Off Axis Guiders?

[Matty P]

I will be upgrading my scope soon and I've been looking at alternatives for guiding setups. I am currently using a piggybacked 70mm refractor for guiding with fairly successful results, however I am wondering if using an OAG would yield better results. The problems with what I believe to be differential flexure give me inconsistent results.

http://www.saratogaskies.com/articles/gs-vs-oag/index.html

The results from this website seem very good comparing the two guiding techniques and without a doubt the OAG technique is much more consistent.

My plans are to upgrade to a GSO RC and I am considering using an OAG with it. Would it be recommended to use an OAG for guiding? Are there any reasons why I should not?

Thanks. :thumbsup:

[Hagar]

Hi Matt,, consistency is directly proportional to the type of guide mounting and the money thrown at it. OAG's have their place but also have their problems. Again the amount of money thrown at an OAG definitely reflects in the usability and usfulness of an OAG.
Things to consider are the type and size of the pickup prism. it's ability to move around the image and the magnitude of the stars or the magnitude your camera can resolve well. You need to think about the ability to correctly focus your guide camera and your imaging camera at the same time. So the use of a par focal ring on an EP or helical focuser for the guide camera.
Last thing I found a problem with was the amount of in focus travel when using a focal reducer/flattener.
I had a Lumicon 2" and sold it quite quickly as the process of focussing both cameras was a real pain.
In my opinion if flexure is your only concern then bolting a set of fixed rings to the top of your imaging scope and making a simple but strong support for your guide scope focuser and camera is a way easier and way less expensive means of removing flexure. Finding a guide star is always easier with say a 70mm guide scope than an OAG even if it is fixed and bolted down. Save yourself some money and see if you can borrow one for a trial first.

My 2c worth.

Good luck with it all.

[Dennis]

Hi Matt

Recently Allan Gould posted a link to MetaGuide (http://www.astrogeeks.com/Bliss/MetaGuide/)and upon following the link, I read that Metaguide (http://www.astrogeeks.com/Bliss/MetaGuide/):

“Measures flexure/mirror flop using two telescopes, two web-cams, and two instances of MetaGuide that link to each other”

So, if you have the time, it might be worth running this test on your current set up to quantify the amount of flexure and the potential source(s)?

I’m waiting on a spell of good weather and some motivation before I try this experiment myself, just to add to my knowledge of how my mount and side-by-side system behaves. Oh and from the website…

"MetaGuide is free, easy to use, works with any type of telescope, and most web-cams".

Cheers

Dennis

[Matty P]

Thanks for the reply Doug. It is great to hear from someone who has had experience with one. The trouble with focusing and finding a suitable guidestar with an OAG seems quite problem some.

I will definitely see if I can get one for a trial run. If not I will see what I can do with securing the guidescope a little better.


:thumbsup:



Would be a great way to test for flexure however I only have one CCD camera. I don't think it is possible to use my DSLR as the second camera. Looks like a nice program though.

Thanks Dennis. :thumbsup:

[g__day]

I use a Lumicon OAG with a Meade DSI II Pro as the guide cam for a C9.25. I tried guiding on two other OTAs attached by Losmandy dovetails (which are fairly heavy duty) but couldn't avoid star trails.

So I definitely had differential flexure happening somewhere. Since using the OAG my shots have been fine - no field movement over several hours.

I recall guiding first using a Meade Colour DSI - gave poor results - the Mono version (well a higher model too) does a far superior job.

With the right tools, techniques and patience I may have been able to isolate and solve the flex problem - but I intuited it was the mirror shifting in the main OTA. Rather than invest all the effort - the OAG bypasses the problem for me.

[Gama]

Unless your guide scope is heavy, i cant see flexure as an issue.
80mm is not heavy, unless your rings are made of straw.
The main reason you get no trails when using an OAG is mainly due to the fact you are using a longer focal length (Main telescope), thus is able to detect errors faster and way before its detected in the guide scope.
The guide scope wont be able to see the errors until its large enough to be seen itself. By then, the main scope would already have that error recorded.

Theo

[AlexN]

Theo's right... it all comes down to resolution of your imaging setup vs your guiding setup...

I've used OAG's and will be using one with my current setup.. I love them. You want to make sure your guide camera is sensitive! really sensitive... I'll be using a QHY5 in my Lumicon OAG for starters, and eventually moving to the Starlight Xpress Lodestar with the SX-AO unit..

I can tell you right now, the images I achieved with the OAG + C11 versus using a guide scope were tremendous.. Mind you I was attempting to guide my 2800mm focal length with a 400mm focal length guide setup.. Not the best setup.. Using the OAG my results were MUCH MUCH better than external guide scopes..

