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Brand new doublet needs collimation.

Collimation Optics Refractor Visual
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#1 HorseBadorties

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Posted 17 May 2024 - 06:58 PM

I just bought a new ED doublet refractor from a reputable dealer. I won't mention the telescope brand or the dealer name because I don't want to cast aspersions -- or encourage anyone else to. Let's just say that the vendor's advertising promises a well-collimated scope. 

 

For lack of a clear night, I did some indoor tests. First, I verified focuser alignment using a plastic 35mm film can with central peepholes, and a paper circle over the dew shield. Using the film can as an eyepiece, the peepholes lined up well with a small central hole in the paper circle. I got the same results when I rotated the focuser.

 

Next, I used a Cheshire eyepiece to test objective collimation. The photo shows a typical view. I have a very limited understanding of collimation, so I'm seeking some opinions:

  1. Relatively speaking, does this look well-collimated?
  2. How difficult -- for a newbie -- would it be to correct the collimation? The telescope does have the necessary adjustment screws.
  3. Would it be reasonable to ask the vendor to replace the telescope?

I'm looking forward to any thoughts you might have.

 

Thank you,

 

Bob

 

 

Attached Thumbnails

  • Cheshire_Refractor.jpg

Edited by HorseBadorties, 17 May 2024 - 07:02 PM.

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#2 BlueMoon

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Posted 17 May 2024 - 07:06 PM

Take it out under the stars and test it then make a judgement. Generally doing an Airy disk test will be more telling if you have problems. You can also examine the field of view for elongated or deformed stars which might indicate a pinched optics problem. As far as replacing, do the star tests first and if they bear out then you'll have a basis for your complaint. Cheers.


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#3 Scott in NC

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Posted 17 May 2024 - 07:24 PM

If it turns out to be miscollimated then don’t try to adjust a brand new scope yourself, especially if you don’t have any experience in this area. You’ll risk making things even worse. Any reputable vendor should accept a return for such an issue. The Cheshire results do look odd, but as BlueMoon said, don’t rush to judgment and definitely take it out under the stars first.


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#4 CHASLX200

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Posted 17 May 2024 - 07:26 PM

Send it back.


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#5 Eddgie

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Posted 17 May 2024 - 09:09 PM

I think it is better to use a laser to collimate because these ED doublets have pretty steep curves in the lenses and if the focuser is off even a small amount, it can cause issues.

 

Next, you have to establish if the primary lens actually has push/pull collimation screws. I have seen models that don't have it so even if it is out of collimation, you may not be able to fix it.

 

Last, what you might be seeing is lens de-centering. Once again, ED doublets have very steeply curved lenses and if the center of the curve on the two lenses are not in perfect alignment, you will see the same thing as you show in your image (and this can also be tilt). To adjust this (if it is adjustable), you would see three radial screws around the outside of the lens cell but to get at them on many scopes, you must remove the dew shield.

 

If there are collimation push/pull and radial screws, it is with the capablity of the owner to correct these errors, but I would say that if the scope is new and under warranty, don't make it your problem, but at the same time, if you can diagnose the problem, that would be useful to the community.

 

The symptom if decentering is that if you look at a bright star near zenith, if the lenses are not centered to one another, you will see a red fringe on one side of a star and a blue fringe on the other.  Tilt of the objective generally will not show this. The primary effect of tilt is to tilt the focal plane so that stars at one side will not be in focus at the same place as stars on the other side. This is not typically much of an issue for visual use, but can be a issue for imaging. Tilt is actually not that serious if it is a slower refractor but most ED refractors today seem to be f/7 and that might be a little fast. 

 

If the necessary adjustment screws are there, it is something that can be corrected with patience, and I would not hesitate to do it on a used scope, but on a new scope, don't make it your problem. 

 

If you do the decentering test or laser test though, you should share with the community what you find out. 

 

Hope it resolves out to your satisfaction.  

 

Please feel free to PM me if you have questions about any of this as I likely will not return to this thread.


