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Aperture ring to sharpen star edges

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7 replies to this topic

#1 dron2015

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Posted 27 June 2020 - 05:08 PM

Fellows,

 

I like my refractor very much.  However, some bright stars have soft edges.

 

Having a closer look at the objective, I noticed two things

 

1. Small metal spacers inside the lens like 1 cm long and 3mm thick - a kind of 120 deg separated

2. (I bought my scope used) - some traces of something on the ID of the objective - meaning most probably here was some sort of aperture stop ring that covered these spacers.

 

So I am thinking now, is it really good idea to make and install some aperture ring - like 4-5mm (I know that decreases the aperture) hoping that it will help with star edges?

 

who can print such aperture ring for me? 6 inch refractor.

 

 

Thank you!

 

Best,

Andrey



#2 Garyth64

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Posted 27 June 2020 - 06:10 PM

The 3 small pieces of metal are there for a reason.  To keep the crown from touching the flint and to make the correct spacing between those elements.

They really do not interfere with anything. 

I am sure that there wasn't any aperture ring missing, nor should you install one.  Just leave it as is.

There are many lenses out there that were made that way.


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#3 John Huntley

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Posted 27 June 2020 - 07:32 PM

All my refractors have those foil spacers including the triplet.

 

I don't think they will be the cause of your soft stars.


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#4 Rutilus

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Posted 28 June 2020 - 06:39 AM

What type of 6 inch refractor is it?  If it is a 6" f/8 achromat, then some of those have been known to have a turned

down edge, and one reason why it might appear to have had a aperture mask fitted at some point.

Or simply someone made one to cover the spacers. I once made a 140mm mask for my 6" achromat,

to see if it made a difference to the star test. In fact, I used the scope for a couple of years at 140mm aperture.  


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#5 gwlee

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Posted 29 June 2020 - 05:33 PM

Fellows,

 

I like my refractor very much.  However, some bright stars have soft edges.

 

Having a closer look at the objective, I noticed two things

 

1. Small metal spacers inside the lens like 1 cm long and 3mm thick - a kind of 120 deg separated

2. (I bought my scope used) - some traces of something on the ID of the objective - meaning most probably here was some sort of aperture stop ring that covered these spacers.

 

So I am thinking now, is it really good idea to make and install some aperture ring - like 4-5mm (I know that decreases the aperture) hoping that it will help with star edges?

 

who can print such aperture ring for me? 6 inch refractor.

 

 

Thank you!

 

Best,

Andrey

I suggest first making a simple ring from paper or cardboard and attaching to the scope with scotch tape to see whether it helps. I doubt the spacers have anything to do with soft stars you described. 


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#6 BillP

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Posted 29 June 2020 - 05:46 PM

I like my refractor very much.  However, some bright stars have soft edges.

Can you please explain what you mean by soft edges??? 

 

When magnification is too low to achieve an airy disk pattern, the star point will not have a sharp edge of any kind as it will just be a pinprick of gleaming light.  Now if your scope is not thermally acclimated that star can look fuzzy, I call that a wooly star.  If seeing is not stable then the pinprick can be globulous and in motion.  Once you pump up the magnification to achieve the airy disk, if the little ball in the center of the patter, the spurious disk, does not have a sharp edge well that can be seeing and the scope not being in thermal stability yet too.

 

Anyway, would appreciate it if you could give a more detailed description of what you are seeing, how the star appears, what magnification you are using so can better understand exactly how the edge, and what edge, is soft. And if that softness is uniform all around like it is out of focus, or if there are spikes, is it white on the edges or some color, does it look like a milky region around the star point, etc.


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#7 dron2015

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Posted 29 June 2020 - 05:48 PM

Thanks much, fellows, for you advice - I appreciate them very much!

 

it is APO triplet 1200 F8 - about 12 YO.

 

The reason I started to think about that  is the pictures of some scopes.

 

E.G.

https://www.cloudyni...-1574665832.jpg

https://www.cloudyni...-1583001726.jpg

 

Best,

Andrey



#8 daquad

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Posted 29 June 2020 - 06:34 PM

OK that picture of the Starwave is my scope.  It does not have the spacers, but that is not why the it produces textbook Airy discs.  I once owned a Jaegers 6" f/5 that had 6 lens spacers and produced perfect Airy discs.  Answer Bill P's questions first and then try to determine the quality of your scope.  BTW make sure the seeing is very good when you try to evaluate your scope.

 

Dom Q.


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