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ETX125 DPAC

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16 replies to this topic

#1 Chuck Hards

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Posted 13 September 2019 - 05:41 PM

I was recently given an ETX125 "Premier" Edition with dead mount.   This is the one with NGC 7000 printed backwards on the tube graphic.   I de-forked the scope and want to use it on a GEM I have on-hand.  First I want to look at it with the Ronchi.  I've searched but can't find any quick ways of getting the mirror spacing correct for infinity focus.  If someone knows about where the focal plane should be with the spacing correct, please tell me!  Or perhaps there's a certain eyepiece with the field stop at just the right place when the scope is focused at infinity?

 

I have the manual as .pdf files but have just barely begun to dive in.

 

Any tips are appreciated. 

 

 

 

 



#2 fcathell

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Posted 13 September 2019 - 06:02 PM

Just about any standard 1.25" eyepiece in the scope focused on a star (or the moon, a planet, etc.) will do it. I made a Ronchi eyepiece by gluing the grating at the field stop of an old eyepiece and removing the lenses.

 

Frank



#3 Chuck Hards

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Posted 13 September 2019 - 06:05 PM

Frank, I want to do it on my optical bench, using my 8" test flat, not on a star.   I need the designed mirror spacing for infinity focus to get the best results.  I'm using a 133LPI Ronchi screen & green LED.



#4 Chuck Hards

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Posted 13 September 2019 - 06:07 PM

I suppose I could just get a focused image with a low-power eyepiece of a distant mountain, then move it in to the bench.  That may be close enough.  Just pivot the flip mirror out of the way once on the bench.



#5 freestar8n

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Posted 13 September 2019 - 06:27 PM

Hmm - why wouldn't you use the DPAC to find the spacing that has least spherical aberration - and then use that as a guide to place the eyepiece?  Focusing on a mountain would be a good starting point - but if any spherical is evident I would adjust the mirror spacing.  Unless it pulls the focus into the OTA.

 

It's not clear to me how well they make these things so the designed spacing really is optimal.

 

Other Frank



#6 Chuck Hards

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Posted 13 September 2019 - 06:52 PM

I tend to agree, Doc, this isn't the most precise optic in the world.  I wanted to set the mirror spacing at factory spec for infinity just to start, then see what happens as I play with the variables. 

 

I just focused on a distant mountain peak, at least 13 or 14 miles, that should be sufficient.  I used an old Edmund 28mm eyepiece with the focal plane about halfway down the 1.25" barrel.  

 

I will set it up on the optical bench without moving the focus knob, and move the flip mirror out of the way to test straight-through.  Thinking about it, it would be nice to know the quality of the flip mirror itself so I'll try to check it both ways.

 

On the GEM, I wanted to use it straight, with a standard 1.25" visual back threaded on and a known high-quality diagonal.  It will be interesting to see if the spacing has to be pulled far from optimal to reach focus like that.

 

 

I've owned the Celestron 5" Mak as well as a couple of ETX90's.  Currently using an Orion 7.1" Mak which I'm enjoying so haven't tested it on the bench yet (see my sig).  Then this one fell out of the sky.  I thought about trying to repair the mount but I'm not a fan of small forks, and the ETX is just a snap to de-fork anyway.  No pun intended.

 

Stay tuned but continued comments welcome.


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#7 freestar8n

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Posted 13 September 2019 - 06:54 PM

Sounds great!  Very interested to hear any findings.

 

Frank



#8 cytan299

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Posted 13 September 2019 - 07:23 PM

It’ll be interesting to compare your DPAC results to mine

 

https://www.cloudyni...-goodbad-is-it/

 

What is the vintage of your ETX125.

 

cytan



#9 Don Taylor

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Posted 13 September 2019 - 07:25 PM

............  It will be interesting to see if the spacing has to be pulled far from optimal to reach focus like that.

 

I'm very interested in your results.  

 

I've had a Terrabeam 125 for a few years and it's fitted with the Wegat back.  I set it up with a Baader QC so I can use my T2 diagonals and they do couple closer to the backplane than if I had used a conventional visual back with the Wegat.  I did that to under the (perhaps incorrect) assumption that use of a conventional visual back and diagonal would drive mirror spacing at focus to be well away from optimum.  BUUUUUUTTTTT.....that was just a guess or a prejudice. I have no idea of where the spacing is optimum. 



#10 Chuck Hards

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Posted 13 September 2019 - 07:38 PM

It’ll be interesting to compare your DPAC results to mine

 

https://www.cloudyni...-goodbad-is-it/

 

What is the vintage of your ETX125.

