Losmandy GM-8 (w/Gemini) - Help save my sanity.

rogerg (Roger) · #1 03-10-2006, 02:49 PM

I'm after help, stories of your troubles and triumphs, and any general information to help me use my Losmandy GM-8 (with gemini) better.

The Losmandy is my portable setup. So it's never setup in the same place twice within a 4 week period. Perhaps this is part of my problem.

Ignoring the drift method, I find it very hard to consistently get good polar alignment. I try using the Polar Align Assist and Polar Align Correction functions of the Gemini. Sometimes I think they work but then soon find they clearly haven't and I still seem to be off.

I seem to consistently have different RA tracking problems - like, tracking at the wrong speed. This weekend for example I got it setup, used Polar Align Correction, then was trying to autoguide on a star near 47 Tuc only to discover after experimenting that if I turned off the RA motor (set tracking to terrestial) the guide star was drifting at the same rate .. so like the RA motor wasn't working but it was turning.. and moving to Orion it was tracking...

Most of my problems seem to have the symptom that the RA tracking is wrong. Dec drift occurs but nothing like RA drift. I don't really understand what affects RA tracking?

Balance is generally perfect, I can move all payloads in all directions and easily counter-balance the setup (it's well under weight limit). I use the standard counterweight.

My LX200 I can easily polar align by hopping between two SAO stars and making appropriate correctsion. I duno why it's so hard on the Losmandy, I think maybe just because I'm always in a new location with that bit more uncertainty on N/S direction, correct level of the mount, etc.

Setting it up for visual use is a breeze, after doing 4 odd additional alignments it'll go straight to each object dead bang on centre no problem, and track them fine. Getting the level of tracking accuracy needed for photography seems to be a completely different story, more so than with my LX.

Any input? tips? pointers? What's your setup proceedure? help!

I'm getting sick of wasting my 400km trips to the country. I feel like a $1000 mount would behave better than my $4000 mount right now.

Maybe I don't set the RA/DEC locks the right tightness? maybe my initial polar alignment guess is so far off nothing works? maybe I need to adjust the worm gears? maybe I just need to understand the manual better. I do refer to the manual continuously when trying to use the different gemini alignment functions to make sure I get it right, out of frustration more than anything.

Oh and there are occasions when it works. Really don't know what the difference is - as I said at the start, observing sessions with it are usually 2 weeks apart at least, 4 weeks for deep sky astro photography stuff so it's hard to remember between goes the finer details a paturn of what works and what doesn't.

Roger.

JohnG (John) · #2 03-10-2006, 03:14 PM

Hi Roger

Couple of things, what Level Gemini do you have. Prior to Level 3 Version 1.13 there were problems with PAC, it was southern hemishere specific and would send your correction in the wrong direction. This was supposed to have been fixed in Level 3, Version 1.13.

I am asuming you have done this as you are experienced in using a Gemini. Have you checked your cables, no obvious faults. How is the 2032 battery, if it has been in the unit for more than 12 months it should be changed, a lot of problems can be traced back to this. Do you have the Gemini in Photo or All Speeds Mode when using the autoguider. How is your power supply, is the Gemini getting enough power.

I know these are the basics and you have probably already eliminated them but it is a start point.

Cheers

JohnG

[1ponders] (Paul) · #3 03-10-2006, 03:21 PM

Jeez Roger, what a pain. And often its something simple

I wish I could help but I don't have gemini. You've checked the obvious I gather? Right mount selected, gemini hasn't defaulted back to its factory setting? The usual stuff?

The only thing I can offer is; we were having trouble with the G11 at the Maplton Observatory, it was doing really weird things, and not consistantly wierd. Like loosing location, wrong time, not slewing to the correct coordinates, defaulting to a different mount at start up. What it turned out to be was the internal battery was shot (in fact it was the wrong battery, and it had been installed in the factory, we had never changed it previously). Change the battery and we haven't had a problem since.

That might be a place to start. How about trying a very quick, hard and dirty, half hour say, dirft alignment and then use the build in corrections? See if that makes a difference.

