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Need help collimating Explore Scientific Mak-Newt

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#1 sneutrino

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Posted 10 July 2012 - 06:37 PM

Trying to collimate the explore scientific 152 f/4.8 mak-newt that I just received today from Astronomics.

I'm certainly not pleased that it was not collimated better out of the box.

At first I assumed the secondary was properly positioned in the factory so I threw in my Orion laser collimator and went to check the tilt of the secondary, but I found that I didn't have enough freedom to tilt it enough to get the laser into the primary mirror's center spot.

I called ES and he said I need to use a sight tube on it, after the phone call I realized my collimation cap should do just fine for now.

I took out the laser and replaced it with one of Orion's collimation caps, and you can see what it looks like in the attached picture taken by my camera phone looking down the focuser tube.

Is this a case of the secondary being improperly vertically positioned? I'm surprised it's so far off. I tried to adjust it but no luck so far since the online pdf instructions are different from how the actual scope is built. They may have been written for an ealier model or protoype.

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#2 TG

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Posted 10 July 2012 - 07:39 PM

You still need the laser. The sight tube is step #1. Rotate and adjust the secondary till it shows a centered profile in the "field" of the sight tube. Normally, the MNs don't have a vertical adjustment for the secondary but if you tighten/loosen the three screws by the same amount, it will move up or down and usually only a small amount is needed. Rotation for my Intes M-N is harder to achieve and requires loosening the retaining ring of the secondary (which itself has a tiny hex set-screw in it). If the Chinese ES scope is a clone of the Intes ones, you should have something similar. At this point, it may still look elliptical but it should be symmetric in the sight tube. After this, you insert the laser and adjust tilt till you hit the center but before you proceed, iterate on the secondary-in-sight-tube again till both are perfect. When this is done, the secondary should look circular in the sight tube and equidistant from the tube walls. Only then start adjusting the primary.

IOW, this is just like adjusting a Newt except in ONE crucial detail: in a Newt, the aperture stop is at the MIRROR. In a Mak-Newt, it is at the CORRECTOR. This means that the Newt can tolerate misalignment between tube and optics (usually requiring an adjustment of focuser tilt) but in a Mak-Newt, the corrector, tube and focuser better be square from the beginning or you will start blubbering from frustration soon :lol:

If you have Suiter's Star Testing book, he's got a good explanation for the principles and method of collimating Newts as well as a method for making sure your focuser is square to the tube.

Finally I should say that for your short focal ratio, the secondary offset may become important for even illumination of the field. However, I've never understood this properly as it's not critical for my needs (my scope's an f/6 and has a small secondary anyway). If you have a concern here, talk to the reflector guys especially starman1.

Regards,

Tanveer.

#3 sneutrino

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Posted 10 July 2012 - 08:15 PM

It seems there is nowhere near enough movement possible to center the secondary in the focus tube... as if there is only 1 or 2 mm of play in the vertical position?

#4 sneutrino

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Posted 10 July 2012 - 09:50 PM

Two things tell me the secondary mirror is too far from the corrector plate:
1) that's how it looks through the collimation cap.
2) if the secondary was too far from the corrector, it should pull the laser spot on the primary toward the focuser, which is exactly what I see. There is not enough freedom in the secondary tilt to get the laser spot to the center of the primary.

I will have to consult with ES and maybe send it back.

#5 spencerj

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Posted 11 July 2012 - 09:30 AM

On my MN66, I can slide the mounting plate for the focuser forward or back to center the secondary. Does the focuser or mounting plate on the ES allow for any adjustment?

#6 bykhed

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Posted 11 July 2012 - 03:18 PM

Google "barlowed laser collimation" and also find some of the great docs for mak-newt collimation on the web. I found that barlowed laser collimation gave me the best results with a mak-newt.

-Matt

#7 sneutrino

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Posted 11 July 2012 - 03:48 PM

I will use a barlow with the laser when adjusting the primary, but I don't think it can help with the tilt and position of the secondary.

Something I don't understand is why the laser spot on the primary cannot get within 1 cm of the center spot, when the vertical position of the secondary may only be off by a few mm. A ray trace sketch suggests to me that the 45degree angle of the 2ndary would result in Xmm offset of the laser on the primary whenever the 2ndary vertical position is offset by Xmm.

