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Home Made Mount - Auto Guiding Help Please


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Hi everyone, finally joined the SGL community :smiley:

I'm hoping someone could give me some pointers to getting me up and running with autoguiding......I have a hommade equatorial mount which I have designed to enable me to get into astro photography.  It's buitls hold a 10 inch reflector that I built as a Dob originally.  The plan was to enable driven observing and later on (like now) to able to image with this scope or any other one, thinking that if the mount is strong enough to hold the monster 10 inch, it would hold something like a 80mm WO Zenith Star (which I am currently using for imaging) easily! 

So far I have only used a Toucam attached to an old Tasco scope approx 700mm focal length to manually guide with a control pad.  As you can appreciate, I want to get going with auto guiding asap as I want to spend some of my imaging time actually looking at and appreciating the night sky rather than huddled over the laptop getting neck ache.....

I now have the GPUSB box all the cables required etc all set up and ready to roll.  I have tried using both PHD and GuideMaster, but so far have been having problems getting the set up to stay accurately on the guide star long enough to get any decent long exposures without any star trailing!!  I'm getting pretty frustrated now and have tried all kinds of setttngs changes on PHD after reading all the advise on both the help pages on the software and elsewhere on the net.  I have just tried GuideMaster the other day and this seems to sort of work ok, but not really any better than PHD and in my opinion not well enough.

Now, the mount has actually got quite a lot of backlash as the 500:1 gearboxes I have used are quite cheap and cheerfull.  These motors then drive a 2 inch wheel that then drives a 10 inch wheel by way of a friction drive.  To take all the backlash out, I have been making sure that I know which way the DEC needs to be corrected and that I overballance the gearbox to ensure that all the corrections are always winding the gearbox up and pulling the weight correcting in one directin only.  The same is also done for the RA drive as the stepper motor as driving slightly too fast, the corrections are always made by stopping the motor and letting the sky movement catch up, therefore there's also no backlash invloved here in theory...

My thoughts are, if I am able to manually guide this set up nice and accurately by using the hand controller, why can't I get either PHD or GuideMaster to cope on their own??  As I mentioned, the polar alignment isn't perfect but pretty close, and I'm thinking that in order to stay away from the mounts backlash issues that it's actually better to be slightly out of polar alignment so the corrections are only ever one way and are always larger than any PEC issues.

Hopefully I have included enough info to enable one or two experts to help out if possible, I have attached a photo of my set up to give you an idea of scale/build etc along with an image I took with the Canon 40D at 210 sec exposure length using the CLS clip filter to give an idea of what I'm getting.

Desperate for help please as I'm really excited about getting into imaging and cannot move past this issue at the moment.

Cheers all!!

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FIrst of all, hearty congratulations on building your own mount. It looks like a piece of work all right!

Reading through your post I can only agree with all the 'mechanical' steps you've taken. (I'm a dead loss on the IT side.) Running out of balance and slightly misaligned sounds about right to me.

Where to go from here?

1) Can we identify which axis is trailing? Of course it might be both but this is important information.

2) You are making life hard for yourself by using a low sensitivity guide camera in a slow focal ratio guidescope. I'd try to get hold of something faster. A firm favourite is the humble and inexpensive ST80 which is F5. A better camera would be the QHY5.

3) How long are your guide subs? Too short and you chase the seeing, too long and the damage is done before the correction can work.

4) Have you confirmed, or can you confirm, that the mount is at least receiving commands in the chosen guide directions and can, in principle, respond to them? Given the pretty slight error seen in the image I'd suspect that it is, but I'll ask anyway.

5) Have you entirely eliminated differential flexure between guidescope and imaging scope? Given your obvious engineering expertise I presume you will have done so but things to check are; The screws which hold the focuser into the main tube of the guidescope. The guidescope focuser itself. The push fit nosepiece. These three are notorious.

IT people will no doubt come along and work through things like the protocols used for guiding but there I'm lost.

Olly

PS, Have you considered a belt drive conversion to lose the backlash in the primary gears? This is flavour of the month in modern mount design.

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Hi everyone, thankyou for all your comments, it's really appreciated!!  Loving the quick responses on this post

Olly, in answer to your questions I will try and be as accurate as I can:

1) Can we identify which axis is trailing? Of course it might be both but this is important information. - I'm pretty sure it's the RA axis mostly, as I did actually take an image without the guiding on to see which direction the RA drift was and it seems to be almost in line with the star trail angle, I say almost the same because the DEC drift must also be playing a part too, but I think the guided image only shows RA drift large enough to be concerned about.

        

2) You are making life hard for yourself by using a low sensitivity guide camera in a slow focal ratio guidescope. I'd try to get hold of something faster. A firm favourite is the humble and inexpensive ST80 which is F5. A better camera would be the QHY5. - Yes I have been looking at these type of camera's as my next addition to my collection of astro gear :grin: Have already primed the wife on this one.......I have been looking at an Orion Starshoot.  Which one do you think would be better, as I don't have any knowledge about these things?  Also I was wonderng about getting the Williams Optics 50mm x 200mm focal length guide scope, but it's only available in the US at the moment, so there's a bit of a delivery charge.  I wasn't sure that stepping too far down on focal length would give less of an accurate guiding situation causing further issues??  I do like the look of this little guider though, as there's an option to purchase it complete with mounting rings and quick release bracket too, and I'm guessing that the correcting signals going to the mount would be fewer, but is this a bad thing, or could this run smoother by making the guiding work less hard, I don't know??

