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Re: Using ST-4 output from ZWO camera with 492 drive
John Bridgman
Thanks Mark. I thought I remembered seeing comments about grounding concerns with 492 drive as well, but IIRC that was for the "no relay box" scenario. Will try it out next non-cloudy night.
If the weather stays like it has been the last week or so (clear & sunny daytime, cloudy at night) I may end up having to use one computer to drive the camera and another to generate a moving starfield I can track :(? If I am able to guide with the ZWO cameras (which now seems like a pretty sure thing) then I'll probably still try to resurrect the ST-237 so I can play with a cooled camera without further investment, but agree it's not going to be worth the hassle/risk to try guiding with it.? |
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Re: Guiding expectations
Tom, You didn't mention what your guiding exposure is. If less than 2 or 3 seconds then, yes indeed, you will be chasing seeing. You also didn't mention if you find the resulting images acceptable or not. One way to see just what is going on is to disable guiding but enable logging. Then you can go into the log (using Excel or Andy Galasso's Guide Log Viewer) and see just what seeing along with mount bearings + gears are doing before guiding gets into the act. With a G11, unless somebody has over-tightened things (which happens a lot when users attempt to eliminate backlash), you should see a smooth trend line in DEC caused by polar alignment being imperfect, tiny errors in the tracking rate in RA, and atmospheric refraction. Superimposed on that you will see what the atmosphere is doing to you. When de-trended RA and DEC should look similar (from a statistical point of view). Having the guide camera oriented so the sides of the chip are parallel to the directions of axis motion makes it easier to interpret the results. When you turn on guiding (with reasonable parameters - find the best usually takes a bit of systematic fiddling) you can then begin to look for problems. You may already be aware of them but some common tricks include biasing the mount weight slightly east heavy. That together with a guide rate of 0.5X or lower ensures the RA gear always stays in contact and never reverses. If you have a small amount of DEC drift then changing the DEC mode so it only corrects in one direction and (possibly) biasing the OTA to keep the DEC gear engaged on the same side of its worm will eliminate backlash in the DEC axis as well. Doing these things, at least, may reduce the damage caused by the seeing. Too much DEC drift, of course, and you'll get field rotation. I routinely image with a much older (20th Year Anniversary model bought used) G11 that I upgraded to the precision (brass) worms and fixed few things prior owners had messed up (like over tightened worm bearing blocks). Otherwise stock and none of the wonderful enhancements Losmandy has made in the past few years. I get about 0.5 arc sec RMS on good nights with a 18+kg load. On bad nights, over 2 arc sec. 1100mm (200mm f/5.5 Newt) is my norm. On tough nights I pull in my horns and go to 400-800 mm with smaller scopes. On great nights, with bright objects, I'll image with a Barlow to get about 3000mm but those nights are few and far between and, of course, the higher f-ratio requires longer exposures. My camera has 6.8 micron pixels. Really high end astro-cameras have 9 micron pixels, by the way. Why? Because the bigger the pixel (more area) the higher the photon counts which reduces the (unavoidable) statistical noise at the lower light levels. Unfortunately the non-astro sensor-makers have been in a mega-pixel arms race for the past decade or so. Fine for general photography or for lunar/planetary work. General remarks: Smaller pixels on the imaging scope, of course, together with long focal length optics, make everything tougher - seeing/guiding that look horrendous with 3.8 micron pixels at 1500mm might look just dandy with 6 to 9 micron pixels at 800mm. Smaller (imaging) chip sizes make things even more demanding since you'll have to enlarge the image more to fill the screen. For example, compared to an APS-C size (nominally 24X17mm) you will have to enlarge the image 50% more to fill the same screen since your chip is about 17X13mm. With your rig a 3 arc sec RMS translates to about 20 microns. Double that (roughly) to get the blur disk you should see on moderately bright stars. So I would expect your stars are probably about 40 microns across, or 10 pixels with your camera. By way of comparison, the diffraction-limited blur disk will be about 11 microns at f/7 (actually, it will be slightly worse since the 11 micron figure does not include the secondary diffraction - 12 microns is probably more realistic). That figure is the absolute best you could ever see and it will never happen - to get that you must have perfect optics, perfect alignment, perfect focus, no scattering of light anywhere (including between the chip pixels and the cover plate of the sensor), no tracking or guiding errors, and no atmosphere. So your images are probably 3 to 5 times larger than the absolute best. Not great but if they are circular not horrid, I would guess. Best Wishes, Mark Christensen St. Charles, IL USA |
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GM811G--pier extension or RA extension?
