OAGs ?

[ZeroID]

I'm looking for a guiding solution for the 8F8 scope. My existing setup is too tall, the scope takes up most of my Ob headroom. I'm working on a low profile adjustable mount system project which should fit but I'm also considering an OAG.
My mind can't quite get around the idea of an OAG. I understand the off axis position and basic optical physics but how do you find a suitable star if it is such a fixed position device in the optical train ? :question:

[Peter.M]

Star suitability is just down to SNR. So all you need is a sensitive guide cam.

[PRejto]

I think most people use a rotator to place the guide star on the chip. Maybe if you don't like that idea too much you could consider an ONAG (In ovations Forsight). Expensive solution, but offers some very unique solutions such as FocusLock and Sharplock automated focusing. My tests show that the focus solution works extremely well and once set up is very easy to use.

Peter

[glend]

This question also puzzles me as finding a guide star is often a frustrating process for me with a dedicated and adjustable guidescope. True the size and f ratio of the guidescope is a limiting factor and the larger aperture of the imaging scope gives greater reach but your always limited to the FOV. Additionally, in light polluted skies, guide stars can be harder to find. My solution that I built into my 10" imaging newt , was to get the Skywatcher guidescope mount which gives good x and y axis control to independently source a guide star. The guidescope mount provides a very large area to find a suitable star. Unfortunately the penalty for that wide field adjustment is weight, the guidescope mount adds about a kilo to the overall weight on the mount/scope. Once you add a larger aperture guidescope the weight goes up again. I use a ZWO 60mm guidescope, which is lightweight and can mount directly on the guidescope mount. As my 10" scope is light the additional weight is not a significant factor and actually assists balance by where I located it. I believe this is a good solution for light newts but would be not so good where your approaching the imaging capacity limit of your mount.
It is also easily moved to other scopes regardless of configuration as long as the weight can be tolerated.

[Peter.M]

I hear this story all the time and it really makes me wonder what people are doing! If you have a guidescope and you are adjusting it to find guidestars you are doing something wrong. Lewis proclaims with his ssag and a finder guider he couldn't find stars so he moved to a lodestar. This just simply isn't the case, I used an ssag with an oag for a while and while that was pushing it, it worked without adjustment. As far as oag being less versatile, when it's focused with the cam in focus you can take it to any other scope and you will be fine.

I like the onag solutions but it simply is not an option for any imaging newts where distance to the tube is an important vignetting consideration

[rustigsmed]

I have an OAG and haven't had a problem finding a star; on the odd occasion I've had to increase the exposure length in PHD2 but that is perhaps made easier by using a sensitive camera like the lodestar. the stars are ugly and not very sharp but it doesn't seem to matter for guiding.

[Paul Haese]

Using an OAG is a cost effective solution over using a guide scope. Not to mention that a guide scope can and does get differential flexure. The main reason that people seem to have trouble with an OAG is getting it focused. It can be a fiddly process but worth the effort. Once you get the OAG focused with the main camera you should never have any trouble finding guide stars. I used an SSAG for years before it died and there were always at least 4 stars in the field. When I spent the money on a SBIG ST-i I found I would get triple that amount of guide stars. Orion now make a look alike to the later guide camera with the same sensor and it is much more affordable.

I have never used an ONAG but from what I hear those appear to be an excellent solution too, albeit one that is more expensive. One of the advantages of course is being able to guide in infrared. Not sure I will ever feel the need to go down that path myself but it would be useful if you have aweful seeing all the time.

[gregbradley]

Guide scopes are really a relic of the Cold War or was it the Paleozoic era?

Serioiusly though OAGs are totally the way to go or self guiding in front fo the filters. Guide scopes work at shorter focal lengths OK but at long focal lengths its a no -go.

Having said that I use a little compact, weighs nothing, Vixen VMC 95mm scope as a guide scope on my CDK17 scope when I use the reducer. That's only because the reducer has no backfocus and only an STL11 would fit. I have to reduce exposure lengths to avoid eggy stars but I can do 5-10,minutes ok.

At long focal length a guide star on something like a MMOAG can occasionally be a bit of a bother and you end up imaging the object away from the centre of the sensor but that is rarish. With an STi guide camera (I have 2 now) I find a guide star 90% of the time close by with no special handling and the rest of the time it simply means looking a bit to the left, right up or down a tad to find one.

At shorter focal lengths the OAG usually has a choice of guide stars.

An OAG is still not a perfect solution as it adds weight, are hard to focus, can flex themselves or get in the road of other components but they are one of the best solutions currently.

Greg.

