Off axis guider or guidescope?

[windjammer]

Hi 

I have come to this a bit late, but here is my 2 pence!

If your mount can hold its act together for 5-10 seconds without a guiding nudge, I would go for an OAG.  A 120MM will pick up a guidestar with a 2-5 second exposure whatever the field of view.  So there would be no issue with finding a guidestar - as others have said, the pain/butt index of rejigging and rotating the image train to locate a suitable star is high and time consuming.  OAGs have the advantage of less flex than a G/S and can fix mirror flop etc.  But, they do flex esp if the image train is heavy - guide cam wobbles on its stalk, the OAG thread attachments to filter wheels and cam flex etc, and everything after the OAG flexes as well.  The shadow of the OAG prism can be a nuisance and visible on bright stars, so practically one has to put it right at the edge of the field to minimise this.

Re flex, I had to bolt mine together - see pic.  It is a bit of a rabbit hole - this is my experience...

My guiding regime is 10-20ms exposures at 100ms intervals, so I use a G/S (90mm F10).  The 120MM is quite a small sensor, so the guide cam sits on an XY stage to move it around the guidescope FoV, if necessary, to find a suitable guidestar - anything brighter than magnitude 7 will do. The XY stage is also useful to align G/S FoV with imaging OTA FoV - not necessary but helpful when confusion sets in.  In a similar vein it is helpful if all the cameras have the same orientation and aligned +/-RA - L/R and +/-DEC - Up/Dwn.

Guidescope does flex - mainly focusser and cable drag issues, as does the main OTA - focusser and cable drag issues. So I use both the g/scope and OAG at the same time.  G/scope controls the mount with one instance of PHD, and the OAG uses a second instance of PHD to control a micro-XY stage that moves the guide camera so as to correct flex at the OAG.  Phew!  It does actually work - the only thing left is field rotation!

Overall moving to PHD guiding is well worth it IMHO, OAG or G/S.  Unlimited-time long exposures are possible, other issues not coming into view....!

Simon

 

 

 

 

 

 

 

 

 

[tomato]

Wow, and thought I had a lot cables and paraphernalia on my rig…👍

 

[Nigella Bryant]

1 hour ago, tomato said:

Wow, and thought I had a lot cables and paraphernalia on my rig…👍

 

My thoughts exactly, lol. 

[Nigella Bryant]

Just now, Nigella Bryant said:

My thoughts exactly, lol. 

 

[DaveS]

I side-stepped the whole guiding problem by going to encoder guiding.

[windjammer]

Nice (and neat) looking kit - you already have a guidescope ?

[AndrewRrrrrr]

kudos! wow that is some setup Windjammer! 2 filter wheels in series is it? And a drill chuck in there? 

 

[Nigella Bryant]

1 hour ago, windjammer said:

Nice (and neat) looking kit - you already have a guidescope ?

Yeah, already got a guide scope but was wondering if an off axis guider would be better. 

[windjammer]

17 hours ago, Nigella Bryant said:

Yeah, already got a guide scope but was wondering if an off axis guider would be better. 

Any particular problem you are trying to solve ?

[windjammer]

18 hours ago, AndrewRrrrrr said:

kudos! wow that is some setup Windjammer! 2 filter wheels in series is it? And a drill chuck in there? 

 

The first wheel is an anti-vibration shutter that flips between a black filter and an open position.  It is triggered by extreme activity on the ST4 control lines to open and close the optical path.   The shutter saves long exposures. I posted on the project here fyi:

 

Yes, more than drill chucks - the gearboxes in cordless drills are a great source of parts.  I posted quite lot about that here if you scroll down the page 2:

 

 

[Nigella Bryant]

15 minutes ago, windjammer said:

Any particular problem you are trying to solve ?

Hiya, I can't reach focus with my zwo asi 120mm on the 70mm f6 Ed refractor. I've tried inward focus, outward focus, but nothing but black. I've even upped the gain, exposure and then I see hot pixels. I've used extension tubes on and off, nothing. 

[windjammer]

Thats weird.  This is the 120MM with the 1.25" snout that goes right in ?  Well, back to basics - if you pop an eyepiece in you can visually get focus with eyeball ?  If so the 120MM in a similar position to the e/p should show something.  How about a) electrical - USB cable/bandwidth or b) software/driver- using sharpcap or a.n.other?  Point the bare cam at a bright light and still get nothing, I would guess s/ware, settings or cam is bust (unlikely unless cooked by the sun or mice).