All: ? So, to avoid this in the future and to give me a margin of error, I figure I need either a pier extension OR an RA extension kit.? ? Any advice on this?? I do NOT plan on getting a longer OTA (famous last words).? My sole interest here is AP--if I want to do visual, I have a perfectly nice Dob.?? ? Also, if a pier extension is the way to go, any thoughts on the 8" extension vs the 12" extension? |
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Re: Guiding expectations
¿ªÔÆÌåÓýHello Tom Not too sure what guiding software you use, but bad seeing drives guiding nuts¡ If you use phd2 use guiding assist and run it for 3 or 4 minutes and it will make suggestions for you. After you restart guiding let it settle down for a minute and then make small adjustments. Make sure you set the guide camera in PHD to 1400mm as well if you haven¡¯t already¡(done that before :) I have an 1996 G11 that I have been working on for a few months and get under 1 RMS and 1 ? arc sec peak to peak at 1300 mm FL (was 4to 6) Make sure you don¡¯t have crazy backlash (easy to adjust out) At 714mm F7 I get about the same until about the last 3 hours then I guide out around 1 arc secs peak to peak at .6 to .8 RMS I also calibrate around 50 degrees? and near the meridian and never at zenith or close to the horizon A lot of mechanical work has been done to the mount and not quite finished yet but New Mexico dark skies are the main reason for decent guiding on my part.(we always try for better) Chasing the seeing is just the beast we all deal with. Ken ? |
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Guiding expectations
Tom McPhail
Hi All, I'm hoping I might be able to reset my troubled brain when it comes to the guiding results I am getting from my backyard. To set the stage, I have a 12 month old G11 which I'm using with a Celestron Edge HD 8" + 0.7 Focal Reducer + ASI1600MM. My guide camera is an ASI174MM mini attached to a Starlight Xpress OAG. This setup is F/7 with a 1487mm focal length. It should be noted that the seeing from my inner city backyard (Brisbane, Australia is pretty terrible). After building a Tpoint model from TSX on the narrow window of E->S sky that I have from my deck I can anecdotally centre on a star unguided and walk away for an hour to find it has barely moved when I get back. This seems to indicate at a macro level that the tracking on my mount is pretty good. However when I pick a target and try to guide I am seeing all sorts of oscillations back and forth by TSX as it tries to constantly correct my mount. At best I'd average 2-4" RMS. I've tried tweaking the settings in TSX to back off the aggressiveness of corrections but to no avail. I've tried 1x1 and 2x2 binning with similar results. I was getting a bit stressed about this last night, but then it occurred to me that I really might just be hard up against the terrible seeing conditions -> Feedback welcomed. |
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Re: Using ST-4 output from ZWO camera with 492 drive
In PhD2 click on the Tools tab. The first item is Manual Guide. If you set the Guide/Slew rate in the 492 panel to 16X you'll be able to see and hear the manual guide corrections. I use the Manual Guide at 8 or 16X to check cables (a common source of field failures for me) and to center on targets. It works fine, as long as I remember to set the Guide/Slew back to 0.5X afterwards! The whole reason for opto-isolators is to get rid of the problems with relays. The SBIG relay box, in my opinion, was a disaster waiting to happen with grounding and other problems - like no anti-kickback diodes with relay coils powered off (according to the SBIG schematic) your camera power supply. Scary as heck and the reason why the old 237 specification included the number of Joules of energy the circuit had to be capable of withstanding. Optos don't do that. I suspect your anxiety arose from hearing old horror stories associated with the original Gemini system, which had grounding conflicts with some guide devices. The Model 492 never had those problems to my knowledge. I've guided with the ST4 interface with optos on the 492 since 2006 with two different guide cameras wit their own optos and the Shoestring USB to ST4 box (GPUSB) with an old mono Meade DSI. Never a hiccup in any of it. So I think you're better off without the relays and the (my opinion) clunky 237 box and software. Send it to be recycled - don't invest in the past by trying to get a computer with a true parallel port. I ditched my old LX modified Celestron planetary camera (the original NextImager modified ala the MexImager mod of Freddy Diaz in Mexico City) that I used for guiding years ago rather than try to keep it running. That and the GP-INT (parallel port to ST4 opto-adapter from ShoeString) has been sitting in the spares box for over a decade. If you ever get a failure in the 237 you will be on your own repairing it and even getting parts. If you want to guide with a camera that does not have an integral ST4 interface, get a USB to ST4 box (called the GPUSB) from Shoestring Astronomy. If you order it direct it is cheaper than orders through astro-retailers. PhD2 (and PhD) will support that as the Mount interface under the Configuration menu. That and my Meade mono DSI-Pro have been my backup system for over a decade - they both still work with Windows 10. Mark Christensen |
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Re: Using ST-4 output from ZWO camera with 492 drive
I don't have a 492 but I repair Gemini 1 & 2's, Cameras.....and everything Astro put in front of me.....