3) How long are your guide subs? Too short and you chase the seeing, too long and the damage is done before the correction can work.  This is something I'm not that clear on......I tried playing around with the settings in Guidemaster and I think the "Delay after Slew" setting controls the guide sub length as it looked like the image was pausing for different lengths of time when I tried various settings.  I couldn't find any other obvious setting for controlling the subs length.....I tried 0.5, 2, 4 and 5 second intervals, but settled on 0.5 seconds in the end.  Any longer than this and I thought the RA error would be too great as the drive is too fast at the moment (which I was rather hoping would be handled by the auto guiding, and still think it may do as long as I can set it up properly, unless someone can prove otherwise).

4) Have you confirmed, or can you confirm, that the mount is at least receiving commands in the chosen guide directions and can, in principle, respond to them? Given the pretty slight error seen in the image I'd suspect that it is, but I'll ask anyway.  Yes the mount is defenitely getting the signals and is responding well.  I can feel and hear the motors making the corrections, so I don't think there are any issued here.

5) Have you entirely eliminated differential flexure between guidescope and imaging scope? Given your obvious engineering expertise I presume you will have done so but things to check are; The screws which hold the focuser into the main tube of the guidescope. The guidescope focuser itself. The push fit nosepiece. These three are notorious. - The screws holding the guide scope into position were wound up pretty tight so there should not have been any movement there, not sure what you mean regarding the focuser and nosepiece - do you mean that they could flex during the movement of the RA axis by the cable coming out of the webcam pulling on them?

Regarding the belt drive, yes I did consider this, but I'm not sure if the stepper motors would work in this situation as the gear ratio would make the movement too jerky.  As at the moment the gear box is 500:1.  If this was removed, then the stepper motor would need to move 500 times slower and I'm not sure this would give a smooth movement as the delays of the pulses sent to the motor would be massive compared to what I have right now.  I know the gearboxes and the way in which it's all set up right now are not ideal, but I'm trying to see if I can make it work without having to re design things first if possible.

Thanks for your questions, hopefully I have given enough info to get closer to a solution :smiley: :smiley:

Oily,  In anser to your question, yes I'm guiding with the Tasco approx 700mm focal length and the scope I'm imaging with is a WO Zenith star 80mm apo, focal length 545mm.

Cheers

Matt

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I have been looking at an Orion Starshoot.  Which one do you think would be better, as I don't have any knowledge about these things?  Also I was wonderng about getting the Williams Optics 50mm x 200mm focal length guide scope, but it's only available in the US at the moment, so there's a bit of a delivery charge.  I wasn't sure that stepping too far down on focal length would give less of an accurate guiding situation causing further issues??  I do like the look of this little guider though, as there's an option to purchase it complete with mounting rings and quick release bracket too, and I'm guessing that the correcting signals going to the mount would be fewer, but is this a bad thing, or could this run smoother by making the guiding work less hard, I don't know??

The QHY5 used to be a very popular guide camera, but I don't think it's made any more and only seems to be available second hand.  I guess the QHY5-II and QHY5L-II are intended to replace it.  By using a colour camera such as the Starshoot you're likely to be reducing the sensitivity of the camera because of the bayer matrix on the front of the sensor.  The pixels are also considerably smaller on the Starshoot compared with the QHY5-II (about 30% smaller?) which doesn't help with sensitivity either.  If you don't like the price of the QHY5-II I'd be tempted to look out for a second hand QHY5 on ebay or Astro Buy & Sell in preference to the Orion camera.  The only plus side I can see for the Starshoot is that it might work ok as a planetary imaging camera should you wish to do that.  I can't recall that I've seen that many images from one though.

At the moment I'm guiding my 127 Mak using a 50mm Skywatcher finder which also has a 200mm-ish focal length.  Mostly because I didn't have anywhere to attach a larger guidescope easily.  It's ok, but it's by no means great and I do have quite reasonably dark skies.  There's often not a huge choice of guide stars and I lose them from time to time.  I have a dual mount now, so if the cloud ever disappears I shall be swapping it for an ST80.  Like the QHY5, the ST80 is a much-favoured instrument for a guidescope, and relatively cheap when they come up used.  At a lot of star parties I bet you couldn't swing a large dob without hitting half a dozen imaging rigs fitted with a QHY5 and ST80.

James

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What do you reckon on the accuracy of a short focal length for guiding?  Isn't it better to go for the longer focal length?  Just to clarify, when you say ST80, so you mean the short tube 80 Orion aka "Orion Awesome AutoGuider?"

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What do you reckon on the accuracy of a short focal length for guiding?  Isn't it better to go for the longer focal length?  Just to clarify, when you say ST80, so you mean the short tube 80 Orion aka "Orion Awesome AutoGuider?"

