stars, I only use the HFD setting, at 1.5 to reject hot pixels.

Also you're aware that you can plan a session with a fov box on a planetarium, so that you can rotate and frame the imagiing camera to include guide stars in the oag area ?

Also I notice that you plan to guide every 20 seconds ? My lowly ASI 120MM picks up nebulae at 2 second exposures,  a 20 second guidecam exposure should pick up every star in the galaxy !

Even if you are really exposing at say 2 seconds and using the delay setting in  PHD2 to delay corrections by 18 seconds, that would mean correcting an 18 seconds old error. Perhaps I'm misunderstanding  your intentions.

Maybe better to expose the usual 1 to 3 seconds, with a suitable Min Move in PHD2 ?

Michael 

[Jkulin]

I have a G2-8300 mk II with Moravians new OAG, it was a pile of pooh, it used a small mirror instead of a prism and yours looks very similar to mine, focusing was a nightmare and comet shaped stars were like massive comets. It was impossible to focus my Lodestar X2 without making my own modifications and when I informed Moravavin they said they only made the OAG to work with their cameras and that I would have to put up with it. They did however make a replacement adapter if you want to convert you camera to not using an OAG and that is what I did and then used my Celestron OAG.

The other thing was that they designed the adjuster for focus so that it fouled the field flattener for my Esprit and so I had to use a thin flat washer to lock it.

The camera is fantastic, but the design of the OAG was rubbish, they claim that the reason they made it so small was so that they could maintain the 55mm Back focus of the camera

[Benklerk]

3 hours ago, newbie alert said:

Rugby ball shaped stars are quite common with oag but no probs with phd as it uses the centroid of the star to guide on..

In my head beyond 1000mm fl is a sure thing for a oag, even below that for some if they're chasing flexture..does a RC suffer with mirror flop like a sct?

Have you got  the mount with or without the encoders,  a friend's just ordered one of those  and is hoping to put a big RC on it in the future

 

Hi newbie

RCs do not suffer from mirror flop. I’ll be getting one with encoders.

 

2 hours ago, michael8554 said:

As Newbie said,  star shape isn't a problem, according to the PHD2 developers.

What did you mean, "none of the stars were usable" ?

I find using Star Mass or Min SNR in PHD2 leads to dropped stars, I only use the HFD setting, at 1.5 to reject hot pixels.

Also you're aware that you can plan a session with a fov box on a planetarium, so that you can rotate and frame the imagiing camera to include guide stars in the oag area ?

Also I notice that you plan to guide every 20 seconds ? My lowly ASI 120MM picks up nebulae at 2 second exposures,  a 20 second guidecam exposure should pick up every star in the galaxy !

Even if you are really exposing at say 2 seconds and using the delay setting in  PHD2 to delay corrections by 18 seconds, that would mean correcting an 18 seconds old error. Perhaps I'm misunderstanding  your intentions.

Maybe better to expose the usual 1 to 3 seconds, with a suitable Min Move in PHD2 ?

Michael 

Hi Michael

When I let phd choose a star, Guiding was all over place. Even after a changed a few settings I did not help. Using a guidescope solves this issue and I can do 10min subs on my current setup.

I’ll be using SGP so It will mostly be automatic.

Yeah, a delayed guide pulse. But I’ll see how it goes when I get there.

[Benklerk]

2 hours ago, Jkulin said:

I have a G2-8300 mk II with Moravians new OAG, it was a pile of pooh, it used a small mirror instead of a prism and yours looks very similar to mine, focusing was a nightmare and comet shaped stars were like massive comets. It was impossible to focus my Lodestar X2 without making my own modifications and when I informed Moravavin they said they only made the OAG to work with their cameras and that I would have to put up with it. They did however make a replacement adapter if you want to convert you camera to not using an OAG and that is what I did and then used my Celestron OAG.

The other thing was that they designed the adjuster for focus so that it fouled the field flattener for my Esprit and so I had to use a thin flat washer to lock it.

The camera is fantastic, but the design of the OAG was rubbish, they claim that the reason they made it so small was so that they could maintain the 55mm Back focus of the camera

Hi jkuln

I’ll be getting the mk 1 version not the mk 2 as I can’t justify the cost. The OAG is slightly different on the G4 but it still uses a mirror to reflect the light. On the G4 the back focus is 61mm.

[Jkulin]

25 minutes ago, Benklerk said:

On the G4 the back focus is 61mm.

Then I would look at the ZWO OAG as it is really thin and a hundred times better that the Moravian

[Benklerk]

17 minutes ago, Jkulin said:

Then I would look at the ZWO OAG as it is really thin and a hundred times better that the Moravian

I have one and it’s crap. The prism is way to small for the 174mm. I’m going to sell it at some point.

[Jkulin]

49 minutes ago, Benklerk said:

I have one and it’s crap. The prism is way to small for the 174mm. I’m going to sell it at some point.

Maybe so but at least they kept the foot print small and used a proper prism with blackened side, I haven't used it yet but will do on my Newt because of the back focus.

[michael8554]

1 hour ago, Benklerk said:

When I let phd choose a star, Guiding was all over place. Even after a changed a few settings I did not help

So you managed to find guide stars -  I'd suggest persevering with the PHD2 settings (binning will give a better guiding image scale)  because with your tightly engineered mount and  no flexture, only Seeing should upset your guiding ?

Michael 

[ollypenrice]

On 07/05/2019 at 13:25, Benklerk said:

Hi newbie

RCs do not suffer from mirror flop. I’ll be getting one with encoders.

 

 

I always think the word 'flop' confuses the issue because mirror movement is tiny on a well made cell, but is an RC less susceptible than any other design since the mirror cannot be rigidly mounted in any system? For this reason I'd always go for an OAG on a reflector. (On the other hand a local 0.8 metre research grade scope runs in a direct drive mount so controlling the mirror must be possible. In this case it's a de-rotated alt az which might reduce the problem.)

At 0.6"PP image scale it doesn't look as if guiding will be your limiting factor in any case. (I found at 0.66"PP that we were seeing-limited. I never found it worthwhile to post images at 100% for this reason, but downsampling a little still left a pleasing image scale on small targets. From my site I can, however, post full size at 0.9"PP.) 

Using an accurate friction drive mount, a Mesu in our case, we seem to have settled on the 4 second guide sub and this never left us without a guide star with the OAG and reflector. While I've never tried longer guide subs I dare say they'd be possible and would bring new stars into play. I think you'll be fine.

Keep us posted on the JTW mount...

Olly