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Unknown 2" Lens - Seeking Info

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#1 Marker 24

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Posted 22 May 2019 - 06:57 AM

Seeking Info.  I have this lens.  It is completely unbranded, although it appears well made.  The lens is a doublet.  It is convex on the end with the male threads and slightly concave on the end with the female threads.  The male threads screw into standard 2" eyepieces.  The female end has threads for standard 2" filters.  The lens appears to do nothing, i.e. perhaps slight but no significant magnification, edge correction, etc.  Could it be a coma corrector, or a field flattener, or ???  Appreciate comments.  Mike.    

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#2 gr5org

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Posted 22 May 2019 - 08:47 AM

What's the focal length?



#3 ccaissie

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Posted 22 May 2019 - 09:35 AM

looks a very long focus unit or maybe a coma/field corrector.  If the view through it shows a distortion around the edge, it's a corrector of some sort.



#4 Chuck Hards

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Posted 22 May 2019 - 09:46 AM

Looks like an old Lumicon coma corrector.  I believe they were optimized for f/4.5.



#5 Marker 24

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Posted 23 May 2019 - 07:47 AM

Response to Q.  It doesn't seem to have a Focal Length.  Light passes through more or less as if it were a piece of plate glass.  Also, no obvious distortion at the edges.  Some very slight "magnifying glass" effect when used with the convex lens up.  None noticed with the concave lens up.  So, if it is some sort of Coma Corrector, how do I evaluate it?  I do have an F5 Dobsonian to work with. Thanks for the continuing interest and info.



#6 Chuck Hards

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Posted 23 May 2019 - 08:20 AM

I actually have a Lumicon corrector, buried somewhere, that I bought from Jack Marling himself decades ago.  I'll try and find it and shoot a pic for comparison.  

 

You can evaluate it visually with a 2" eyepiece, if you space it correctly in relation to the scope's focal plane, even though it was designed for photography.  I believe the spacing has to be correct for it to work as advertised.  



#7 gr5org

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Posted 23 May 2019 - 09:11 AM

If it's a coma corrector, wouldn't it have some kind of label?

 

If it's the objective of a cheap (or expensive) telescope then no label is normal as it's only part of the product.

 

Try going outside under direct sun and stand over some pavement or asphalt or other flat surface and aim the axis of the lens towards the sun and move it from 0 feet to 6 feet from the ground and see if it focuses to a very bright dot at some point.  Note the distance from lens to ground.



#8 Chuck Hards

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Posted 23 May 2019 - 09:23 AM

The Lumicon coma-corrector did not come with any stickers or engraving.  Got mine with a 2" Easy-Guider.  Still have that somewhere, as well.



#9 Marker 24

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Posted 24 May 2019 - 07:55 AM

Additional:  OK, so I took the advice that gr5org passed along.  When held some five or six feet from a wall, the lens projects a circle of about 5" diameter with what appears to be a fairly sharp edge - kind of like looking at an image of the sun through a scope with a filter.  Moving the lens farther away causes the edge of the projected image to become fuzzy but not much larger if any.  As I move the lens closer to the wall, the image diminishes in size.  However, it never comes to a point.  This is a bit hard to explain/visualize, but the projected circle slowly shrinks to the diameter of the lens as the lens reaches the wall.  Does the same thing no matter which way I hold the lens - concave up or convex up.  Based on Chuck's comments, I'm going to do some reading up on how to correctly use a coma corrector (was that a pun??) and see if it produces some sort of useful effect.  Appreciate the thoughts and will gladly work with additional ideas.     



#10 Marker 24

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Posted 24 May 2019 - 08:30 AM

I think the combination of your comments and a bit of research has broken the code.  I did a Google search, and came up with this from Excelsis.  It's likely a Lumicon Coma Corrector/Field Flattener.  It is described in part as being "a-focal", i.e. "zero power" and an air-spaced doublet.  So far, right on.  Excelsis goes on to say that it is optimized when the centered of the last optical element is 3.8" from the film plate.  Not being an astrophotographer, I'm not sure what that means, but I'm equally sure it would be clear to the right people.  Excelsis also makes it clear that although the unit may provide some visual advantage (coma correction) in fast Newts, the item is definitely built with astrophotography in mind.  It also points out that when screwed into the Lumicon 2" Newtonian Easy-Guider, the distance is automatically correct.  So, if you look at the photo I downloaded from the Internet, it appears I have the lower element that screws into the Easy Guider.  It all seems to make sense.    

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#11 Chuck Hards

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Posted 25 May 2019 - 05:32 PM

I actually found my own Lumicon Coma-corrector.  I'm fairly confident it's what you have.   I was pretty sure when I saw your pic.  Designed for f/4.5 Newts of the day.

 

I have the Easy-Guider as well.

 

Mine:

 

Coma corrector 001.jpg   Coma corrector 002.jpg  


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#12 Chuck Hards

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Posted 25 May 2019 - 05:41 PM

And here it is in place on the Lumicon Easy-Guider.  That T-ring is probably for my old Pentax K-1000 SLR.

 

Easy Guider with Coma Corrector 001.jpg

 

 




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