[1ponders] (Paul) · #4 03-10-2006, 03:22 PM

Snap John

JohnG (John) · #5 03-10-2006, 03:32 PM

OK Paul, I'll bite

JohnG

rogerg (Roger) · #6 03-10-2006, 03:38 PM

Quote:

Originally Posted by JohnG

Hi Roger

Couple of things, what Level Gemini do you have. Prior to Level 3 Version 1.13 there were problems with PAC, it was southern hemishere specific and would send your correction in the wrong direction. This was supposed to have been fixed in Level 3, Version 1.13.

I am asuming you have done this as you are experienced in using a Gemini. Have you checked your cables, no obvious faults. How is the 2032 battery, if it has been in the unit for more than 12 months it should be changed, a lot of problems can be traced back to this. Do you have the Gemini in Photo or All Speeds Mode when using the autoguider. How is your power supply, is the Gemini getting enough power.

I'm happy to start at the basics, believe me. I'm sure it is something really basic that I should have worked out by now.

Gemini Level: L4 (installed a couple of months ago .... which makes me wonder, as the last 2 trips have been particularly bad... hmm, I haven't checked what bugs there are in it's version)

Power: About 16.5v. I have one of these multi-voltage things made for plugging into a car cigaret lighter sockent which has settings for 12v, 15v, 17v, 19v. I have it set to 15v and with no load it registeres at 16.5v, haven't checked it when there's load on it. Battery is 40Ah so lots of juice available in general.

2032 - I will replace this, good idea. It's been something like 9 months.

When tracking I have the gemini in Visual Mode. I'm tracking using GuideDog and a webcam, interfacing to the scope through the standard RS232 connection not the autoguider port, so as I understand it (and have seen from experimentation) the mode doesn't matter for this... Yet to really get the hang of this setup (guidedog with webcam to gemini) though, still new at this.

I haven't checked voltages on pins of cables, but it will GoTo accuarately, so assume the cables are OK. I sure knew I'd plugged the RA and DEC in around the wrong way when I did that one time recnetly

, that I could diagnose!

Thanks,
Roger.

JohnG (John) · #7 03-10-2006, 03:49 PM

I am wondering about Level 4, there have seemed to be a lot of problems with the L4 release. Someone else from IIS was on the Gemini Group recently complaining about PAC problems, just wondering if the bug was re-introduced.

Do you use ASCOM at all, there was a special release for L4.

I am still using L3, v 1.13 and have just done a battery replacement after I was getting some weird behaviour, all is right again now.

Do you still have your L3 chip, if it keeps playing up, change it back, There was some talk about dud chips!!!!

Cheers

JohnG

rogerg (Roger) · #8 03-10-2006, 03:59 PM

Quote:

Originally Posted by JohnG

I am wondering about Level 4, there have seemed to be a lot of problems with the L4 release. Someone else from IIS was on the Gemini Group recently complaining about PAC problems, just wondering if the bug was re-introduced.

Interesting.... I'm now feeling suspicious of L4.

Quote:

Originally Posted by JohnG

Do you use ASCOM at all, there was a special release for L4.

No.. I just have an older ASCOM (downloaded it a while back), I don't remember having to download a new one recently. Perhaps this is a major source of my problems for auto guiding. I will take a look at this as soon as I get home.

Quote:

Originally Posted by JohnG

Do you still have your L3 chip, if it keeps playing up, change it back, There was some talk about dud chips!!!!

Yeap, still have the L3 chip safe and sound - I had a feeling I might want to put it back in some time....

Thanks heaps... all progress, hopefully some of it will make life easier...

JohnG (John) · #9 03-10-2006, 04:05 PM

Quote

"When tracking I have the gemini in Visual Mode. I'm tracking using GuideDog and a webcam, interfacing to the scope through the standard RS232 connection not the autoguider port, so as I understand it (and have seen from experimentation) the mode doesn't matter for this... Yet to really get the hang of this setup (guidedog with webcam to gemini) though, still new at this."

I have just re-read this, not having tried this at all but what about your guide speed, I know that for normal autoguiding it should be around 0.8 or 0.5, have you changed that when you are autoguiding through the RS-232 port. Maybe there is over-correcting which is causing your RA drift.

When you use the autoguider port and when set to Photo Mode, the guidespeed is automatically reduced.

Just a thought.