#8 TG

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Posted 11 July 2012 - 05:01 PM

I will use a barlow with the laser when adjusting the primary, but I don't think it can help with the tilt and position of the secondary.

Something I don't understand is why the laser spot on the primary cannot get within 1 cm of the center spot, when the vertical position of the secondary may only be off by a few mm. A ray trace sketch suggests to me that the 45degree angle of the 2ndary would result in Xmm offset of the laser on the primary whenever the 2ndary vertical position is offset by Xmm.


You're correct, a barlowed laser will help only in collimation of the primary. However, I never had success with it on my Mak-Newt due to (1) my mirror having a central plug, and (2) the return of the laser not being visible from the front. Due to the corrector there's no way to use a small hand-mirror do view the return as you would do for an ordinary Newt. However, doing the usual collimation with the sight tube resulted in perfect collimation.

I found Explore Scientific customer service excellent. What have they to say about it?

Tanveer.

#9 sneutrino

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Posted 11 July 2012 - 08:19 PM

ES said that they want this to be confirmed with a sight tube and basically the collimation cap is not good enough to confirm anything. I ordered the celestron Cheshire Collimating Eyepiece which will be here Friday with a little luck.

He mentioned the focusing draw tube can be adjusted. There are a couple of hex screws I loosened which allow me to move the draw tube to center the secondary, and get the laser into the primary center doughnut, but so far can't figure out how to lock it down.

#10 Patrick

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Posted 11 July 2012 - 08:43 PM

Hi Neutrino,

I have the ES Comet Hunter also. You basically collimate it the same way you would a standard Newtonian. I wouldn't be too surprised if the scope arrived out of collimation simply because of the shipping and rough handling it may have gone through to get to you. It's not something that Explore Scientific has any control over.



Is this a case of the secondary being improperly vertically positioned?



You should be able to eyeball the vertical position but sometimes it's difficult to get the light just right so you can actually see the edges of the secondary mirror. From you picture it looks like the secondary is not centered in the draw tube. To move the secondary towards the corrector plate, loosen the three adjustment screws "a little" and tighten the large center screw. Keep doing this until the secondary is centered in the drawtube. It may not take as much as you think. Then work with the laser to adjust the tilt of the secondary, and then thirdly, the tilt of the primary.

I reworked my collimation tonight just to see how much travel I have within the secondary itself. In order for me to get the tilt correct I had to loosen two of the thumbscrews completely and tighten one of them pretty tightly. I eventually got the laser (I'm using a Hotech 2" laser) in the center of the donut and snugged up the other two screws lightly. After that I centered the primary with the laser.

Patrick

#11 sneutrino

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Posted 11 July 2012 - 09:23 PM

Hi Patrick,

Thanks a lot for your description and inspiration. I got the thing collimated.

The secondary has a big center screw which I believe is just the 'rotation lock screw' which puts the 3 plastic rotation locking pins in place. However, my 2ndary seems to be able to rotate whether or not those are locked unfortunately ... At some point the design of this system changed so yours might be different.

Anyway, behind the big center screw (which can be removed) is a small hex screw, and on mine, I turned it counter-clockwise all the way until it hits a hard stop, this allows rotation of the 3 thumbscrews adjustment screws to bring the 2ndary as close to the corrector plate as possible. In previous attempts I may not have gotten the center hex rotated all the way due to the 3 screws preventing that?

Then, like yours, two of the thumb screws had to be loose and the 3rd pretty tight to get the laser in the donut.

Then I could collimate the primary as usual.

If only I could make some small adjustments to the draw tube direction, I could probably make it so my 2ndary adjustments are not so maxxed out.

It is cloudy last night and tonight, a very unusual situation for these last few months here in socal... so first light will have to wait.

#12 Patrick

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Posted 11 July 2012 - 10:12 PM

behind the big center screw (which can be removed) is a small hex screw



I was not aware of the small hex screw behind the big center screw...so I suspect our scopes are the same. I was trying to be careful not to drop the secondary mirror onto the primary...something that can happen with a typical Newtonian secondary, so I didn't think of removing the center screw.

Glad you got the scope collimated though! I hope you like it! I've enjoyed mine.

Patrick

#13 rtanton

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Posted 12 July 2012 - 12:25 PM

This image may help:

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#14 rtanton

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Posted 12 July 2012 - 12:28 PM

Images of a collimated Comet Hunter:

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#15 rtanton

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Posted 12 July 2012 - 12:39 PM

more images:

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#16 rtanton

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Posted 12 July 2012 - 12:42 PM

another image:

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#17 Patrick

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Posted 12 July 2012 - 12:48 PM

Good information Russ...thanks for posting.

Question:

I don't understand about the 'recent update' statement. What is the secondary center pivot point? Does it refer to being able to adjust the secondary mirror so it's perpendicular with the focuser draw tube?

Apparently I've been collimating the secondary backwards because I've been loosening the center screw to collimate, and then tightening it when done.

Patrick

#18 rtanton

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Posted 12 July 2012 - 12:52 PM

collimation procedure:

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#19 Patrick

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Posted 12 July 2012 - 03:38 PM

Russ,

These descriptions need to ship with every Comet Hunter! The manual I have is out of date.

Patrick

#20 sneutrino

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Posted 12 July 2012 - 08:37 PM

Is the draw tube direction adjustable?

#21 rtanton

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Posted 13 July 2012 - 10:49 AM

Simple answer - yes, but it is HIGHLY unlikely that you will need to do so. By differential adjustment of the 2mm set screws the draw tube can be tilted (see image below). I have never found this necessary with the Comet Hunter. It is more likely that tension adjustment is required with heavier loads (cameras, CCDs, etc.). Refractors, due to their much longer draw tubes (attached image), sometimes require differential adjustment. If you have questions, please give me a call on the phone number sent by PM.

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#22 Hothersale

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Posted 10 May 2013 - 05:57 PM

Got my Comet Hunter today. Collimation out of the box is actually pretty good, but my sight tube tells me the secondary should be pulled up closer to the corrector to be properly centered under the focuser. I know there is a hex screw located under the secondary's center slotted screw. I am GUESSING that I should loosen all three collimation knobs and then tighten this center hex screw to pull the secondary closer to the corrector. Anybody know for sure before I start messing around? 8^)

#23 Hothersale

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Posted 10 May 2013 - 09:17 PM

Belay that request. It was user error. My sight tube wasn't inserted far enough. With it pushed in all the way and my eye jammed up close, the secondary position looks perfect. I did have to tweak the angle of the focuser slightly, but that was easy enough. (FYI, the center hex screw does NOT appear to change the stalk length, it only rotates the secondary.) Now I have to decide if I'm brave enough to remove the primary mirror cell to check the center spot accuracy. Not much else to do since the clouds are just rolling in....

Oh, and a big thanks to Russ Tanton who responded to my PM right away and even gave me his cell number. Now that's customer service!

#24 Fred1

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Posted 10 May 2013 - 09:59 PM

Removing the primary is simple. Just unscrew the 1/2 dozen or so hex head screws surrounding the back of the Comet Hunter. The mirror comes out quite easily and holds well inside its cell.

#25 myotherforumname

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Posted 07 November 2013 - 01:28 PM

I am not sure how old this thread is but the last comment has pushed me from lurking.
I have been trying to collimate my Comet Hunter today for the first time. From which I have found that the locking ring on the secondary is loose and that none of my Metric or Imperial hex keys fit!! Result: Rotating Secondary.
Moving onto the primary. I wanted to remove the second finder base bracket to another scope to use the excellent finder, so i try to remove the primary.
All but one of the hex head screws come off fine, the last is worn to shapeless and wont come loose for love nor money!
The words of the downloaded manual come to mind, "The comet hunter uses standard metric hex wrenches and a flat-head screwdriver to make MOST of the adjustments..." At other times try imperial or somewhere in between?
I am counting 6 different tools needed to collimate this thing. and it is sitting next to a scope 1/3 of the price that can be collimated with an opposable digit!
Not happy!!
Ok, rant over....






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