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#6 betacygni

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Posted 17 May 2024 - 10:53 PM

The absolute test is of course under the stars, but yes, assuming nothing is out of the ordinary with your cheshire that definitely looks to be out of collimation. Even if you could in principle fix it yourself, don’t, buying new is partly to assure the scope arrives without issues. Don’t accept one that does have issues. More than likely the scope just took a big drop in transit and it wasn’t the vendors fault, but they still need to make it right.
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#7 HorseBadorties

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Posted 18 May 2024 - 01:16 PM

Thanks to everyone who responded. Your comments were quite informative. I'd have done a star test already, but the weather has not cooperated and probably won't for several days. So I've ordered a cheapish artificial star; we'll see what that reveals.

 

I don't mind doing more tests, as they will provide learning experiences. Trying to collimate the scope myself would also be instructive, but I'll save that lesson for when the stakes are lower. I've opened a support ticket with the vendor, and will probably insist on a return.


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#8 Orion68

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Posted 18 May 2024 - 04:40 PM

Your scope is out of collimation. A star test at high power will most likely confirm that. You might want to notify the seller while you are waiting for good conditions to do a star test. Let them know the results of the Cheshire test and that you will do a star test as soon as possible.

 

Let us know how the star test goes. Good luck.


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#9 Orion68

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Posted 18 May 2024 - 04:45 PM

Make sure your star test includes an in focus view at high power. Lower power views may not show the extent of the mis-collimation.


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#10 Heywood

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Posted 18 May 2024 - 11:04 PM

Thanks to everyone who responded. Your comments were quite informative. I'd have done a star test already, but the weather has not cooperated and probably won't for several days. So I've ordered a cheapish artificial star; we'll see what that reveals.

 

I don't mind doing more tests, as they will provide learning experiences. Trying to collimate the scope myself would also be instructive, but I'll save that lesson for when the stakes are lower. I've opened a support ticket with the vendor, and will probably insist on a return.

 

Return it, while you still have time.


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#11 davidc135

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Posted 19 May 2024 - 01:40 AM

The artificial star is a good idea. Placing it at 25m (if a 4'') shouldn't add too much back focus and the small amount of added SA won't interfere with collimation. 50x per inch magnification at focus and just either side will be revealing. Especially at focus.

 

Even at this distance seeing can still be poor.

 

Judging by my 4'' apo that's corrected for coma and has three pairs of push/pull screws for collimation, it takes a lot of tip and tilt of the objective to make any difference to the image. Which is one reason why some are supplied factory set and without means of adjustment.

 

Looking forward to your results.


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#12 davidc135

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Posted 19 May 2024 - 01:50 AM

It could be worthwhile checking the Newton's rings between surfaces 2 & 3 under a fluorescent or similar light. If they are un-centered there would be a problem with the spacing between the two lenses and the scope should be returned.

 

David


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#13 Skywatchr

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Posted 19 May 2024 - 11:52 AM

I just bought a new ED doublet refractor from a reputable dealer. I won't mention the telescope brand or the dealer name because I don't want to cast aspersions -- or encourage anyone else to. Let's just say that the vendor's advertising promises a well-collimated scope. 

 

For lack of a clear night, I did some indoor tests. First, I verified focuser alignment using a plastic 35mm film can with central peepholes, and a paper circle over the dew shield. Using the film can as an eyepiece, the peepholes lined up well with a small central hole in the paper circle. I got the same results when I rotated the focuser.

 

Next, I used a Cheshire eyepiece to test objective collimation. The photo shows a typical view. I have a very limited understanding of collimation, so I'm seeking some opinions:

  1. Relatively speaking, does this look well-collimated?
  2. How difficult -- for a newbie -- would it be to correct the collimation? The telescope does have the necessary adjustment screws.
  3. Would it be reasonable to ask the vendor to replace the telescope?

I'm looking forward to any thoughts you might have.

 

Thank you,

 

Bob

Using film canisters is way too imprecise.  Not only are they very difficult to insert squarely, and stay square, but the center holes of the pair could be way off from each other.  Way off can be even a fraction of a millimeter causing a "false positive", or negative in this case with a doubled image.

 

However as was mentioned, be SURE not to wait too long for a return window to close!  After that you just may be stuck with it, and need to get it repaired properly if you cannot do it yourself.