I had in mind the Skywatcher ST80, but as far as I'm aware Orion sell the same scope under a similar name (and Celestron may do/have done as well).  The Orion autoguider package looks pricey to me (perhaps because I buy most things used), and I'd probably be tempted to fit a dovetail directly to the tube (or perhaps use clamshell type rings and a dovetail) rather than the adjustable rings provided in the autoguider package.  I know some people don't get on with autoguiders, so I'd ask around for opinions before committing to a purchase.

I don't have enough experience with different rigs to really comment on accuracy of different configurations, but my understanding is that modern guiding software works to "fraction of a pixel" accuracy even with a short focal length.

James

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Ok, that seems like sound advise, thanks!  I will keep an eye out for an ST80 in one form or another.  I won't bother with the starshoot camera then as I would rather spend a bit more and get something I will be happy with, I will keep my eye on the usual used selling sites.  Obviously the ideal would be to have a guiding set up where you don't have to move the guide scope to locate a guide star after the shot is already framed in the imaging camera and ready to go...All I would want to do is click on a star in the FOV and let it do it's thing.  Clear skies around here are few and far between, so time is valuable when imaging as you all know I guess.  I want to turn around my 95% messing around and 5% capturing decent light frames the opposite way!!

In the mean time though do you think I'm flogging a dead horse trying to guide with the Tasco and Toucam?  Should I ditch them and get the above and do you think this would solve my guiding issues?  I can have the motor driving speed on the RA slowed down slightly to come closer to perfect, so the guiding set up won't need to work so hard, maybe this will also add to improving things?.....

Any more help on this would be appreciated.  Is there anyone out there that has had similar guiding issues with trying to cope with an RA drive that's too fast?  Would a fast RA drive ever work properly when auto guiding??

Cheers

Matt

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Hi Astronymonkey!  Thanks for that, I have been wanting to put all the details of this mount onto a website or something for a while now, but have never gotten round to it....Perhaps I will take this opportunity to share it with the SGL community,  it would be nice to be able to help anyone else that may be thinking about making their own mount.  I'm by no means an expert, but it would be a pleasure to share my experience if that helps anyone out, whatch that space...

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In the mean time though do you think I'm flogging a dead horse trying to guide with the Tasco and Toucam?  Should I ditch them and get the above and do you think this would solve my guiding issues?  I can have the motor driving speed on the RA slowed down slightly to come closer to perfect, so the guiding set up won't need to work so hard, maybe this will also add to improving things?.....

I suspect you're making things harder for yourself than they need to be, certainly.  Even just swapping the scope to the ST80 would probably give the camera a bit of an easier time.

James

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Yes I reckon you are right James.  I think this has to be the first change I need to make to see what improvements it brings.  Then in the mean time I will look out for a replacement camera second hand....

Cheers

Matt

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Yes I could do this, but it would be hit and miss doing it this way, my mate Tim is the designer and builder off all the electronics on the mount and I'm hoping he can modify it so it can easily be tweaked (maybe even while it's running live with the webcam so it can be perfected, he has also been watching this thread with interest :smiley:

Cheers

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What do you reckon on the accuracy of a short focal length for guiding?  Isn't it better to go for the longer focal length?  Just to clarify, when you say ST80, so you mean the short tube 80 Orion aka "Orion Awesome AutoGuider?"

http://www.firstlightoptics.com/startravel/skywatcher-startravel-80-ota.html

Oooops - already beenn said...

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Using either an Atik 16ic or a Lodestar in an ST80 I have never, ever, in seven years of imaging in a good climate, needed to search for a guide star. My guide scopes are bolted down hard and can't be adjusted. The number of guidestars usually varies between 5 and far too many!

Olly

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How much do the Atik 16ic's cost, I'm guessing they are way more expensive than the humble QHY5?

I've been looking at the QHY5 II and the QHY5L II version in mono, not the cheaper colour version as it's less sensitive.  Would the QHY5 II guarantee me a choice of stars every time, or do you think I would need to spend the extra £20 and get the more sensitive QHY5L II?  Even though this version would enable some entry level imaging, I'm guessing it would still not be as good as the Canon 40D's capabilities, giving no additional benefit, or have I got this wrong??  As long as the QHY5 II does the job as a guide camera, should I go for that one?  I'm struggling to decide..........

Unless anyone has further advice regarding this?

Thanks

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How much do the Atik 16ic's cost, I'm guessing they are way more expensive than the humble QHY5?

I've been looking at the QHY5 II and the QHY5L II version in mono, not the cheaper colour version as it's less sensitive.  Would the QHY5 II guarantee me a choice of stars every time, or do you think I would need to spend the extra £20 and get the more sensitive QHY5L II?  Even though this version would enable some entry level imaging, I'm guessing it would still not be as good as the Canon 40D's capabilities, giving no additional benefit, or have I got this wrong??  As long as the QHY5 II does the job as a guide camera, should I go for that one?  I'm struggling to decide..........

Unless anyone has further advice regarding this?............Well that line made no sense :grin: That will teach me to not try and watch the match and type this at the same time...

Thanks

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