For what its worth, it makes setup easier too... Im lazy.. 2 scopes is a hassle! :D

[Paul Haese]

Some interesting thoughts. I have tried using OAG's in the past but it did not work that well.

However, the QSI comes with one installed onto it and will be using it to guide my scope. Theo is correct on errors being recorded on longer focal length main scopes before the guide scope records the problem. Sorting flexure issues can be very tiresome, it means spending a lot on very solid mountings too. So this is where OAG guiding comes into its own. If you think about it SBIG self guided camera's work on the same principle. It plucks off some light and uses this to guide with. I would say a really good OAG is what you should buy if you are going to get one. Buy the best, that way you are assured of the best results.

[Lester]

Good topic. I will be going down this road in the future.

So the good OAGs are?

[AlexN]

The Astrodon MOAG is far and away the best one available... The Taurus Tracker III is also a great choice... I've used the Celestron radial guider, and its not a bad cheaper option, the Lumicon Newtonian easy guider is another great cheaper option...

I also hear the new Orion deluxe OAG is a great bit of gear... there have been a few sold with some manufacturing issues, but on the whole, they are seen to be a great budget option at $209 AUD from bintel...

Rigidity is the key with the OAG. Theres no point getting rid of the differential flexture of your guide setup if you're introducing more flexture with an OAG... The more money you spend, the better off you'll be.

[pjphilli]

Hi Matt
Great topic and the saratoga article gives some food for thought in tracking down the dreaded flexure. With my current setup I am getting about 0.7pixel
per minute flexure movement on the imaging scope and all my attempts at stiffening the mountings to no avail. Also I agree with Matthew regarding the guiding camera. I too tried my old DSI Colour but disgarded this for a QHY5 mono which is excellent and gives me rock steading guiding using PHD.
The comments on off axis guiding are interesting but I find it useful sometimes to move my separate guiding scope off collimation with the imaging scope to get a good guide star.
Cheers Peter

[g__day]

Must say - even when using a focal reducer on the C9.25 and guding with a side by side 5" MAK at 1500mm focal lenght - I got alot of star field shift - like the RA was running too fast. And I mean alot!

Since using the OAF at full prime focus on the C9.25 there is no discernible star field shift after several hours. All my gear was mounted using Losmandy gear - so I wouldn't call it flimsy gear causing shift. Personally I thought the main mirror in the C9.25 was moving slightly as the scope moved about the sky. Just a guess - either that or guiding with a colour DSI was just faulty. I might try guiding again with the Mono DSI II pro attached to the 5" MAK and see if that too exhibits star field shift.

Once the problem is isolated - fixing things is alot easier!

[Matty P]

Thanks for the information guys. It has been a great help. :)

I am contemplating purchasing the Orion OAG as a test to see what kind of results I will get with a moderately cheap OAG. If all goes well I may upgrade to a higher quality OAG later.

Another item that will go on my TO GET list. :doh:

:thumbsup:

[pjphilli]

Matthew - isolating the flexure is certainly the problem. I too only see it in RA as I suspect that the scope rings are far enough apart to prevent the rotation on my flat platform that would be equivalent to a DEC shift. However, I think that my 150mm Mak imaging scope is a bit heavy for the rings supplied with it which could result in sag in the sideways direction which would give me RA flexure. I did reduce this by about 1/2 by changing from a piggy back to a flat platform and I have plans to stiffen the ring attachment to reduce possible side sag. The search goes on!
However, life's too short, and your success with OAG indicates that this is perhaps the way to go. Hence Matty, I would be very interested in your trials of an OAG, please post any results that you get.
Cheers Peter

[RodM]

Hey Matty,

I'll be interested to hear how you get on with the OAG, as I might try one myself at some point. I also had a lot of trouble with flexure, but recently made a bit of a breakthrough.

Having securely mounted on scope on top of the other I was still getting quite a bit of flex until I realised it was being caused by play in the scope focusers. I was corresponding with a guy in Sweden on another forum that had the same problem - I tried propping up the focus tube on my guide scope and he took it a set further by clamping his with another scope ring (http://www.istarion.net/Picturepages/Autoguiding.htm). I've since done this to mine and the difference is HUGE.

I've still got a bit of play in the focuser of my FLT-110, which is a lot worse when I add a 2x PowerMate, but when just the camera is in place I'm getting only 0.2-0.3 pixel movement for a 5 minute shot :D.

Let us know how you go.

