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Frankenscope: ETX70 on reworked Meade 4504 GEM

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#1 MtnGoat

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Posted 10 August 2008 - 03:06 PM

I picked up a Meade 4504 4.5" reflector off of Craigslist for $45. It was in pretty clean shape, though missing a few screws. Since it will accept the 497 handset I decided to see what I could make of it. I am so sick of fighting RA clutch slip on the ETX when it's in polar mode, I wanted to try a cheap GEM and see what I could make it do.

There was a dreadful amount of backlash in both axes due to the down and dirty way they motorized the scope. Both shaft couplers had a lot of slop and the funky plastic motor mount system was not rigid at all.

I decided to remount the motors and toss their setup. The output shaft on each motor has a square hole in it which I could plug the worm shaft into perfectly after filing a new flat opposite the original setscrew flat on the other side. I then put a small machine screw through both output shaft and worm shaft at that location.

I mounted the motors parallel with each axis and fabricated a couple small angle brackets. After opening up the motor and seeing that there was no place to put blind nuts or any easy way to screw mount the case, I just CA'd some spacer/mounting blocks straight to the front of the motor case. These blocks are what the actual mounting screws bite into.

Then I ran into the typical motoGEM issues of clearance. The motors now miss each other just fine, but the scope cradle was whacking the RA motor big time. So I simply cut off the offending portion of the cradle. It's not like this is a Losmandy or something. To heck with it.

That cured the interference problem from the cradle. I checked it with the ETX70 OTA on it...and now the scope hits the RA motor. Great. I decided to stay with down and dirty and fabricated a couple risers to get the OTA off the cradle by an inch. Clearance problem solved. Nothing hits nothing no matter what, not as easy as I'd thought on a GEM.

I then pulled the tripod legs and drilled bolt holes through the three mounting points. These bolts are vertical and pick up blind nuts on the backside of the upper pier adapter plate.

Plugged in the 497 and selected the LXD75 mount. Changed gear ratios to 4.56296 as shown at Weasner's gold mine of a Meade site. I had to reverse ratio polarity on one of the axes, don't remember which.

Trained the drive and then decided to pretend it was midnight last night. Did a two star alignment and everything went fine. Did some meridian crossings to different targets and the GEM smarts are working, it flips the front of the OTA across the pole as it's changing RA, just like it's supposed to.

So hopefully tonight I'll get to try it out. I noted a couple things already.

- Gear ratio is higher so fast slew isn't very fast.
- lower speeds actually have a clear and controllable effect on the motion of the image now, something I was never very happy with from the ETX.
- the amount of backlash visible in the system is vastly improved over the stock setup.

Here are some pics...

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#2 MtnGoat

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Posted 10 August 2008 - 03:08 PM

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#3 MtnGoat

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Posted 10 August 2008 - 03:09 PM

more

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#4 MtnGoat

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Posted 10 August 2008 - 03:09 PM

detail

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#5 MtnGoat

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Posted 10 August 2008 - 03:10 PM

last one

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#6 MtnGoat

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Posted 11 August 2008 - 12:28 AM

Just did my first ever GEM home position setup and align for real...pretty danged good results. First GOTO after a Mizar/Vega two star align is M13...dead nuts centered. Suweet! Now I'm going to pop the DSI on it and lets see the nutz and bolts of how a sub bargain basement GEM can perform. Or not.

#7 MtnGoat

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Posted 11 August 2008 - 03:24 AM

GOTO's are more accurate, periodic error is worse..I think. I am not certain I have an accurate polar align, I didn't drift it.

However, my overall impression is that this mount works better than the ETX forks, at least for polar mode where the side load is troublesome for the fork mount.

I messed around with FITS3P image format and for deep sky, this appears to be the one. I have to 'stretch' the images far more, but they come out better and the star 'streaks' on brighter field stars (due to saturation) are much reduced compared to my previous work. probably due to what looks like a vast increase in dynamic range vs the BMP format.

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#8 MtnGoat

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Posted 11 August 2008 - 03:27 AM

NGC6946

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#9 Michael Miles

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Posted 11 August 2008 - 09:21 AM

Woo Hoo! Nice seat-of-the-pants setup. Anybody who uses hose clamps is my kinda ATM (hose clamps held my 4.5" test scope on my mount for years).

Curious to hear how it operates after drift alignment,

Michael

#10 MtnGoat

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Posted 11 August 2008 - 01:01 PM

oh yeah, forgot this one, kind of cool. Satellite crosses M31 during a 15 second subframe.

As you can see, when the RA gear isn't PEing me, the tracking is not too bad for a mount that makes cheap sound upscale.

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#11 MtnGoat

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Posted 12 August 2008 - 02:13 AM

i did some more tuneup on the mount and it is freaking ticking over like a watch. I haven't been watching every sub, but if there is any PE at this image scale, it's pretty slight. every time I check back in with the subframes, M101 is in the same danged spot.