 

cytan

I found that thread while searching, before posting this one.  Interesting thread.

I have no idea of the time frame this one was manufactured.  It has the backwards-printed NGC 7000 on the barrel.  This was given to me by a friend, I'll try and pin down the manufacture date.

 

 

 

I'm very interested in your results.  

 

I've had a Terrabeam 125 for a few years and it's fitted with the Wegat back.  I set it up with a Baader QC so I can use my T2 diagonals and they do couple closer to the backplane than if I had used a conventional visual back with the Wegat.  I did that to under the (perhaps incorrect) assumption that use of a conventional visual back and diagonal would drive mirror spacing at focus to be well away from optimum.  BUUUUUUTTTTT.....that was just a guess or a prejudice. I have no idea of where the spacing is optimum. 

You'd think that design spacing would be one of those bits of data that would be engraved somewhere, or at least easily obtained online.  I know that production methods introduce small variations between individual units, but there's still a spec for the design that would be nice to know, if just as a starting point reference.



#11 cytan299

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Posted 13 September 2019 - 08:32 PM

Hi Chuck,

 

With my Wegatized ETX125, I’m sure the mirror is not at its optimum point because the measured f# is 16.5 rather than 15. But this is where I’m operating the scope and so the measurements for this mirror location is the most relevant to me.

 

cytan


Edited by cytan299, 13 September 2019 - 08:33 PM.

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#12 fcathell

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Posted 14 September 2019 - 03:02 PM

I think most of these small Maks tend to have a true FL slightly longer than their specifications. I'm not sure what effect it will have on the testing if any.

 

The Frank in Tucson.



#13 Chuck Hards

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Posted 15 September 2019 - 01:04 AM

I'm still testing it visually on the sky.  So far I'm not impressed but it could be the seeing, though I should be getting more from 125mm of aperture than this.  Planetary and lunar detail are going to mush over 90X .  It's hard to tell when you are focused, there's no "snap" when you go past it or nail it.  It's been cooling all afternoon in the shade.

 

 A side-by-side with my C5+ and/or 100mm ED would tell a lot.  I'm using the internal flip mirror so that definitely needs to be looked at; I need to borrow a visual back from another scope to try it with a known high-quality star diagonal.  I really want to evaluate it visually as far as I can before I look at it with the Ronchi, but may do that tomorrow anyway.  If I don't, I may not get the chance for another week.  



#14 Asbytec

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Posted 15 September 2019 - 03:10 AM

I'm still testing it visually on the sky.  So far I'm not impressed but it could be the seeing, though I should be getting more from 125mm of aperture than this.  Planetary and lunar detail are going to mush over 90X .  It's hard to tell when you are focused, there's no "snap" when you go past it or nail it.  It's been cooling all afternoon in the shade.

Yea, something is bad wrong. Need to rule out seeing, thermal stability, and collimation. You said it was cooling in the shade all afternoon, still I'd check for thermals just to be sure that was sufficient cooling to reach thermal equilibrium under whatever conditions existed that day and after dark. It might not be a thermal problem, but it's worth absolutely ruling it out as a potential source along with seeing and collimation. 



#15 Chuck Hards

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Posted 15 September 2019 - 10:27 PM

Collimation is dead-on with a laser and the one time I got a decent Airy disk it seemed good.  When the moon rose, it was tinted strongly yellow from area wildfires and I could see a high, thin cirrus in spots.  So the seeing was not good.  I waited it out until nearly 3 AM but it never improved in the eyepiece, despte looking quite decent to the naked-eye.  I'm confident the scope had well-cooled by then.

My wife was ill today so I didn't get a chance to test it on the bench.  A storm is moving in tomorrow night but should be cleared-out by next weekend.  I want to mount the ETX better so it's more stable at higher powers, I've just been using it on a large tripod & pan/tilt head.   Would the foundations of the universe be shaken if I mounted an ETX on a CGEM?  ;)



#16 Asbytec

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Posted 16 September 2019 - 06:05 AM

So, you got a good look at the Airy disc at least once. I wonder if the fires are causing problems with turbulent air rising and creating a local seeing condition. 



#17 Chuck Hards

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Posted 16 September 2019 - 08:59 AM

Heat off of local roofs and pavement is usually the problem in summer for me, especially with targets as low as Jupiter & Saturn have been, and a rising moon this past weekend.  I have only had two nights all summer where the seeing was crisp and good.  It's been bad this year, with images undulating and bloated for hours after sundown.  

 

My one quick look at the Airy disk showed a roughly identical pattern each side of focus, no sign of astigmatism at least.  Stay tuned.


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