All modern ST4 outputs are either opto-coupled out or transistor TTL "low" signals.?? Aka...the signal is a "low level" the opposite to a "high" (high= +5V).? So, ST4 these days outputs is "a signal low".?? The 492 Drive Hand-controller buttons switch ground (or "low") out the appropriate pin to command movement.? See attached diagram. Thus "yes" you can use your ZWO camera as an ST4 for the 492 drive as the HC and ST4 signals are essentially the same.? You will need that parallels these ZWO ST4 signals with the HC signals and your ST4 guiding will be limited to what G/S rate you have set in the system...the top LEDs:.3x; .5x; 2x....etc? But it should work fine at .5X.? Lastly you need to pay attention to the wiring of your ST4 to HC adapter cable to ensure it goes to the correct positions wrt to the HC buttons.? If you miss-wire it...don't fret it won't cause damage....just work the wrong way!? may help a little more in this. Cheers tell us how you get on. ? -- Brendan |
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Re: Using ST-4 output from ZWO camera with 492 drive
¿ªÔÆÌåÓýYup I use a asi 120mc for guiding with my 492 using PHD2. ?Best move since st4 camera.Sent from my iDork On Sep 1, 2019, at 5:58 PM, John Bridgman <john.bridgman@...> wrote:
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Using ST-4 output from ZWO camera with 492 drive
Hi all,
I have a ~18 year old G11 with 492 drive. Used to guide it with an ST-237 + relay box, but (a) the relay box disappeared at some point and (b) I no longer have a computer old enough to run the 237, although I am trying to change that. I do have a ZWO ASI290MC and wanted to try guiding with that. It's a one-shot colour cam which I know is not ideal for guiding but if it works OK I would pick up an ASI120 mini for guiding and use the 290 for imaging. According to ZWO all the ST-4 outputs are opto-isolated so it seems like this should all be safe to hook up - I'm just surprised that I haven't found a single example anywhere of anyone using ST-4 from a ZWO camera with a Losmandy non-Gemini mount. I haven't seen anyone saying it *won't* work, just... nothing.? Is anyone using this combination (ZWO ST-4 output with 492 drive) successfully, and if not does anyone see any big risk of damage ? I can live with it not working but with 492-based mounts out of production and 492 electronics out of stock I'm a bit more wary than usual.? Thanks, John |
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Repairing Stripped Motor Mounting Threads on the Losmandy Gearbox
JohnS
Recently, I found that the servo motor on my GM811G declination head was loose - that I can move it by hand against its gearbox.? Then I found?that I couldn't tighten the motor's 4-40 cap head mounting screw snugly anymore.? The threads in the screw hole had become worn.? Being made of?plastic, I suppose they wear with repeated removal and adjustment of the motor despite care not to over-tightening the cap screws.
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I read about the McLennan replacement but decided there was too much work involved.? Besides, my mount is just two years old and the gears?inside should still be okay.
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So I decided to see about repairing the stripped screw threads.? I considered two options: 1) completely fill the screw hole with epoxy then tap?new 4-40 threads, or 2) partially fill the screw hole with epoxy, and quickly insert the 4-40 cap screw, then remove the cap screw after the?epoxy has set by undoing the screw thus leaving threads behind.
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I decided to go with the second option but the challenge was how to be able to remove the cap screw after the epoxy has set.? One solution I?found was to coat the cap screw thread with Vaseline petroleum jelly (I read cooking oil could also work).? So here is what I did.
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1.? Enlarge the screw hole to allow enough epoxy to line the walls of the hole and be able to form threads along it.? The hole can only be?enlarged very slightly because the screw hole is actually a U-shaped cavity inside the gearbox casing separating the screw threads from the area?where the gears are, and the walls of this cavity must not be breached when enlarging the screw hole.
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I have attached a drawing (not to scale) with a cut-away showing approximately the original screw hole (yellow shaded area), and the enlarged?hole (red shaded area).
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2.? Coat one of the 4-40 cap screw completely with Vaseline petroleum jelly, threads and cap head, then melt the petroleum jelly (for example?with a hair dryer) to make the petroleum jelly more fluid and flow into the screw threads.? Then shake off the excess so only a thin film of the?liquid coats the threads.