I got three successive GOTO's right near the center of the imager field, the last a hop from Vega to M101. Not bad.

this is the best 45 bucks i've spent so far. it appears I may have lucked into turning dog *BLEEP* into ice cream.

#12 desertrefugee

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Posted 12 August 2008 - 11:06 PM

Hey MtnGoat,

I've followed your build-up with great interest. I also recently acquired (for a steal) a Meade 4500 EQ mount. It's identical to the 4504 minus the motors. However, the motors appear very similar if not the same as those used on later DS-114 Autostar alt-az units. (I happen to have one of those setups, too). You don't suppose those motors could be used on the 4500 to convert to 4504 automation?

Could you please, maybe, snap a photo of the business end of one of your motors? I'm referring to the drive output end.

Those old Meade 4500-class mounts are decent units. I love mine, but thoroughly dislike the tripod. The pier sure does give me pause to think . . .

Appreciate the tutorial on your "experimentation".

Clear skies, and huge dilated pupils!

#13 MtnGoat

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Posted 13 August 2008 - 11:48 AM

I have a set of each type of motor, the DS and whatever we're calling the motors for the 4504. I have no idea which one has technically better performance.

They may even be very similar inside the case..but the difference is in the output drive. On the DS motors, power exits the motor assembly via a gear buried down inside the case. A matching spur gear on the DS motor mount mates with this. In other words, to mount the DS motors your matching piece must have a gear that reaches inside the DS case to the right depth and mate to the output gear properly.

On the 4504 motor, there is an actual output shaft that is easy to connect to since it has a rectangular hole in the end of it. This is what I filed down the worm shafts on the mount to fit into. If you look closely at the 3rd pic you can see the worm shaft going right into the output shaft on the motor, and the head of the small machine screw I drilled through both.

It would be interesting to figure out which motor system actually has better stats, but probably a fair amount of work. I've asked some questions about mounts in these classes over in the Mounts forum, but never gotten an answer. I realize it's not a Vixen or something, but I have a history of really poor luck in getting anything answered in that forum. don't know why

#14 desertrefugee

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Posted 13 August 2008 - 01:49 PM

On the 4504 motor, there is an actual output shaft that is easy to connect to since it has a rectangular hole in the end of it.



I thought this might be the case, and with the offset, there's no way to make a clean conversion. Oh well, it was a thought - although I'm not yet ready to throw in the towel. You've provided a bit of a spark of inspiration - and again thanks for sharing your exploits!

#15 MtnGoat

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Posted 13 August 2008 - 01:57 PM

keep your eye on your closest metro area Craigslist for 4504s. I just search the entire site for "Telescope". (Only problem with this is...you'll see waayyy too many other tempting deals. )

I saw yet another one go for fifty bucks last weekend. That's two in a month. I'm sure there's nothing magic about Portland OR in terms of people who get a scope, look through it twice, and then sell it 6 years later.

#16 desertrefugee

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Posted 13 August 2008 - 08:41 PM

keep your eye on your closest metro area Craigslist for 4504s



Way ahead of you, Mtn. That's exactly how I ended up with the eclectic stable I'm currently sporting - many of which are not reflected in my brag . . . er tag line. I'm in Metro Phoenix and there are lots to choose from on the "c" list.

But, we should not be giving away those good secrets . . .

#17 ccs_hello

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Posted 13 August 2008 - 11:21 PM

Hi,

The blocky Meade motor unit is called DH4 motor.
I adapted in the SVP GOTO mount, see this CN post.

DH4's gear ratio is twice as many as DS motor unit
(410.667 vs. 205.333) thus will move slow but has better precision.

DH4 motor now is hard to find. It has its own output shaft as oppose to DS's just has a output spur gear buried inside the motor unit.

Clear Skies!

ccs_hello

#18 desertrefugee

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Posted 14 August 2008 - 11:14 PM

Thanks ccs,

Another great thread - now on the SVP. Unfortunately, all my parts and pieces don't go together . . . although I have visions of GoTo on an Astroview dancing in my head now . . .

#19 ccs_hello

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Posted 14 August 2008 - 11:55 PM

Darrell,

These two pictures (DH4 motor mod) may help a bit:

Posted Image

Posted Image

Clear Skies!

ccs_hello

#20 MtnGoat

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Posted 15 August 2008 - 12:19 PM

51 minutes of M31 from last night. Thick haze, full moon.

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#21 desertrefugee

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Posted 15 August 2008 - 02:45 PM

Great work, ccs.

Now, if only I could get my hands on a couple of DH4s.

Meanwhile, there's a fellow in Phoenix selling a complete 4504 rig...(but I only need his motors). I offered HALF what he was asking for the whole rig just for the motors, but he's not biting . . .






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