Cheers

JohnG

JohnG (John) · #10 03-10-2006, 04:09 PM

There was a special Gemini Driver released on the ASCOM site specifically for L4, it is an executable file and will only work with L4.

Cheers

JohnG

Lee · #11 03-10-2006, 06:33 PM

That was me on the Yahoo Group with the PAC problem - I still think that component of Gemini L4 is buggy, and am about to email Rene Goerlich with some data from last night.

I drift aligned (my first time - thanks to your website Roger!) - then did an 8 star alignment - Gemini reported I was off by -28' in azimuth and -20' in elevation - after a 50% or so correction with PAC the figures were A = -11 E = -34 - I'm sure that it gives elevation correction backwards - and had similar problems last week at home.

I'm going to stick to drift aligning I think! seems easier...... a pain, because PAC seems so elegant and simple.....

JohnH · #12 04-10-2006, 11:46 AM

Rodger,

Assuming you have a laptop and webcam available this tool is great as it calculates the amount of adjustment required for ALT and AZ and provided a screen to let you make the adjustments with precision. I use it to polar align my mount each use as I do not have a permanent setup - and gets me close enough for 5 mins exps without field rotation at f9 with a Canon 20d in about 20-30mins. I could never quite figure the drift method - this tool is visual so using it is quite easy - takes a couple of goes to get familiar with it....

http://wcs.ruthner.at/index-en.htm

rogerg (Roger) · #13 04-10-2006, 02:38 PM

Excellent. Thankyou all. I have downloaded the new ASCOM drivers (I had version 3 something) and will download the program you have recommend John (H). I'll see how I go.

I'll also not use the Gemini's PAC, I'll stick to other methods instead, probably using two stars and hopping betwen them making corrections, like the Polar Align Assist does but I do it manually (silly how they have such a small list of stars available).

Hopefully I'll improve the situation.

Roger.

[1ponders] (Paul) · #14 04-10-2006, 02:54 PM

Quote:

(silly how they have such a small list of stars available).

I know what you mean Roger. You'd think you would be able to use any star in the database, but realistically at least all common named stars down to mag 4 could be included in the alignment list.

Lee · #15 04-10-2006, 06:12 PM

I think you can use any of the objects in Geminis database for alignments - it is just that the alignment stars are more accurately positioned - what difference it would make I have absolutely no idea....

jase (Jason) · #16 06-10-2006, 03:24 PM

I'm using L4 v1.01 on a Titan with no problems assocated with PAC. If you've got a CCD camera and time, a CCD drift align works best. PAC got me very close to the pole though. Probably close enought to use an 800mm FL instrument for 5-10 minute unguided exposures.

Indeed you can use any object for an alignment, just select the object from the object database, slew to it. The select align telescope and the last object chosen is selected. This seems to work well. You can also use TheSky to do a similar task. If you want to get really serious, get yourself a copy of TPoint and then perform an 180 point model. Takes some time, but if you want to precisely know how far from the pole you are, this is very very accurate. TPoint will also ensure any object you slew to is always on the CCD chip. Nice for robotic operations.

rogerg (Roger) · #17 09-10-2006, 02:43 PM

Quote:

Originally Posted by jase

I'm using L4 v1.01 on a Titan with no problems assocated with PAC. If you've got a CCD camera and time, a CCD drift align works best. PAC got me very close to the pole though. Probably close enought to use an 800mm FL instrument for 5-10 minute unguided exposures.

That's pretty impressive unguided times. Seems perhapf the PAC problems depend on a few factors - hence sometimes working and sometimes not.

Quote:

Originally Posted by jase

TPoint will also ensure any object you slew to is always on the CCD chip. Nice for robotic operations.

Yeah, TPoint has been on my list for a while, to use with my LX200 though I have to admit. It'd be extremely nice. Just a little pricey for my budget (but fairly priced for the product).

I use TheSky so just use that for picking stars to align on.

Still haven't had time to try the new ASCOM version for the Gemini, hoping to do that this week.

