John Stone:
Daniel Petzen:
I then went on to do 2h19m at 0.4" RMS and then 1h 0.33", so I'm not sure how serious to take this.
As the new owner of a Mach2GTO I can say you're getting comparable performance when using a "floppy" telescope (i.e. one that has moving optical parts that makes repeatability impossible).
Something to consider that I've been having great luck with is to try the ZFilter algorithm with super fast guide exposures (I'm using 0.25 seconds) but then tune the filter to respond to only changes in the star positions that you'd notice if you took 4 second exposures (Xfac : 20)
This way you get a chance to "bump" the mount every quarter second but it will only correct for movements that are apparent in 4+ second exposures.
This works better because you get to issue corrections during the entire next 4 second exposure based on the results of the prior 4 second exposure.
So you're bumping the mount gently but constantly instead of abruptly but occasionally.
NOTE:
I have to warn you that you will have to completely ignore the guide graph ... it will show all of the seeing effects; instead you should turn on trend-lines and look for completely flat lines... That shows you're tracking perfectly.
Also, you can "eyeball" the bullseye and see of all the points look to be centered. I'm thinking of modifying PhD2 to add a "center of gravity" display to the bullseye so you can see any slow moving trends. (You'd want that "center of gravity" to be pinned exactly in the center of the bullseye.)
You can test it to check the FWHM and Eccentricity of successively longer exposures. They should stay pinned at what you get with say a 1-3 second exposure.
So far tonight I'm up to 21 minute exposures with no change.
 Wow. OK. That is super interesting. I read up on the ZFilter algorithm. I must admit I didn't fully understand it at the time, but your description makes sense. Are you running the ZFilter algorithm on RA and Dec? I may need to reduce my Dec backlash before moving away from the ResistSwitch on Dec (even though the Dec backlash compensation seemed to work surprisingly well for me). I also have a low signal in my guidescope, as I'm using a Barlow to increase the focal length, making it an f/10. I'd be keen to try it, though.
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Daniel Petzen:
Are you running the ZFilter algorithm on RA and Dec? Yes, because unlike your mount, mine has encoders on DEC as well as RA so there's no backlash to deal with. Off the top of my head, I can only think of the CEM70EC2 and CEM120EC2 as other more affordable mounts with encoders on both axes. I think the CEM70EC2 comes in around $6k and the CEM120EC2 comes in around $8.6k while the Mach2GTO is $12k. I would have purchased the CEM70EC2 if this Mach2GTO hadn't appears used/NIB like magic just as I was about to buy the CEM70EC2. There is a DEC backlash adjustment box you can fill in if you can measure backlash accurately.  And if you get an AO unit it also has a DEC backlash box:  Also, be sure to set that "Bump steps" box to as small as possible. It controls how much movement the mount does and big numbers lead to jerky movements that affect the AO unit's guiding. Set it just big enough to allow the mount to keep the star on the AO's tilt plate.
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John Stone:
I picked up a Starlight XPress Active-Optics That is awesome. I dismissed AO outside professional observatories after seeing a few unfavourable videos about it, but this is very impressive. Thanks for sharing. The video was really good with a lot of facts rather than opinions, so I'll definitely do some more research into this. It'll be some time before I can invest in one, but not having to get a Celestron OAG already put a dent in the total cost... Tempting... :-)
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Hello
I just wanted to report back that I've successfully upgraded my EQ8-Rh with an anti-backlash upgrade from Shibumi Astro-Engineering and my Dec backlash is gone.
Well, PHD2 is still reporting 400ms backlash, but I used to have somewhere between 3,500ms and unmeasurable before the upgrade, so let's call it significantly reduced at least.
I'm still finding the right settings with the help of Shibumi AE, but guiding for 3 hours at 0.4" RMS error with the Dec backlash compensation switched off was amazing.
I'm now started doing 8 minute subs. Not because the guiding is better than the best guiding I (momentarily) had before, but because it's _consistent_. I know it won't have a Dec walk-about for a minute destroying the sub now. 0.4" is actually sub-pixel guiding with my ASI294MC.
I'm going to experiment a bit more with the guide settings. I gave ZFilter a go, but couldn't get it to work. I'm going to give it one more go.
The next step will be to find a way to fit my Starlight Xpress AO to my image train ;-)
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Daniel Petzen:
Hello
I just wanted to report back that I've successfully upgraded my EQ8-Rh with an anti-backlash upgrade from Shibumi Astro-Engineering and my Dec backlash is gone.
Well, PHD2 is still reporting 400ms backlash, but I used to have somewhere between 3,500ms and unmeasurable before the upgrade, so let's call it significantly reduced at least.
I'm still finding the right settings with the help of Shibumi AE, but guiding for 3 hours at 0.4" RMS error with the Dec backlash compensation switched off was amazing.