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3.? I used Gorilla brand "GorillaWeld" epoxy that is sold in two tubes (hardener and resin) and is black in colour when mixed, instead of the?clear epoxy in an injection tube.? The epoxy starts to cure in 10 minutes so the steps described below must be done quickly.
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4.? If the gearbox's other mounting screw hole is still good, thread the other cap screw in.? This can serve as a guide to how deeply the screw?should go into the screw hole.
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5.? Mix thoroughly a pea-sized amount each of epoxy hardener and resin.
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6.? Apply an amount of epoxy into the screw hole and, using a toothpick or similar implement, spread the epoxy into the hole to coat the bottom?and sides of the hole.
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7.? Apply a thin coat of epoxy to the cap screw which is coated with petroleum jelly.
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8.? Insert the cap screw coated with epoxy into the screw hole using the other screw as guide to know how far this screw should go in.? Ensure?that this screw is centred into the hole and is upright like the other screw.? If excess epoxy is squeezed out of the hole, don't clean it up?at this stage.? Just ensure the cap head and the Allen key hole is free of epoxy to ease the screw's removal after the epoxy has set.
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9.? Allow the epoxy to set, about 10 minutes.? Then using an Allen key, carefully undo the screw.? The petroleum jelly should keep the epoxy?from binding firmly to the screw thread.? The screw should turn with little pressure on the Allen key.? Remove the screw completely and?allow the epoxy to completely cure overnight.
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10.? Using a box cutter or thin blade, shave off excess epoxy so the screw hole is flush with the gearbox casing.
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11.? Install the motor and tighten the 4-40 cap screws only enough so the motor is snug on the gearbox.
I hope this helps. JohnS ?
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Re: New Losmandy Owner
¿ªÔÆÌåÓýWelcome aboard Rick ? G11 is a great mount, and there¡¯s lots of great people here to answer any questions you may have ? Thanks ? Brian ? portfolio https://www.brianvalentephotography.com/ ? From: [email protected] [mailto:[email protected]] On Behalf Of Richard Paul via Groups.Io ? All,? |
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New Losmandy Owner
All,?
Joined this group a while back when I was trying to decide on my next mount. You¡¯ve helped me decide on getting a G11.? I have started ordering it ¡°in reverse¡±. Getting all the small stuff first (knobs, power supply, etc). Will order the G11 next month.? -- Rick Paul Tucson, AZ |
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Re: Trouble after installing internal battery
Thanks.? Yes, I think I just need to start anew and tweak some things.? 9 times out of 10, at least in my case, it turns out to be "operator error". Jim On Thu, Aug 29, 2019 at 1:42 PM John Kmetz via Groups.Io <jjkmetz61=[email protected]> wrote: Jim, |
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Re: Trouble after installing internal battery
Jim,
After installing a new battery you must do a Cold Start from the Hand Controller. The make sure time and location are correct. You may need to redo your pointing model. Just went through this myself. You can reload all your other particulars from the Gemini app. Regards, John |
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Re: Trouble after installing internal battery
Thanks.? I'll give it a shot. J. On Thu, Aug 29, 2019 at 6:53 AM David C. Partridge <david.partridge@...> wrote:
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Re: Trouble after installing internal battery
¿ªÔÆÌåÓýSet to UTC and set local offset IIRC ? D. ? From: [email protected] [mailto:[email protected]] On Behalf Of Jim Ribble ? One thing I have NOT tried yet is to set everything to UTC time with a zero offset.? I will do that.? Fingers crossed.? (I had it set to local time with a -04 offset for EDT.) ? On Thu, Aug 29, 2019 at 12:06 AM Teddy Wang via Groups.Io <tedcwang=[email protected]> wrote:
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Re: Trouble after installing internal battery
One thing I have NOT tried yet is to set everything to UTC time with a zero offset.? I will do that.? Fingers crossed.? (I had it set to local time with a -04 offset for EDT.) On Thu, Aug 29, 2019 at 12:06 AM Teddy Wang via Groups.Io <tedcwang=[email protected]> wrote: Maybe check the scope type from the HC?? Make sure you select the G11 as your mount type.? |
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Re: Trouble after installing internal battery
Yep, G11 is set as well.? I'm stumped (although it's likely something simple.) On Thu, Aug 29, 2019 at 12:06 AM Teddy Wang via Groups.Io <tedcwang=[email protected]> wrote: Maybe check the scope type from the HC?? Make sure you select the G11 as your mount type.? |
Richard Paul
David C. Partridge