Roger.

jase (Jason) · #18 10-10-2006, 12:54 AM

Roger,
When you do additional aligns after a cold start and one star sync, do you see the A and E numbers vary after each alignment? I've seen this and believe it is particularly common on a SCT due to very slight changes in the primary mirror movement. It doesn't have to move much especially if your using a 9mm guide eyepeice. This is utimately what the Gemini pointing model is trying to determine i.e, mirror flop and flexure in the set up. This is generally a good thing as it will improve goto slew accuracy, however when it comes to PAC the accuracy isn't the same. If you have a refractor (such as your Megrez 80) or a decent finder scope, then I'd highly recommend using this compared to the SCT. Also when centering the star, I usually go to photo mode and ensure the alignment is dead centre with the eyepeice cross-hairs. I'd be interested to hear how you get on with the refractor and if the accuracy changes.

Indeed PAC can get you close. I'm impressed with the capabilities of the Titan mount. Big payload capacity and superb tracking. I will admit that the mount does take some tweaking though. Fine adjustments of the worm block with an automotive feeler gauge was need to minimise backlash in both Dec and RA. A few other tweaks also. Works a GEM now (no pun intended).

rogerg (Roger) · #19 10-10-2006, 08:27 AM

Jase,

Thanks very much for your input. I can see what you mean with the SCT and how movements such as mirror flop could adversley affect the PAC. I'm keen to try it out again and see how I go, to be honest I haven't had the opportunity to use the setup since my last encounter which prompted this thread, so I haven't tried any of the suggestions here yet. I'm hoping to this week, Saturday at the latest.

It was only the last time that I used the scope which I started to think "hmm, i wonder what those numbers actually mean, perhaps I should be looking at them". They were huge some times (I remember seeing numbers like 300!) but can't remember what measurement they were in. It also sounds like I need to be more careful with the PAC - I typically have just put my 22mm Panoptic in the Megrez 80 (480mm focal length) and centered the star using that. I've done this because it's easy to find the star and center it. I am usually too lazy to put the illuminated crosshair eyepiece in. I think that's just not good enough and I should now definitely use the 9mm illuminated eyepiece, perhaps even with the doubler. Using the 22mm is just too lazy on my part. Your reply has made me realise that.

One thing that I still find a little confusing is that I can more often than not achieve a pointing model which results in every GoTo being dead on accurate. I'm very very impressed with it's modelling capabilities in that respect. Yet at the same time, the tracking will be way off - completely un-usable for guiding.

Roger.

jase (Jason) · #20 10-10-2006, 08:57 AM

Hi Roger,
Wow, A and E numbers up into the 300's is rather far off alignment. I started around -60, 30 and worked from there. I did cheat a little though. On the initial set up, I used a GPS compass to tell me the direction of true north/south, not magnetic. Once I had a visual, I used a tree in the general direction to get close alignment. I refined using PAC. I'm assuming you've got a permanent set up (considering you sig. also mentions an observatory), so it would be wise to spend a while perfecting the alignment of stars. When I ran PAC now and then I usually get A: 1 E: -2 so I'm pretty close to being polar aligned. Don't forget the hidden enemy - flexure. Make sure everything is secure. In some cases there is not much you can do about this and something the pointing model will assist you with.

The pointing model will continue to work even if you're not dead on the south celestrial pole. In fact, you simply have to be facing the axis south - nothing else. This is great for visual work, but as you've found out tracking isn't very good. The point model will continue to place objects in the field of view despite the misalignment.

On Page 39 of the Gemini L4 manual you'll find some interesting figures on your pointing model. A snipet of the info is below. There is some really nice information to check out on how well you set up is working. Keeping an eye on these values will assist you.

3.5.3 Modeling Parameters
Gemini models the following mount and telescope parameters:
• Azimuth misalignment from the pole
• Elevation misalignment from the pole
• Axis non-perpendicularity at the pole
• Axis non-perpendicularity at the equator
• Gear play
• Mirror flop
• Index error in hour angle
• Index error in declination
• Counterweight flexure
You can examine each of these parameters immediately after an additional alignment. The azimuth and elevation misalignment (A and E) are displayed right after the “Additional Align” is executed. You can also use the “Show Information” menu item to display the other parameters: NP = non-perpendicularity at the pole, NE = nonperpendicularity
at the equator, FR = gear play (in RA), FD = mirror flop (in Dec.), IH =
Hour Angle index error, ID = declination index error, and CF = counterweight flexure.
All values are given in minutes of arc. These values give you valuable information about your alignment, your mount and your telescope.

Hope this helps.