[Bassnut]

Why stop at the focuser :shrug:, bolt the cam down, like this, no flex, anywhere :P.

[multiweb]

I got the cheapest OAG I could buy. It's the Meade T-piece. It's a 2" inside diameter for the QHY8 with a 1" 1/4 off axis for the QHY5. Works very well with my C11. IMO big mirrors flop. You can glue, weld, bolt anything piggy backed on top of your imaging scope, you'll never get rid of flexure because the big primary will always be "lose".

[marki]

Unless you have a LX200 with a mirror lock that is :P. Suprised Celestron have not done something similar. Actually I am suprised they all don't just bite the bullet and put the primary on a roller bearing setup similar to the crayfords . Would have to be a whole lot better then sliding it up and down a greased plain bearing.

Mark

[multiweb]

Not familiar with the LX200 but I'd assume that if the primary is heavy and designed to move in anyway to get focus bearings or not it would move to some degree. All high end gear seem to have all optical elements linked in a rock solid truss/tube system. The only moving part is the focuser.

[srchadwi]

Hi Matty

I've recently started using an OAG with a SX Lodestar as an autoguider. So far I've never had a problem finding a guide star at exposures of 0.1 seconds! I know they are expensive but if you add up all the accessories you need in order to use a guide scope (and you still need a guide camera anyway) then it's certainly worth a consideration.

Steve

[AlexN]

Agreed Steve, the Lodestar is by far the best autoguiding camera for using with an OAG... I'll be getting one of them as soon as I can afford it.. :D

[Merlin66]

I've used almost every OAG available over the years. Never found the Holy Grail but a couple came close.
For 2" focusers (and or Newtonians etc) the Lumicon Newt OAG is very good. It has a large movable pick-off prism.
For SCT threads, the Celestron Radial guider is hard to beat. Allows almost 120 degree rotation of the prism without moving the camera.
OAG is very different from using a guide scope, but when mastered will give much better results.
BTW I use a QHY5 for guiding, and a Vixen flip mirror system modified to a 70/30 beamsplitter for spectro work.
Hope this helps.

[marki]

The LX 200 has a mirror lock. You adjust coarse focus then lock the mirror via a second knob. I have a crayford behind this so once I am in the ball park the mirror is locked and the rest done on the crayford. If you look closely at where your mirror moves on the baffle you will see a gap stuffed with grease which is used to prevent mirror flop. If this was changed to roller bearings it would reduce much of the slop present in the standard systems.

Mark

[RodM]

On the subject of guide cameras - I use a Imaging Source DMK21 USB camera and find it great. I bought it with the intention of also using it as a plantary imaging camera (inspired by IceMan), but haven't as yet gone there. Using a WO ZS66SD as the guide scope I've never failed to find a guide star at 1 second, and generally guide at 0.5-0.7 sec exposures.

I was imaging M104 the other night and could actually make out a blur of the core of M104 itself when guiding at around <1 sec. :thumbsup:

[Bassnut]

Guide exposures in urban skies less than 2 secs is not a good idea, your then chasing seeing, which no mount can follow without AO. A jumpy star at 0.7 seconds will give erratic guide corrections, 2 secs smooths them out.

[Matty P]

I find the shorter exposures I use when guiding, the better results I get. I tend to use 0.5 second exposures and when guiding on a moderately bright star. I get fairly decent results.

I don't think seeing conditions make too much difference in short focal length scopes though. :shrug:

Back to OAGs, I have looking at the Lumicon GEG and would anyone recommend it over the Orion for any reasons?

:thumbsup:

[Merlin66]

I used one for many years. I don't think they are currently available from Lumicon.
If you're using, say, a 10" or 12" LX200 the "Eyeopener" adaptor gives an increase in FOV. The built in 3" compressor lens is OK but certainly not as good as a dedicated reducer like an Optec. If you can find one at a good price certainly a nice piece of gear. I replaced mine with the Celestron Radial.
BTW getting the guide camera in focus... usually needs either a 50mm spacer or better a cheap PRISM diagonal between the OAG and the guide camera.
As I mentioned before the Lumicon Newt OAG is good for 2" focusers and it has the same pick off prism adjustments as the GEG.
Hope this helps.

[srchadwi]

In general I agree (well, it's about 1 second minimum for me for normal guiding). As it happens I do use AO and that's why I guide at 0.1 seconds. I was just pointing out how sensitive the Lodestar was.

Steve

[Moon]

Steve

I don't want to hijack this thread, but I would be very interested to know a little about your AO setup (perhaps in a new thread?)

[srchadwi]

Hi James

I've started a thread "AO"

Steve