9 replies to this topic

[KLWalsh]

Why track past the meridian?
I have two mounts, a CGEM on a permanent pier that can view a lot of the sky, but due to my neighbor’s trees, can’t see towards the southern horizon. Thus, much of the southern part of the ecliptic and the Scorpius-Sagittarius region is blocked.
My second mount is a old CG5GT, which I recently mounted on a pier in a different area on my property that has an excellent view toward the southern horizon, but very limited views toward the east and west, also because of neighbors’ trees.
Annoyingly, the CG5 has a built-in stop that prevents it from tracking past the meridian. (The CGEM will track 20 degrees past the meridian.)
I quickly got tired of starting an observing session with the CG5 and almost immediately having to do a meridian flip. Then I noticed that only 2 M6 bolts hold the Dec section to the RA section. And I realized that if I could rotate the Dec section by 30 degrees from its ’normal’ position, I could track an object from 1 hour before the meridian to 2 hours past the meridian. I’d just have to adjust the time input when I started the mount.
So I made an adapter that fit between the RA and Dec sections that holds the Dec section at a 30 degree angle.
I bought a 3.5 inch diameter disk from McMaster-Carr and drilled 2 countersink holes to hold the disk to the RA section. (Shown as the B-B holes.) And I drilled and tapped two holes to hold the Dec section to the adapter at 30 degrees from its’normal’ position. (Shown as the A-A holes.)
When I start the mount, the counterweight rod is now angled 30 degrees from plumb. Facing South looking at the mount, the scope is rotated 30 degrees clockwise from its ’normal’ home position. To compensate, I have to subtract 2 hours from the actual time when I start the mount. Ie., if it’s 9:05 PM I input 7:05 PM.
So happily, I can now begin following an obect like Saturn one hour before it transits to 2 hours after it transits without being interrupted by the need to meridian flip.

 

(There’s a second set of countersink holes C-C that would let me mount the Dec section at 30 degrees counterclockwise, if I ever need to.) 

The second photo shows the adapter disk mounted between the Dec and RA sections.

As far as the CG5 knows, it starts at the normal Home position. It is of course unaware that the Dec section has been rotated.

 

By the way, in case anyone is wondering: Before I added the adapter, I had tried just starting the CG5 with The Dec section pre-rotated by 30 degrees. But that didn’t work. When the mount reached the Meridian it stopped.

Thus the need for the adapter.

 

Edit: The photo of mount got turned 90 degrees here. Don’t be confused by its odd orientation; that’s not part of the modification.

Also, notice the 1/2 inch center hole. That allows the polar scope to see Polaris and the surrounding part of the sky.

 

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Edited by KLWalsh, 24 September 2023 - 09:14 PM.

[AstroApe]

That's a pretty neat mod for adapting the mount to do what you need. I've got a CG5-ASGT too, and although I'll probably never need to mod it for your specific situation just knowing I can if the need arises is cool. 

 

What is the notched area in the bottom left of the plate?

[SkipW]

Wouldn't starting it with the PA in a position offset from the regular index position accomplish the same thing without the need to machine and install an adapter? 

 

I did that with a CG5 ASGT and an AVX for the 2017 eclipse using blue painter's tape to make my own index mark, then lying to the system about either my location or the time (but not both - I forget which I did) when starting up. I will probably do the same for the upcoming eclipses since mid eclipse for both will be near the meridian where I expect to be in Texas.

 

BTW, the CG5 will track past the meridian, but not as much as some other mounts; I think it's up to 40 minutes (the AVX can go up to twice as far beyond the meridian). If yours stops at the meridian, check the limit settings.

 

Do be aware that there is more likely to be a clash if you do this, so check to see if (and where) the telescope might hit a tripod leg or parts of the mount will hit each other!

Edited by SkipW, 25 September 2023 - 12:24 PM.

[KLWalsh]

That's a pretty neat mod for adapting the mount to do what you need. I've got a CG5-ASGT too, and although I'll probably never need to mod it for your specific situation just knowing I can if the need arises is cool.

What is the notched area in the bottom left of the plate?

Thanks.
Good question. The disk I got is 3.5” in diameter. I needed one 3.25” in diameter to match the mounting surfaces between the Dec and RA sections, but that size wasn’t available at McMaster-Carr. The notch is for a small section on the Dec section that is 3.25”. Without the notch, there’d be mechanical interference.

[KLWalsh]

“ Wouldn't starting it with the PA in a position offset from the regular index position accomplish the same thing without the need to machine and install an adapter? ”

As I’d mentioned in my post, I tried just starting the mount with the scope at 30 degrees past the meridian, but when the scope got to the meridian (or, just barely past it) it stopped. I couldn’t find any setting in the menu to change the stopping point.
My CGEM will go 20 degrees past the meridian.

Your warning about a pier crash is good advice. I’ve built a pier for the CG5 where that’s unlikely to happen, but for other piers it might.

[SkipW]

“ Wouldn't starting it with the PA in a position offset from the regular index position accomplish the same thing without the need to machine and install an adapter? ”

As I’d mentioned in my post, I tried just starting the mount with the scope at 30 degrees past the meridian, but when the scope got to the meridian (or, just barely past it) it stopped. I couldn’t find any setting in the menu to change the stopping point.
My CGEM will go 20 degrees past the meridian.

Your warning about a pier crash is good advice. I’ve built a pier for the CG5 where that’s unlikely to happen, but for other piers it might.