[glend]

Perhaps I should have pointed out that my issues with OAG arise because of my DSLR - Canon 450D cold finger mod (where the heatsink/fan extend out the side of the camera) which precludes rotation of the camera in the focuser and the use of a Baader coma corrector and focus range issues. With the OAG it would have to be mounted so that the guidecamera was placed below the bottom of the dslr (due to the body shape) and no rotation would be possible. The Lacerta OAG EOS model comes close but as stated the ring could not rotate in my situation due to proximity to the tube surface. Threads I have read on other forums (CN) suggest separate guidescopes are preferred in this configuration .

Given the OP (Brent) uses a DSLR, the same model actually, these are relevant issues for this thread. In constructing his scope, if clearance is adequate then the Lacerta EOS would be the choice but his ASI guide camera could only clear the body if mounted below and the rotation issue remains.

[rustigsmed]

I use a dslr also, the TSOAG9, it has 3 positions available but really only works in one position due to the shape of the dslr. but it does work its just a bit fiddly. obviously any cold finger mod would have to be in the 'right' location.

I did have a MPCC but moved onto a RCCi for some extra spacing.

[ZeroID]

Some extra details for the discussion.
I am using two DSLR's ( 450D and the SONY ) plus I use my ASI120mc for planetary imaging and hopefully DSO later. I currently guide with a 80F5 ( Good old shorty scope ) and a Phillips SPC900 which in the Lunt refractor configuration was mounted on a chopped off AZ3 head. Heaps of adjustment and very stable on a common mount plate. Unfortunately it is too tall to fit to the 8F8 and would probably cause some off balance issues.

I also have to contend with some LP problems. The 80F5\SPC900 works well once you have the exposure values right in PHD2 although it suffers sometimes with LP induced contrast issues. But 95% ok to go but I need some latitude to find suitable guide stars.

I am hoping my guide mount project will resolve the problem by low profile and weight but still be stable and have sufficient adjustment to find a star.

I queried about OAG as it is the next possible solution but I see limitations in the ability to find and hold target adequately.

I have another possible partial solution also, the resurrection of my beam splitter project from an earlier idea. I acquired a better, bigger quad prism cube and stabilised construction and got it parfocal. It would be suitable with the ASI for added length but not the DSLRs being only 1.25".

Heheheh, keeps me busy and amused for sure. The Lathe\Mill is already proving it's worth in accurate manufacture and better finish. :thumbsup:

Thanks for all the input folks, all good stuff to hear. :thanx:

[gregbradley]

Fair enough. There are quite a few OAGs on the market though. I am sure one can be made to work with the right adapter.

Guide scopes do work but are often problematic and OAGs are not problematic so its a matter of tracking accuracy at the end of the day. I've used both many times and would always opt for OAG if there were a choice.

Greg.

[g__day]

A few thoughts on OAG's.

1. For longer focal length scopes they (or an ONAG) or a imager with build in guider are a must. Every other solution finds it hard to combat residual differential flexure and /or mirror shift.

2. The selected OAG must be able to hold all your gear really firmly. Do all the attachment bolts up real tight or you will get coma and your camera/s will be sloped to the imaging plane! Check the tightness of all the bolts regularly!

3. Make sure your imaging train can take the weight! I think in the near future I will upgrade my Meade motor focuser connected to a large Lumicon OAG to a motorised, PC controlled Moonlight focuser into a ONAG. The Moonlight seems a lot better built than the Meade!

4. Make sure your imaging train has enough back focus. With an SCT you are probably fine - for everything else double check before you buy!

5. Nail the focus of both cameras! I used a bright star with a Bathinov mask to focus the main camera then slewed to the full Moon centred in my guide camera using the same Bat mask to check focus on the guider. This gave me sufficient light to properly focus the guide camera and haven't touched it since.

6. Get a really decent, mono guide camera. A colour Meade DSI won't do - the mono Meade II DSI is alot better but still just sufficient. Get a sensitive, mono camera and you won't regret it.

7. For you guiding learn your software and match it to your location's seeing and the arc seconds per pixel on your guide camera to ensure you don't chase the seeing and end up yo-yo'ing everywhere! For me I found setting a guide star must be 0.93 pixels displaced for a guide instruction to be issued to be optimal - given this equated to about an arc second - as I am over sampling and it I had it set too low - I would be ridiculously chasing the seeing (and for me 1 hour subs still look great - my guiding at 2.3 metre focal length is really good).

8. Balance and re-tighten your gear really well once again. If it is all set up right, you will be giving yourself every chance not to introduce stray factors that can ruin your imaging.

Hope this helps.

Matthew

[LewisM]

Not quite the case - I just found it SOMETIMES not finding very suitable guidestars in my severe light pollution. The Lodestar conversely offers me DOZENS to chose from, even in WORSE LP since moving. Maybe my SSAG was a dud. Not sure, but the Lodestar has been worth the outlay for sure! (I got my latest one when Claude was selling them at $600 NEW)

I use SOLID rings - no lateral or vertical adjustment possible. What my finder- guidescope sees, the main scope sees. Being close in f/ratio they see similar FOV.

