Differential Flexure - cure it or auto-guide from an off-axis guider?

[g__day]

Apologies if this should be in tips / how to's versus equipment set-up!

Bascially I diagonised the degree of RA trails I was seeing in my longer duration guided shots and determined my image shift is 1 arc second a minute. It may not sound like alot but this means stars have visible trails on 10 - 20 minute shots.

After asking what is the likely source of this error (and thinking it was seeing) Ron Wodaski posted to me on his Yahoo group that is was far more likely that it's differential flexure causing this problem for me...

So basically as my scopes which are mounted on Losmandy side saddle bars move across the sky, the mirrors in the SCT might be moving or the MAK might be shifting against its metal plastic Delphin pins by an amazing 0.0079 of an inch!

I am going to really try and tighten all the gear holding my MAK guide scope to see if that helps, but if it doesn't then its plan B.

I ponder has anyone had success autoguiding using an off axis guider on their main OTA into a CCD like a Meade DSI?

Can you share your thoughts on how this might work and secondly how you bring the DSI into focus? Is there a focal adjustment bar for the off axis focuser?

Many thanks,

Matt

PS

If this all fails then eventually I guess plan C is I go with Robin Cassidy saddle plate or plan D the S-BIG route of guider in built into the main imaging chips light train!

PPS

Don at BinTel just suggested really nail balance - if balance is even slightly out - then this too can cause differential flexure! So that gives me two very free things to try. Step one - check balance on a plank with two ball bearings mounted on it, especially for a side saddle rig. Step two - tighten everything so nothing even remotely appears loose!

[Bassnut]

I did a lot of testing on this. Although the OAGer (or SBIG internal guide) is ultimately the best way, it is possible to get acceptable results with external guide up to about 15min at a FL of 1500mm SCT). Unfortunately, in my tests, the Losmandy side by side set up gave the poorest results.

One problem is, the guideing refractor is heavy at the front end and an SCT is heavy at the back end, so they flexure is opposite.

In the end, I found the guide scope mounted on the main OTA eliminated flexure in the side by side plates. I hard bolted the guide scope to the main OTA with clamps (no adjustable rings) and bolted the guide cam to the guide scope with a steel bar, so I couldnt focus it. The wide field of the ST80 guide scope assured there was always a guide star without pointing adjustment, and focus stayed good enough over the long term without the need for refocus.

In short, 1st mount the Guide scope on the main OTA if you can, and if you find the above dodgy or too hard, get an OAGer ;-).

If you have a shorter FL and use DSLR (wider field of view), then the problem maybe solvable by just making things more ridged.

Fred

[g__day]

Fred,

First thank you – actually Don at Bintel mentioned your set-up (not the Paramount) the G11 to me today and the approach you took as an excellent example of how it can be achieved.

He was also quick to point out OAG is great – if and only if you can find a star in your field of view – given you are on the periphery of the light path from your main imaging target.

The big step forward for me is that as of today I have a diagnosis I can work with to improve imaging. Before this I had six or seven suspects to consider. Now its basically see if I can solve a rigidity (and/or a remaining balance problem).

Balance I can do very shortly- weather independent. Rigidity will need some experimentation!

I would be interested to hear if any users of OAGers with large SCT’s find it difficult to find a guider star with this kind of a set up. I had assumed the OAG would still see the same subject that the main CCD sees. But according to Don – this is not the case.

Matt

[citivolus]

Since the pick off mirror is located at the side of the light path, the OAG will "see" an area off to the side of the object you are imaging. As a result, if you are imaging a planetary nebula at a long focal length for example, there may be no star in that area bright enough to guide on with suitably short exposures. Rotating the OAG/guide camera assembly around until a suitable guide star could be found would be necessary, while you will need to be aware of what (albeit small) impact this may have on your balance and also plan your geometry to prevent mount collisions.

You may also need to retake flats if the pickoff mirror causes partial vignetting of the imaging sensor due to obstruction of the light path, as the position of that vignetting will have changed with the rotation. However, I think the pickoff mirror is usually far enough to the side that this isn't an issue, however if your optics have a particularly large amount of coma they may blur the field enough that an OAG is not suitable.