When did you pre-rotate the polar axis by 30°? Was it before "Align the index marks and press Enter"? What you describe hasn't been my experience with the CG5. I pulled the offset index marks off the mounts a while ago; maybe I need to do this again to make sure my memory isn't even worse than I think it is [shudder!]

[SkipW]

To prepare for the annular eclipse in a couple of weeks (and check my sanity), I tested the AVX's ability to continuously track well past the meridian in the backyard yesterday. I'll want this capability for both upcoming eclipses from south Texas.

 

Starting it up with the index aligned 15° west of the meridian instead of straight up, the time set to 1 hour earlier than actual, and with both RA limits set to -20° (20° beyond meridian), it tracked the sun to an hour angle of just over 2h21m (35.25° past the true meridian) before stopping at about 3:36 PM CDT. That HA is according to Stellarium for this location at this time and is consistent with the location that the mount was physically pointing. The HC read "Azm" axis (the polar axis) position as 199°59'43" when it stopped tracking; almost exactly 20° beyond where the mount "thinks" the meridian is (Azm = 180°). Since it thinks the meridian is 15° west of the true meridian, that is about 15° + 20° = 35°, as expected.

 

 

"Why, yes... the index marks are aligned (the way I want them)."

 

 

Mount tracked to about 35° past the meridian, then stopped.

 

 

[SkipW]

I set my CG5 ASGT up indoors with its RA limits at -6° (maximum) and index set 30° west of normal before startup. The longitude was fudged to 30° west of actual, and the correct local time here was entered; it tracked 30° (2 hours) past its normal limit.

 

Because the motor housings on the ASGT are so clunky, it may be necessary set the declination indexes 180° apart instead of pointing at each other to avoid the housings clashing as the polar axis goes beyond the usual limit of 6° past meridian, so watch out!

 

I'm not sure why this did not work for you.

[KLWalsh]

As SkipW describes, I thought the CG5 would track well past the meridian. But, *my* actual experience with it recently was that my CG5 does not. About 2 months ago I was imaging M20. I started with M20 slightly E of the meridian. When the CG5 got to the meridian the mount stopped tracking, so I had to do a meridian flip.
As a work-around a few days later, I tried starting the CG5 with the RA axis pre-rotated 30 degrees W of the meridian. But that didn’t help. When the mount reached the meridian it stopped tracking.
So apparently there’s either a mechanical stop or a firmware setting to prevent my CG5 from tracking past the meridian. Thus… my efforts to make a adapter to ‘fool’ the mount.
*Is there a firmware setting that will let my CG5 track past the meridian*? I don’t know. I looked through all the options in the HC and couldn’t find one.

I’ve considered replacing this 14 year old CG5GT with either a Celestron AVX or a Losmandy G811. Both of these mounts are capable of tracking through and well beyond the meridian.
For now, the CG5 is tracking really well. I’m going to continue using my 30 degree adapter and see how well it goes.
I’ve done a ‘hypertune’ on the CG5. If I got an AVX I think I’d have to hypertune it to get decent performance. Otoh, I don’t think there’s any need to ‘hypertune’ a Losmandy; from what I’ve seen they’re a major step up in mechanical performance.

Edited by KLWalsh, 29 September 2023 - 07:35 PM.

[SkipW]

As SkipW describes, I thought the CG5 would track well past the meridian. But, *my* actual experience with it recently was that my CG5 does not. About 2 months ago I was imaging M20. I started with M20 slightly E of the meridian. When the CG5 got to the meridian the mount stopped tracking, so I had to do a meridian flip.
As a work-around a few days later, I tried starting the CG5 with the RA axis pre-rotated 30 degrees W of the meridian. But that didn’t help. When the mount reached the meridian it stopped tracking.

That's odd. My CG5 does exactly what you say yours won't do.

 

AFAIK, the only concept the mount has about where the meridian is, is relative to where the index mark is when you start the alignment process. If you put that mark 30° west of the true meridian and hit ENTER after being told to align the index marks, the mount doesn't know anything other than you told it that is the direction to the meridian.

 

Very strange.

 

So apparently there’s either a mechanical stop or a firmware setting to prevent my CG5 from tracking past the meridian. Thus… my efforts to make a adapter to ‘fool’ the mount.

*Is there a firmware setting that will let my CG5 track past the meridian*? I don’t know. I looked through all the options in the HC and couldn’t find one.

There are mechanical obstacles - like the motor housings - that will keep it from tracking far beyond the meridian when oriented in certain directions, but those don't stop it from *trying* to track and will likely break something if you try to bypass the limit built into the software, like I just did, and aren't careful. 

 

Some mounts have internal hardware that does identify how the axis is physically positioned ("switch positions"). I know, because I've tried, that this technique won't work with a CGX because it apparently senses the switch even if you lie to it about where it's actually starting.

 

One thing... I ran today's test using a NexStar+ (v5) HC, not the original NexStar* (v4) HC that came with the 12-year-old CG5. I can't imagine how that could make a difference, but ya' never know. I will try again with the old hand control soon.

 

[Edit]

Same result as above using the old NexStar* HC with the CG5. If I set the index mark 30° west of normal before pressing ENTER at the index prompt, it tracks 30° further west past the meridian than normal, then stops tracking.

 

Are you sure you set this up correctly when you tried it?

Edited by SkipW, 30 September 2023 - 02:13 PM.