I'm now started doing 8 minute subs. Not because the guiding is better than the best guiding I (momentarily) had before, but because it's _consistent_. I know it won't have a Dec walk-about for a minute destroying the sub now. 0.4" is actually sub-pixel guiding with my ASI294MC.
I'm going to experiment a bit more with the guide settings. I gave ZFilter a go, but couldn't get it to work. I'm going to give it one more go.
The next step will be to find a way to fit my Starlight Xpress AO to my image train ;-) Do you have some pictures and a write up of fitting the new anti backlash gears,?? also just an FYI check the belt tension is correct, I say this because my mount had 2200ms of backlash, and just by getting the belt tension correct, it went down to 310ms…without touching the worm mesh at all..👍🏻
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AstroStew:
Do you have some pictures and a write up of fitting the new anti backlash gears,?? also just an FYI check the belt tension is correct, I say this because my mount had 2200ms of backlash, and just by getting the belt tension correct, it went down to 310ms…without touching the worm mesh at all..👍🏻 I actually filmed the entire upgrade with the intention of making an EQ8-Rh Dec anti-backlash upgrade video. I've not gotten around to it yet, but I'd be happy to start that now. I'll post back here when I'm done. I'll work on it this weekend. It's funny you would mention the Dec belt... When I took the Dec worm wheel housing off, the belt tension was extremely bad and some of the screws were actually loose. I don't think I would have gotten the same performance by just tensioning a screw up, but the extreme backlash was definitely because of this. I'm not an engineer, but the Shibumi replacement is a beautiful piece of engineering and when you campare it, side-by-side, with the original, then it's just wow. I just found a picture from when I had everything fitted before putting the covers back on:  |
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If you were to graph your guiding error against the resolution of your optics you'd find that at some point the FWHM and the eccentricity just flat lines. You can still see errors in the guiding but they are too small to effect the image which averages out the error. If you're getting 0.35" RMS now then you really won't have a problem until your resolution on the chip gets close to that value. I don't expect that you'll have a real issue until you upgrade to an aperture of >12" and under almost perfect seeing.
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Tony Gondola:
If you were to graph your guiding error against the resolution of your optics you'd find that at some point the FWHM and the eccentricity just flat lines. You can still see errors in the guiding but they are too small to effect the image which averages out the error. If you're getting 0.35" RMS now then you really won't have a problem until your resolution on the chip gets close to that value. I don't expect that you'll have a real issue until you upgrade to an aperture of >12" and under almost perfect seeing. Yes, you're right. That was what I noticed the first nigh with consistently good guiding at 0.4" RMS error. I only threw away two subs (of 40 I think), and that was because of clouds. All the other subs were really sharp (well, as shar as it gets with a C9.25 XLT SCT!). I've been tempted to image (at full focal length) with my new MiniCAM8 with 2.9um pixel size, but I don't think my nerves are ready yet :-) I know I was chasing amazing guiding, but taking it beyond this (with this setup) is more of a technical challenge rather than a necessity now.
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Daniel Petzen:
AstroStew:
Do you have some pictures and a write up of fitting the new anti backlash gears,?? also just an FYI check the belt tension is correct, I say this because my mount had 2200ms of backlash, and just by getting the belt tension correct, it went down to 310ms…without touching the worm mesh at all..👍🏻
I actually filmed the entire upgrade with the intention of making an EQ8-Rh Dec anti-backlash upgrade video. I've not gotten around to it yet, but I'd be happy to start that now. I'll post back here when I'm done. I'll work on it this weekend.
It's funny you would mention the Dec belt... When I took the Dec worm wheel housing off, the belt tension was extremely bad and some of the screws were actually loose.
I don't think I would have gotten the same performance by just tensioning a screw up, but the extreme backlash was definitely because of this.
I'm not an engineer, but the Shibumi replacement is a beautiful piece of engineering and when you campare it, side-by-side, with the original, then it's just wow.
I just found a picture from when I had everything fitted before putting the covers back on:
Yes they do make nice upgrades, I bought and fitted there replacement DEC clutch ring on my old EQ8, made from brass, and it was so much better than the original, do you still have to adjust this new worm, or it it just a case of fitting and forgetting as it’s spring loaded..?
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AstroStew:
do you still have to adjust this new worm, or it it just a case of fitting and forgetting as it’s spring loaded..? Once the new worm wheel housing has been fitted, you need to adjust the spring tension. There is a scale that makes it easy to get it right:  There is then a burnish (wearing in the worm wheels) procedure that takes a couple of hours. ...and that should be it! I've gotten to removing the casing and saddle in my video so far, so it'll be a bit longer :-)
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For the record: I've not forgotten that I said I'll cobble together a video of the upgrade. I had loads of footage, so it's taking a bit of time. I shall report back when it's done.
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