[Bassnut]

Matt,

The OAG "picks" off a portion of the image going to the CCD with a tiny mirror, at the edge of the FOV. Finding finding a star can be a real pain in the butt. IMO its important the OAG can be easily rotated to find a star, or it is used with an instrument rotator. The difficulty in finding a star (and the SBIG internal guide, same problem) is what made me make external guide work. On the other hand, a perfectionist would not consider this downside a hassle, in the pursuit of the best guiding possible with very long exposures.

[avandonk]

This is how I guide my Tal200K 1800mm FL with reducer with a 90mm MAK. There is no flexure! I use a Gstarex for guiding so finding a star is no problem. The Tal200K has a fixed primary mirror.

Bert

[Bassnut]

Now thats ridged, nice work ;-).

[g__day]

WoW!

[avandonk]

If you dont believe me here is a pic
http://avandonkbl.bigblog.com.au/data/2/13839/image/OCL6496020071017145329.jpg

By the way the wood is Queensland Red Cedar light but strong and now rare.


Bert

[beren]

I have a Taurus tracker 111 off axis guider {with F/5 reducer} that I have used with a C9.25 and a St-4 autoguider. My experience is limited to one outing so far but I was pretty happy with what I achieved first go. It took some time to get all the ports {guider/eyepiece/camera} parfocal and get used to guide star acquisition but I managed to get a few trail runs with the St-4 guiding with results that weren't to bad {:whistle: for me anyway}. Heres a link http://www.taurus-tech.com/tracker.htm

[Merlin66]

In a previous life I used to guide with a piggy back 4 1/2" f10 reflector on top of a SOLID 12" f5 using the old Oly gear and hyper film. No problems with 20-25min exposures, other than the risk of "nose bump" from a cold and tired operator!! When I went over to the dark side and got my first SCT I found the usual problems of guiding vs flexing vs Mirror slumping vs inadequate focal length of guider!

My old rule of thumb was to guide at x3 the photo focal length with a 12mm cross wire eyepiece. So as a minimum for an 8", F10 system this would mean a guide FL of 240", think about it for a moment; a 3" f8 guide scope is 24" FL this ratio would require a x10 barlow! If you can't see the movement then it can't be "corrected"

Flexing between the guide and the main optics due to a variety of reasons is a major cause of headache and the solutions being mentioned ie rigid connections etc etc are certainly a step in the right direction.

Now today, with the current CCD's, the resolution is down about 10-15 micron which puts even more "pressure" on guiding ( the old Hyper 2415 was about 20 micron on a good day!, colour films even less) so my comments are directed towards getting a LONG enough FL to the guider, whether that's a MkI eyeball or a ST-4 CCD guide chip; it doesn't matter.

Yes, you can persevere and get the guide scope nice and tight on the main tube of your SCT, BUT when the main mirror slumps around, and I know it will, then the image on the CCD will definately move.

I have tried it; on numerous occasions, infact many, many times. My latest challenge is to remain "focussed" on a 20 micron slit while a spectroscope which functions at about 30% efficiency builds up an image of a VERY faint spectra.

My frustration is I KNOW the answer..... I successfully use a Lumicon OAG purchased last year because of the flexing/ mirror movement problems on my 12" LX200 for DSO type stuff, and it works 100% for me.

With spectro slits it severely limits the "flexibility" of looking around for a guide star; I must hold my "star" on the slit not just 5 sec arc away.
So for my current work, I'm back to square one. The latest trials are based on an earlier experiment with a beamsplitter, so I can focus and guide on the same star as the spectroscope...........

Summary:
Newtonian/ fixed main mirror = long FL guide scope
SCT(any!) = OAG.

[john.pullen1]

HI,

My laptop just died and my new one does not have a parellel port. I am trying to run a SBIG ST 237 autoguider on a USB port using a USB to parallel adaptor. Has anybody tried to get the current generation of laptops and XP to operate a SBIG parallell port camera.

I suspect that you need to play around with the port settings?

Regards,

John:doh: