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6" F/5 Objective (from a Cheshire?)

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#26 brian dewelles

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Posted 05 February 2021 - 06:43 PM

So i put this almost all jaegers 6 inch f-5 together several years ago. I got lens and cell in tucson, it has six spacers i think its a nice doublet. The tube and dew shield are left over pieces from a previous project and are jaegers as is the focuser. Not sure on GEM but i think it fits scope well. These are wonderful scopes that shine when doing widefield low-medium power observing. You will love yours when you put it together. They are so much fun to use.

 

I also have a cheshire 5 inch f-15 refractor and i always believed the objective and focuser was from jaegers. I have never seen anything to document the belief but it too is a fine achromat. Ive had the scope for maybe 9 years. The only draw back with their build in my opinion was an incredibly heavy tube, must be close to fifty pounds!

 

 

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#27 CltFlyboy

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Posted 05 February 2021 - 07:26 PM

Brian - that is incredible and very beautiful! Thanks for sharing that, it just adds to my desire to get this lens mounted and observing. 

 

Y'all are setting a pretty high bar for me. I hope that I am up to the challenge... 



#28 Jeff B

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Posted 05 February 2021 - 10:23 PM

 When you get straight lines in DPAC when testing an objective that means that the spherical aberration is well corrected. With fast lens like 5" f/5 or 6" f/5  and being an achromat it  still has a huge amount of chromatic aberration that is worse then 1/2 wave. So the POLYchromatic wavefront is still around 1/2 wave or worse.  With a lens ones has to  take into account both spherical aberration and chromatic aberration  to determine the total performance.   A singlet lens can test perfect in monochrome light in DPAC but as we know has a huge amount of chromatic aberration. So just keep that in mind. These larger aperture fast achromats do make for nice Richest Field scope when used at low power. 

 

                                - Dave 

This is why I made a 4" aperture stop for my 6" F5 Jeagers (yours Henry, looks a whole lot like a Jaegers, crinkle finish dew cap and all).  That really cleaned up the longitudinal color differential a lot giving notably sharper images at high power (very sharp actually) despite the reduced aperture and remaining residual color differential.  Mine had a nice figure in green at full aperture in DPAC, a little over corrected overall, good edge and yes, spherochromatism in red and blue.  Yellow was the best correction, almost neutral.  Again, stopping it down to 4" aperture cleaned up the lens to basically neutral in green/yellow but the real story was the big reduction in red and blue sphero.  My sample also showed no on-axis astigmatism or coma.

 

BTW Henry, the lens is, I believe,  if a Jaegers, a Fraunhofer aplanat design, being free of spherical and coma aberrations at one wave length, typically green.   

 

Enjoy!

 

Jeff 


Edited by Jeff B, 05 February 2021 - 10:25 PM.

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#29 CltFlyboy

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Posted 06 February 2021 - 09:17 AM

Thanks for the info, Jeff. I'm certain this is a Jaegers now - there are too many similarities with know examples and the fact that it was in a complete OTA that HAD to be a Cheshire pretty much seals that up. It'll be interesting to see for myself the amount/type of CA I get out of this. For the light bucket visual observing I'm targeting (instead of bright objects) I don't think it will be very noticeable. After all is said and done I might do some testing just to see how good it performs across RGB.


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#30 DAVIDG

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Posted 06 February 2021 - 03:54 PM

 These lens were designed for low power work. At low power the chromatic aberration won't  be very noticeable visually. It is only when you start getting to medium and high power will you see it . The Moon  Venus and bright stars will show a blue halo don't but most won't mind. If you use it for imaging as pointed out the  CCD  will record the halo  since it is sensitive  to these wavelength.

    Just remember  when doing low power viewing , you want the magnification to be such that you have a 7mm exit pupil and not  larger to get the brightest image  That would be an eyepiece that gives around 22x  for 150mm aperture and since you have 30" focal length that would be a 35mm eyepiece. 

 

                  - Dave  


Edited by DAVIDG, 07 February 2021 - 10:29 AM.

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#31 Terra Nova

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Posted 06 February 2021 - 07:31 PM

 These lens were designed for low power work. At low power the chromatic aberration won't  be very noticeable visually. It is only when you start getting to medium and high power will you see it . The Moon  Venus and bright stars will show a blue halo don't but they won't mind. If you use it for imaging as pointed out the  CCD  will record the halo  since it is sensitive  to these wavelength.

    Just remember  when doing low power viewing , you want the magnification to be such that you have a 7mm exit pupil and not  larger to get the brightest image  That would be an eyepiece that gives around 22x  for 150mm aperture and since you have 30" focal length that would be a 35mm eyepiece. 

 

                  - Dave  

And with any F5 instrument regardless of aperture (or focal length) as long as the focal ratio is F5, a 35mm eyepiece will give you a 7mm exit pupil.

 

F x ep = fl        And        fl/F = ep

 

Where:

ep is exit pupil diameter in mm

fl is eyepiece focal length in mm

F = the ‘optical speed’ or Focal ratio of the objective


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#32 steve t

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Posted 07 February 2021 - 10:54 AM

In case anyone is interested, here is that 6" F/5 glass that was on eBay a few weeks ago. I ended up getting it after the first buyer backed away, and after seeing it, I am dang happy that I did get it. This is going to make a heck of a nice yard cannon! Now I have to source a good tube and focuser...

I'm looking forward to your first light reportwaytogo.gif

 

I remember back in 1975 or 76 there was a school observatory in North Little Rock, Arkansas that had a 6" F/5 Jaegers mounted piggy back on a 5" Unitron. The wide field views through that 6" were amazing.


Edited by steve t, 07 February 2021 - 10:57 AM.

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#33 DAVIDG

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Posted 07 February 2021 - 11:04 AM

 The 6" f/5 Jaegers is most likely a Fraunhofer design which is anastigmat  design. That means it is corrected for off axis coma and astigmatism. So you get sharp star images off axis BUT again you have a large amount of chromatic aberration. In a newtonian you have perfect color correction but off axis coma. You can correct the coma with a coma corrector or an eyepiece designed to correct for it.  So it is trade off of optical errors that one system has vs the other .

    You also have to consider the design of the eyepiece used with any fast systems since the off axis errors in them can be as large or large then the main optics.  In the past these fast lenses  were used with WWII surplus Erfles that Jaegers also sold. I have a couple of these along with a two 5" f/5 Jaegers lens.  If you know what to look for you can see the off axis errors in the eyepiece and the chromatic aberration from the lens. In reality thou the wide angle view overshadows any of these optical issue and you just enjoy the view. 

  In the Summer you can get some specular views the North American Nebula or see both M8 and M20 in the same field of view and in the Winter all of the Sword of Orion and Pleiades with the nebula around the stars.

 

                    - Dave  


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#34 starman876

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Posted 08 February 2021 - 12:16 PM

I found the tube I was thinking about and it is 17" long and 6" in diameter.  Looks like it was machined from a single piece of aluminum.  I think it may be to short for your application unless you put an extension on it.  I think the original tube for the cell you have was 22".


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#35 CltFlyboy

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Posted 08 February 2021 - 12:45 PM

I found the tube I was thinking about and it is 17" long and 6" in diameter.  Looks like it was machined from a single piece of aluminum.  I think it may be to short for your application unless you put an extension on it.  I think the original tube for the cell you have was 22".

Well poop. Thanks anyway. IIRC the seller had the original tube listed, she measured it at 20.5" or 21". That's using the stock focuser, which mounted flat to the rear of the tube. I don't know if the designs of different focusers give more leeway here; I need to (as suggested) draw the light path and lay eyes on it. I appreciate the offer and might ping you if I figure out that 17" is long enough.



#36 Terra Nova

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Posted 08 February 2021 - 05:07 PM

Depending on your cell and a ray trace of its light cone this might work. It’s the company I use for my metals orders.

6" OD x 0.125" Wall x 5.75" ID Aluminum Round Tube 6063-T52-Extruded: $52.29 for 24”

 

https://www.onlineme...ruded/pid/18127


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#37 CltFlyboy

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Posted 08 February 2021 - 05:34 PM

Thanks Terra - that's one I've got bookmarked. The price is right but they want $75 + $15 processing flat fee to ship that. A bit... high.. I've got a few people here that have some tubing, and I've located a few shops around Charlotte that might be able to supply this. Still, any/all leads always welcome!

 

smile.gif


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#38 brian dewelles

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Posted 08 February 2021 - 06:33 PM

I have 32 inch length plastic tube. 5-7/8inch ID. 6.25 inch OD. It has wood grain veneer on outer surface. You can have it free if it will work for you but shipping might be expensive.

Here is my 5 inch cheshire, f/15. I still think lens, cell and dew shield are jaegers but looking at 2 inch focuser now i think focuser is  from Japan.

Their scopes (tubes) were so heavy your possibly better off starting with just the lens and cell.

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#39 CltFlyboy

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Posted 08 February 2021 - 06:37 PM

I have 32 inch length plastic tube. 5-7/8inch ID. 6.25 inch OD. It has wood grain veneer on outer surface. You can have it free if it will work for you but shipping might be expensive.

Here is my 5 inch cheshire, f/15. I still think lens, cell and dew shield are jaegers but looking at 2 inch focuser now i think focuser is  from Japan.

Their scopes (tubes) were so heavy your possibly better off starting with just the lens and cell.

Thanks Brian! This particular cell is just over 6-1/8" ID, so a 6" tube is what I'm after. Appreciate that offer. The good thing is that since 6" OD is a pretty standard size I can get an aluminum tube readily, but I want to find it locally. If I had time and the drive to do so, I'd layup a fiberglass one - that would be stable and light weight.



#40 John Fitzgerald

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Posted 08 February 2021 - 06:48 PM

You will need to do a ray trace before you settle on a focuser.  A 2 inch focuser will likely vignette the lens if it has a long drawtube.  You will probably need a larger focuser, or a very short draw tube if you use a 2" focuser.


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#41 CltFlyboy

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Posted 08 February 2021 - 06:57 PM

You will need to do a ray trace before you settle on a focuser.  A 2 inch focuser will likely vignette the lens if it has a long drawtube.  You will probably need a larger focuser, or a very short draw tube if you use a 2" focuser.

Yep, I'm working on that now actually. But what I'm planning is to set this up for a typical visual use for now; I can modify later after I have a chance to enjoy it. And I'm toying with using interchangeable draw tubes that will get me the back focus I'll need - of course that introduces potentials for vignetting as you say, but by having a shorter draw tube for visual work that doesn't have a lot of metal going back inside the OTA I'm hiping to limit that.



#42 Kasmos

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Posted 08 February 2021 - 07:07 PM

I have 32 inch length plastic tube. 5-7/8inch ID. 6.25 inch OD. It has wood grain veneer on outer surface. You can have it free if it will work for you but shipping might be expensive.

Here is my 5 inch cheshire, f/15. I still think lens, cell and dew shield are jaegers but looking at 2 inch focuser now i think focuser is  from Japan.

Their scopes (tubes) were so heavy your possibly better off starting with just the lens and cell.

I know their ads only show the 3", 4", and 6" so I'll have to check my catalog, but I don't think Jaegers offered a 5" f/15. 



#43 clamchip

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Posted 08 February 2021 - 07:07 PM

Will your cell fit inside 7" OD 6-7/8" ID tube? the Jaegers cell was made to work this way.

If not your cell may have been modified by Cheshire, you might measure and see.

 

Robert



#44 clamchip

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Posted 08 February 2021 - 07:11 PM

I know their ads only show the 3", 4", and 6" so I'll have to check my catalog, but I don't think Jaegers offered a 5" f/15. 

I've never seen the 5 inch f/15 lens advertised but I'm fairly certain I recently acquired one.

Jaegers did advertise a 5 inch f/5.

Edmund and Jaegers would occasionally pickup and sell small quantities that never made it into

the catalog, just a note, maybe a catalog stuffer. 

 

Robert 



#45 CltFlyboy

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Posted 08 February 2021 - 07:46 PM

Will your cell fit inside 7" OD 6-7/8" ID tube? the Jaegers cell was made to work this way.

If not your cell may have been modified by Cheshire, you might measure and see.

 

Robert

Robert,

 

This cell is definitely custom made y Cheshire. They had it attached to the tube on the inside of the cell, as opposed to a traditional refractor objective being on the inside of the tube. At least that's what I've sleuthed from examining the cell. Check the pictures at this link, they are full resolution with a ruler.

 

I really appreciate you and everyone else helping with this. My CN community ROCKS!

 

Here are the pics on my secured OneDrive share:

 

https://1drv.ms/u/s!...bEDz3Q?e=6cPEyS



#46 clamchip

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Posted 08 February 2021 - 07:54 PM

It looks Jaegers on the inside, and I think you are right the outside has been modified.

It would normally have a step like this one.

The cell goes inside the tube and butts against the step, same with the dew shield on

the other side of the step.

You should see if your cell is still 6-7/8" OD.

Robert

 

IMG_9892.jpg


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#47 CltFlyboy

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Posted 08 February 2021 - 08:07 PM

It looks Jaegers on the inside, and I think you are right the outside has been modified.

It would normally have a step like this one.

The cell goes inside the tube and butts against the step, same with the dew shield on

the other side of the step.

You should see if your cell is still 6-7/8" OD.

Robert

 

attachicon.gifIMG_9892.jpg

Yes, it definitely measures 6-7/8" OD. But, unless I'm missing something (that's known to happen a lot!) the tube sits inside the cell, not the other way around. I believe that the dew shield slips over the smooth 6-7/8" OD part of the cell. I'll know for sure this week as I managed to get the eBay auction for the dew shield. Someone actually bid it up to $34.00 and unless you are an astronomer with a very specific need for this I can't see why anyone would have bid on it. I was smart enough to make my mid higher than I thought anyone with half a brain would go. Er... What does that say about me? tongue2.gif tongue2.gif


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#48 tim53

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Posted 08 February 2021 - 09:35 PM

a 6" f/5 achromat is a joy to use with a wide field eyepiece.  This one was made by my late friend Dick Zanteson 20 or so years ago.  Blue anodized aluminum tube.  Has a huge focuser made with a dovetail and rack from an old binocular microscope, probably a big petrographic microscope.  It didn't have a dew shield, so I made one from a paint bucket:

 

3494017-Scope2.jpg


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#49 CltFlyboy

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Posted 09 February 2021 - 08:24 AM

That is a dang fine looking instrument Tim!!



#50 clamchip

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Posted 09 February 2021 - 12:27 PM

I buy most of my tubing from Hastings:

https://www.hastings...laneous-tubing/

If you download the order form shipping is less than stated on

the site.

 

Robert


Edited by clamchip, 09 February 2021 - 12:29 PM.

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