 

So yes, definitely get in touch with the vendor ASAP to open a "return window".


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#14 Oldfracguy

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Posted 19 May 2024 - 02:18 PM

I just bought a new ED doublet refractor from a reputable dealer. I won't mention the telescope brand or the dealer name because I don't want to cast aspersions -- or encourage anyone else to. Let's just say that the vendor's advertising promises a well-collimated scope. 

 

For lack of a clear night, I did some indoor tests. First, I verified focuser alignment using a plastic 35mm film can with central peepholes, and a paper circle over the dew shield. Using the film can as an eyepiece, the peepholes lined up well with a small central hole in the paper circle. I got the same results when I rotated the focuser.

 

Next, I used a Cheshire eyepiece to test objective collimation. The photo shows a typical view. I have a very limited understanding of collimation, so I'm seeking some opinions:

 

Thank you,

 

Bob

One thing to check is if your Cheshire is actually aligned properly.  I learned that the hard way.  I was using my Cheshire for Refractors as a quick check to on the Primary Mirror alignment on a Newtonian, and made a few adjustments.  Later that evening when doing a star test I could see there was somthing amiss.  So, I recollimated the scope using my other longer Cheshire, the one with crosshairs.  After that the star tests looked good, with the dark region centrally positioned within symmetrical, concentric diffraction rings.  I took the rubber eyecup and the top off the Cheshire for Refractors, and found that the shiny portion was not seated all the way down inside the outer body, but was cocked upward to one side.  I then just pushed it all the way in so that the top of the internal shiny portion was flat with the top of the outer black Cheshire body.  After that I checked the collimation of that Newt again using both Cheshires, and saw the same results finally.

 

One quick way to test if your Cheshire for Refractors (the one without the crosshairs) is properly aligned is to put that paper circle back on the front of the dew shield and use it instead of the 35mm film cannister home-made Collimation Cap.  If the hole in the paper circle is centered, then your Cheshire for Refractors is aligned correctly, and you can use it with confidence to perform a collimation check by observing the pair of reflected circles.

 

I also have one of those cool Hubble Artificial Stars.  I duct-tape in on top of the handle of a bicycle pump.  Raising and lowering the bicycle pump shaft and locking it in place with a large clamp-type paper clip is a lot easier than having to adjust the tripod legs of a mount a fraction of an inch at a time to get the scope aligned with the artificial star located some distance away.


Edited by Oldfracguy, 19 May 2024 - 02:26 PM.

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#15 Alan French

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Posted 19 May 2024 - 05:10 PM

 I find the seeing can be excellent across a lawn on an overcast day after a good rain. 

 

Clear skies, Alan



#16 HorseBadorties

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Posted 19 May 2024 - 05:10 PM

Using film canisters is way too imprecise.  Not only are they very difficult to insert squarely, and stay square, but the center holes of the pair could be way off from each other.  

Yeah, I couldn't get the film can seated all the way. So I'm not too sure about that result. However...

 

One quick way to test if your Cheshire for Refractors (the one without the crosshairs) is properly aligned is to put that paper circle back on the front of the dew shield and use it instead of the 35mm film cannister home-made Collimation Cap.  If the hole in the paper circle is centered, then your Cheshire for Refractors is aligned correctly, and you can use it with confidence to perform a collimation check by observing the pair of reflected circles.

 

I also have one of those cool Hubble Artificial Stars.  I duct-tape in on top of the handle of a bicycle pump.  Raising and lowering the bicycle pump shaft and locking it in place with a large clamp-type paper clip is a lot easier than having to adjust the tripod legs of a mount a fraction of an inch at a time to get the scope aligned with the artificial star located some distance away.

Yep, the hole in the paper is centered in the Cheshire, and stays there when I rotate the Cheshire -- and/or the focuser -- 90 degrees.

 

The bicycle pump is a good idea. I'll try it if the artificial star arrives in time. Meanwhile tonight, my local sky is expected to be partly clear*, so I'll try for an honest-to-goodness star test with Vega or Spica.

 

* "partly clear" sounds better than "partly cloudy."


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