I will state that I never had to move the scope with the SSAG either - I used Borg mini-rings back then - no movement. The stars with the SSAG just were always VERY dim and PHD would lose the star a LOT.

[PRejto]

Hi Matthew,

Good post!

Re #3, I might be wrong about this but wouldn't you be a lot better off with a R & P focuser (Feathertouch?) than a Crayford? I think a R & P would preclude and shifting due to weight and might work a lot better with electronic focusing that works on knowing absolute position. Does Moonlight make a R & P?

Peter

[g__day]

I have a feathertouch 10:1 manual focus on my 9.25 SCT - for rough / quick adjustment (haven't touched this in a year or more), at the back of this SCT is a Meade motofocuser (controlled via a JMI USB box by PC normally running BYE for imaging control). The Lumicon OAG hangs off the Meade motor focuser then it all connects to a cooled DSLR.

Side by side with this I have two Williams Optics refractors - the larger one is a 110 FLT triplet that uses a Moonlight high precision encoder focuser with both fine tocus hand controls and USB override to again let another instance of BYE (or any other suitable program) manage precision focusing of the second DSLR.

It's just by casual observation that I say the Moonlight looks way higher build quality and capable of rigidity under fair load than the older Meade. I have never tested them, and the Moonlight I believe costs double or triple what the Meade did. Mechanically I haven't tried to evaluate (even mentally) which design should work better. When you consider the tolerances needed, and that all metals flex under load, you might have to think through thoroughly how all surfaces are supported and their strength, length and the turning moments associated with their designs before assuming rack and pinion has direct contact so will be more stable. Good question thought.

One lesson astro-imaging has taught me is start from the assumption that all metals bend under load like plastic under a hot sun, and ask is your gear set up in such a way for this not to be an issue for you! I used to assume its steel its strong, those attachment points must be enough etc. One semi loose attachment point can be a PITA to identify and correct if you don't get a bit paranoid at the start about unwanted flexure!

[SteveInNZ]

Matthew,

That 0.93 pixel is at 2.3m focal length, correct ?
How big are the pixels on your guide camera and imaging camera and what is your seeing ?
Just trying to put your setup into some context.

Steve.

[PRejto]

Matthew,

I posed the question due to something I read recently. (names removed!)

"If I am using a Crayford style focuser using the typical friction drive configuration both on the cross shaft and the ball bearing reduction mechanism, which is typical, can and does the focusing program make adjustments based on absolute values, reported by step count, or if there is slip in the mechanism does it constantly make adjustments to negate the slippage.

Suppose you generate your V curves in the normal manner characterizing your system and then you go to do a focus but for a number of reasons there might be slippage between the ball bearing reduction drive and or the cross shaft and the friction plate on your Crayford Style focuser. Now since the actual position of the draw tube and the numbers displayed will not be the same does the focus program constantly adjust for this or is the effect cumulative?"

Answer(s):

"If you are relying on the motor steps to position your draw tube then you are out of luck - slippage may/may not be linear and it may/may not be predictable depending on load. BUT, you may have more success if you are monitoring the actual draw tube position with an external measuring system such as a precision digital .0001" resolution dial indicator. You would then have the challenge of developing an ASCOM compliant driver that would move the focuser until the desired reading is achieved. The best solution is to engineer the focuser so that slippage is not a concern with a given payload."

and

Interesting in reading your question regarding slippage on a Crayford focuser. We were plagued with that problem on our remote telescope in Spain. The focus would change a small amount depending on where the telescope was pointing. Was worse near the zenith as expected with gravity acting down helped by the weight of the imaging kit!

The solution was to change the Crayford for a FeatherTouch which uses a
rack and pinion focuser. That driven by a Lakeside (or a Robofocus)
motor focus works fine. No slippage with that. They're damned expensive
though, but its quality engineering you're getting for your bucks ;-)"

You can see why now I asked the question!

Peter

[Peter.M]

Televues solution is exactly this. An external measurement of focuser position to make a closed loop. I opted for the moonlite with stepper and have had no issues with slippage.


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[PRejto]

Thanks for pointing that out. I wasn't aware of that system.

Peter

[g__day]

My Arc seconds per pixel is 0.44 - so for my very average seeing conditions I am way overseeing the sky, that is why I dial down the sensitivity on the PHD guide algorithm to 0.93 pixel movement - which is still about .4 of an arc second.

I use modded Canon 40D DSLRs and the guide chips is a Meade DSI II Pro Mono - so I will have to look up the size of the pixels (40D 32.6 µm² (http://snapsort.com/cameras/Canon/EOS_40D/specs/sensor/pixel-size) and DSI II 8.3 microns (w) x 8.6 microns (h)) but at guess I would say - they are too small and the electron wells are too shallow to be optimal for my gear and location!