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Refractor double image.

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#1 RobertMaples

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Posted 16 September 2019 - 09:38 PM

I bought this 80mm, 720 fl refractor telescope the other day and images crap out before I even get to 100x with it it.  I started out on Jupiter and it's so blurry you can't make out any cloud bands.  When I moved to Saturn, I realized that it was actually creating a double image (Saturn's small enough that the double images were next to each other, whereas with Jupiter they were overlapping).  Could this be a collimation issue?  Could it be something else fixable or is it just crappy lenses.  Even though "Discover with Dr. Cool" is obviously geared toward kids, it seems like a well made scope.  It's not the lens or diagonal because I've tried swapping them out with no luck.  Unfortunately, the lens cell is not collimatable, but if it might help I'm sure I can come up with a fix, I just don't want to bother if there's no hope for this telescope.  I got the scope cheap from Goodwill and really bought it for the mount - as far as I can tell the mount is the same as an Orion VersaGo E-Series mount, so it's not a big lose if there's no hope, but I'd like to salvage it if I can.



#2 Don Taylor

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Posted 16 September 2019 - 10:07 PM

This is a guess - but probably someone had the objective out of the scope and re-assembled it incorrectly. I'll try to find some artwork here on CN. This has come up many times in the Classics forum. Usually the images are poor - but its possible to get a double image if the two elements are grossly misaligned with each other.   


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#3 Don Taylor

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Posted 16 September 2019 - 10:19 PM

Look at post #11 in this thread:  https://www.cloudyni...ns-orientation/

 

Also - there should be a thin ring separating the two lenses or more often - 3 little pieces of what looks like aluminum foil (rectangles) to hold the two lenses apart.



#4 RobertMaples

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Posted 17 September 2019 - 10:53 AM

Thanks for the link.  I don't think anybody ever disassembled the objective, in fact, I'm pretty sure the scope had never even been out of the box, but it's entirely possible it was assembled wrong in the first place.  I'll check it out this evening.  If it checks out OK anything else to try?



#5 Eddgie

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Posted 17 September 2019 - 11:08 AM

Well, I would say that if you can return it, do so.

 

As for what the problem might be, don't dismiss reflections of the planet off of your eye.  Many of these cheap telescopes will be shipped with lenses that are uncoated and you can get reflections off of your eyeball.  Try a lower power eyepiece to see if the condition is present.

 

If it is still present with the lower power eyepiece, try removing the diagonal and putting the eyepiece into the focuser.

 

Also, check for focuser slump. With the diagonal removed, rack the focuser out all the way, then look through the end at the objective, and see if the objective appears to be centered in the focuser tube.  If not, the focuser is out of collimation.  Again, if that is the case, return it. 

 

And last but not least, 100x is a lot of power for an 80mm scope.  You read that people use 50x per inch of aperture, but that is of questionable benefit and a lot depends on the observer and their contrast sensitivity threshold, which can very considerably from one observer to the next.



#6 Redbetter

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Posted 17 September 2019 - 03:10 PM

It is a Goodwill purchase so I doubt return is an option.

 

Let's back up a little bit since it is believed the scope might not ever have been out of the box. 

  • First, remove the dew shield and check the retainer ring.  While this doesn't sound like pinch (very tight retainer ring producing a triangular pattern) it could be that the ring is loose and tilted, or tight but tilted/decentered if it was wedged somewhat during assembly. 
  • With the ring loosened a little and the OTA pointed straight up, tap the tube and check for some sound of rattle when tapped.  Rattle at this point is good.
  • Now tap it a few times while still pointed straight up to let the glass settle into place. 
  • Tighten it until slightly snug (enough that the ring won't vibrate loose.)  It shouldn't rattle when you are done.  This is counter to what most people say about leaving it loose enough that it will rattle when shaken, but my experience is that a little snug doesn't pinch, while leaving some rattle results in the objective lenses shifting relative to one another and eventually causing severe miscollimation/decenter type problems.

When done, turn it to a bright, well placed star (preferably over 35 degrees elevation) and see if the airy disk pattern is uniform.  You can do a more rigorous star test, but just checking the airy disk at best focus will tell you if something is seriously amiss as you described earlier.



#7 Redbetter

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Posted 18 September 2019 - 01:50 AM

I wouldn't say that 100x is a lot of power for an 80mm f/9 assuming the seeing is not too poor.  As a test with an 80 f/5 achro I turned to Saturn tonight at 26 deg elevation over the neighbor's roof.  Seeing was poor enough that I could see rapid undulation in the image of the planet.  Cassini's gap was detected faintly at 80 and 100x.  Above that was too much for the scope and conditions for my eye, and I am not one to use excessive magnification for planetary or lunar observing.  It was not something that jumped out, and had that averted vision effect to it on both sides of the planet.  It was seen only a little better at moments when the seeing was a little more stable.



#8 RobertMaples

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Posted 18 September 2019 - 08:22 AM

100x shouldn't be too much, I've got a 70mm/400fl refractor (Explore Scientific Sun Catcher with solar filter removed) that'll give sharp views of Jupiter with easily detectable cloud bands at 100x.

 

Again, I bought it for the mount, so even if I could return it I wouldn't, though I doubt I could with it being a Goodwill purchase.  I really didn't expect much from the scope, but I was surprised when I got it that it just seemed like a quality product, so that got my hopes up for some good views.  The problem's not the eyepiece or diagonal, because I've used them on other scopes and they are fine.  There is virtually no slop in the focuser (one of the reason's it seemed like a quality product - the focuser works much better than the one in my Sun Catcher or my Orion Starblast 4.5) so that shouldn't be an issue.  I've checked the lenses and they seem to be installed correctly.  I'm really starting to think it's just bad optics.

 

Something else I've noticed is as the image moves across the field of view it will go more in or out of focus and the double image seems to come and go.  I tried a star test and it seemed pretty good except there seem to be a dark line across the image on one side of focus.



#9 spencerj

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Posted 18 September 2019 - 08:31 AM

That does sound like an internal reflection.  Maybe even something protruding into the light path.  What about stars?  Do you see the double image on magnitude 2-3 stars?



#10 SeattleScott

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Posted 18 September 2019 - 11:33 AM

Very strange, I guess now we know why it ended up at Goodwill. Maybe you can swap the focuser and use it on your 70mm?

Scott


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