Any other symptoms that might give us a clue ?

 

[ollypenrice]

22 hours ago, DaveS said:

I side-stepped the whole guiding problem by going to encoder guiding.

Some of my robotic shed clients did likewise but nobody is still doing so...  It's like anything else, when it works, it works. Go with it when it does.

What is a guiding problem? I don't have one. I just bung on a cheapo ST80 as guidescope and get on with it. Changes to the imaging scopes, camera rotation, etc etc, have no effect on anything.

In over ten years of imaging, my venerable Mesu 200 never dropped a sub. That's a pretty good definition of not having a problem. To be fair, the same mount ran an OAG for the first three years and, once I'd stopped the thing rocking about by adding a strap to the top of the guide cam, it was as reliable as a guidescope.

Mini finder-guiders? Very pretty and super neat, but a royal pain compared with the ratty old ST80. They lose focus easily and have this deeply unconvincing  'easy focus' ring wottsit. Why? My ST 80s haven't been refocused in 10 years.

Olly

Edited October 23, 2023 by ollypenrice
typo

[Elp]

1 hour ago, Nigella Bryant said:

I can't reach focus with my zwo asi 120mm on the 70mm f6 Ed refractor

Have you tried setting it during the day looking at a distant tree, rooftop or something?

[Nigella Bryant]

2 hours ago, windjammer said:

Thats weird.  This is the 120MM with the 1.25" snout that goes right in ?  Well, back to basics - if you pop an eyepiece in you can visually get focus with eyeball ?  If so the 120MM in a similar position to the e/p should show something.  How about a) electrical - USB cable/bandwidth or b) software/driver- using sharpcap or a.n.other?  Point the bare cam at a bright light and still get nothing, I would guess s/ware, settings or cam is bust (unlikely unless cooked by the sun or mice).

Any other symptoms that might give us a clue ?

 

Yeah, did the focus with eyepiece during the day, taking the 120mm out of the eyepiece holder it brightens to white so it's working. I'll have to try it during the day through the scope. I'm baffled as to why to be honest. Unless the camera has had it's day. I'll try my 462mc and see if that gives any results through the scope. 

[DaveS]

@ollypenrice Probably "problem";was a poor choice of word, "question" would be better, as between guide scope and OAG.

[Nigella Bryant]

1 hour ago, Elp said:

Have you tried setting it during the day looking at a distant tree, rooftop or something?

Only with an eyepiece, I'll try with the camera perhaps tomorrow weather permitting. 

[Grant Fribbens]

I only use an OAG on my Edge 8 HD. Although before I got the OAG I used the Altair 60MM guidescope without any issues. I have the ZWO OAG-L due to the larger prism size which I think really does help along with an ASI174MM 😄

[AndrewRrrrrr]

4 hours ago, windjammer said:

The first wheel is an anti-vibration shutter that flips between a black filter and an open position.  It is triggered by extreme activity on the ST4 control lines to open and close the optical path.   The shutter saves long exposures. I posted on the project here fyi:

 

Yes, more than drill chucks - the gearboxes in cordless drills are a great source of parts.  I posted quite lot about that here if you scroll down the page 2:

 

 

thank you, I know I'm going to enjoy reading those posts

[Nigella Bryant]

Well I tested the guidescope with the Zwo Asi 120mm and I got focus after taking off a 2inch extension tube. Inward focus should bring it to focus now so I'll see how I go with the guidescope before a fork out any money on an off axis guider. Nice pic of the neighbours monkey puzzle tree top, lol. 

[AMcD]

Apologies if this has already been covered, but is there any maths (or, as my American wife would say, math) to consider in optimising the telescope / OAG / guide camera combination? 

I am planning to move to QHY OAG-M, which has an 8mm x 8mm prism, on my TS Optics Photoline 130, which has a focal length of 910mm.  I am considering the ASI174MM or QHY5III678M as the guide camera, but wonder if there are any calculations I should be doing before hand to make sure it is the optimum camera to use?

Edited November 4, 2023 by AMcD