OAG

[Starwiz]

I suspect I'm getting a bit of flexure during parts of my imaging sessions.  There is slight star trailing despite the polar alignment and guiding data looking very good.

I'm thinking about getting an OAG.  Does anyone have any recommendations and is this one any good?  https://www.firstlightoptics.com/off-axis-guiders-oag/zwo-off-axis-guider-oag.html

Thanks

John

[Spaced Out]

I have the ZWO OAG, it works fine for me. I have a fairly sensitive guide cam (lodestar X2) and I haven’t tried any other guide cams with it.

I use a Baader MPCC mkIII and I found it a bit of a faff getting CC spacing + OAG + filter wheel + camera all lined up and working well. The problem I had was CC spacing. At the correct spacing (I think I added about 1.6mm) the prism of the OAG was sitting over one corner of the imaging sensor, which I didn’t like very much ! To get it to line up nice and level with the flat side of the sensor I used 2 thinner spacers adding up to the total required spacing instead. I put one between OAG and camera and one the other side between the OAG and MPCC, then it lined up nicely. 

The only negative I can think of about this OAG is that the stalk with the prism on feels a little bit wobbly sometimes, not as sturdy as I imagined, doesn’t affect operation tho.

[Starwiz]

8 minutes ago, Spaced Out said:

I have the ZWO OAG, it works fine for me. I have a fairly sensitive guide cam (lodestar X2) and I haven’t tried any other guide cams with it.

I use a Baader MPCC mkIII and I found it a bit of a faff getting CC spacing + OAG + filter wheel + camera all lined up and working well. The problem I had was CC spacing. At the correct spacing (I think I added about 1.6mm) the prism of the OAG was sitting over one corner of the imaging sensor, which I didn’t like very much ! To get it to line up nice and level with the flat side of the sensor I used 2 thinner spacers adding up to the total required spacing instead. I put one between OAG and camera and one the other side between the OAG and MPCC, then it lined up nicely. 

The only negative I can think of about this OAG is that the stalk with the prism on feels a little bit wobbly sometimes, not as sturdy as I imagined, doesn’t affect operation tho.

Thanks.

I'm using a Zwo ASI1600mm-pro plus filter wheel for imaging and a Zwo  ASI120mc for guiding, so I guess it should be compatible.

John

[discardedastro]

7 minutes ago, Starwiz said:

 

I'm using a Zwo ASI1600mm-pro plus filter wheel for imaging and a Zwo  ASI120mc for guiding, so I guess it should be compatible.

You may need a more sensitive camera for OAG - the 120MC is OK with a guidescope, but might be hit-and-miss on an OAG. https://www.talkunafraid.co.uk/2019/04/adventures-in-differential-flexure/ I ended up replacing my guidescope rings with some solid machined blocks of aluminium and that eliminated my flexure problems almost entirely as a cheaper fix.

[Starwiz]

2 minutes ago, discardedastro said:

You may need a more sensitive camera for OAG - the 120MC is OK with a guidescope, but might be hit-and-miss on an OAG. https://www.talkunafraid.co.uk/2019/04/adventures-in-differential-flexure/ I ended up replacing my guidescope rings with some solid machined blocks of aluminium and that eliminated my flexure problems almost entirely as a cheaper fix.

Thanks.  An interesting article on the differential flexure, so I'll have a good read.

I scrolled down on the content of the link I originally posted and the example set-up shows a Zwo 120mc being used with the OAG, so I guess my question is, does anyone else use it with this OAG?  I'm also wondering if if makes a difference with what scope is being used (F-number) as the OAG is tapping in on the light from it?  I'm using an F5 SW200p.

John

[ollypenrice]

A reflector is, in principle, best guided with an OAG. Are you sure you have the backfocus? And, if using a coma corrector, can you maintain the right chip distance from it?

Olly

[alacant]

1 hour ago, Starwiz said:

Zwo 120mc

Hi

We successfully guide a 6" f8 with a t7m (a 120mm mini clone) on our oag. Never yet had to abandon for lack of suitable stars. Is the mc much less sensitive?

The disadvantage of the 120 is that unless it's the new mini model, you can't use it with a helical focuser. Unlike a short focus guide telescope, the oag camera focus must be exact. A non rotating focuser makes this easy.

HTH

[Starwiz]

1 hour ago, ollypenrice said:

A reflector is, in principle, best guided with an OAG. Are you sure you have the backfocus? And, if using a coma corrector, can you maintain the right chip distance from it?

Olly

I'm not using a coma corrector at the moment.

I'm currently using a 21mm spacer with the 1600mm-pro, which leaves me with spare 15mm outward travel and 21mm inward travel on the focuser (as per the diagram below).

The OAG is 16.5mm thick, so would replace the 21mm spacer, leaving me with spare outward travel of 10.5mm once focused, unless I'm completely missing something (which hasn't been unknown in the past). 😀

Thanks

John

[Starwiz]

32 minutes ago, alacant said:

Unlike a short focus guide telescope, the oag camera focus must be exact

That's probably the point I'm missing.  I need to get both the main camera and the OAG in focus.  I'm using the standard focuser that comes with the SW200p, but I'm now wondering about the extra distance caused by the filter wheel, so would I need to mount the OAG between the wheel and camera?

Thanks

John

[Davey-T]

1 minute ago, Starwiz said:

would I need to mount the OAG between the wheel and camera?

Usually mount the OAG in front of the wheel otherwise filters would reduce the light, especially narrow band ones.

Dave

[Starwiz]

4 minutes ago, Davey-T said:

Usually mount the OAG in front of the wheel otherwise filters would reduce the light, especially narrow band ones.

Dave

OK, thanks.  That makes sense.

John

[Erling G-P]

On 16/02/2020 at 15:09, ollypenrice said:

A reflector is, in principle, best guided with an OAG.

 

Why is that ?

Not disputing it; just wondering why.  To my eye, all types of telescopes basically does the same thing, so why is one specific type (reflectors) best guided with an OAG ?  Reflectors comes in a wide variety of apertures & focal lengths, just like refractors, so why are they different with regards to guiding methods ?

[Davey-T]

Just now, Erling G-P said:

Why is that ?

Not disputing it; just wondering why.  To my eye, all types of telescopes basically does the same thing, so why is one specific type (reflectors) best guided with an OAG ?  Reflectors comes in a wide variety of apertures & focal lengths, just like refractors, so why are they different with regards to guiding methods ?

Because they can suffer from mirror movement which wouldn't be accounted for when guided by a separate scope.

Dave

[Erling G-P]

23 hours ago, Davey-T said:

Because they can suffer from mirror movement which wouldn't be accounted for when guided by a separate scope.

Dave

Ah thanks, but would a (OAG) guider really be able to compensate for this ?

I assume the movement would be the mirror shifting in the mirror cell, because it's not to be held too tightly, but wouldn't that movement be rather abrupt, and thus difficult for a guider to compensate for ?

[Davey-T]

You can also get what's referred to as differential flexure between the guide scope and the main scope, reflector or refractor, however this can be mitigated by using solid connections between the two rather than adjustable guide scope rings.

All this can be further aggravated at longer focal lengths as any slight movement is magnified I have however guided my 10 inch,  2500mm focal length SCT successfully using a separate guide scope bolted firmly to the main scope and the main scope having a mirror lock.

Dave