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focal reducing lense for lumicon giant easy guider

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#51 Octan

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Posted 28 July 2017 - 10:13 PM

To be honest I have no idea and I am not taking it apart to find out, its not easy to take a photo of a curved peice of glass as you can see by the reducer sitting behind it.

All I can say is its not convex on both side you can see that in the reflection and the back is not 3 x longer than the front having a close look at it when it was out the back  is about - .5 to 1 x concave from the front the type of len it is cemented or not I dont know.

I did compare it with the 6.3 reducer on a lamp nothing to scientific and it acted the same way if anything it may have been a little sharper.

But to say its a simple doublet IMHO would be a bit of a stretch.

I have had the unit for about 8 weeks and it has transformed my c11 it is far superior to my 6.3 yes as you have stated it does get a bit of coma if pushed to f4 but its easily cropped out but non at f7 I use min arou d the f5 to 6 ish

They make your scope very flexible.

The only down side I see with one of these is your collimation must be very good as at fast speeds the coma gets a lot worse if its out and its not to bad if its good 

They only work on the 3 inche back of the c11 c14 and equivalent meade but if you have one of these scopes its a great bit of kit and gives great wide veiws no false colour that I can see.

I would also add this the only reason that I would think that Lumicon would stop making these is that it is a bit of a limited market the 6.3 work very well in all other scopes as well as the c11 and c14 they just dont work as well in the c11 as this one does also the 6.3 is cheaper.

 

Brenden



#52 Ken Sturrock

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Posted 30 July 2017 - 08:18 PM

I know that this thread was started a while back, but PLEASE stop posting your personal email addresses. You will get junk mail (plus it's against the site's Terms of Service). Rather, use the site's personal messenger system and exchange emails that way if you must.

 

Thanks.



#53 mvas

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Posted 06 September 2017 - 02:27 PM

A few things that I do not like about the Lumicon GEG ...

 

Issue #1
=======
It is very heavy

 

Issue #2
=======
The pick-off prism is sufficiently off-axis to prevent any shadow,
but it is way too far off axis where the light rays are very distorted.

We cannot lower the prism into the more focused light.

Sometimes, this makes finding guide stars more difficult.

 

Issue #3

=======

I do not think it is a Coma Corrector like the Starizona SCT Coma Corrector

 

Issue #4

========

Moving the lens between the front location and the rear location, is a royal pain.

 

4a) First - You need the correct size tiny Allen wrench to loosen those very tiny set screws to swap the lens position.

      Socket Heads would be slightly easier.

      Large Thumbs Screws would be best.

      The rule is ... no tools should be required in-the-field.

 

4b) Second - You have to remember when you need the guider extension tube or not.

 

4c) Third - You must add or remove layers of electric tape, as spacers, from under the guider tube to change its ANGLE!

 

This is not something I like to do in-the-field

This issue and the electrical tape "fix" for changing the angle of the guide tube appears to have been realize after manufacturing & shipping.

 

Everything else is great!


Edited by mvas, 06 September 2017 - 03:28 PM.


#54 mvas

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Posted 06 September 2017 - 08:56 PM

I was just reading this post looking for info on a GEG, it seem its been stated here that the lens could be a simple lens from a binocular then it becomes the same in the finder scope and binocular made by the Lumicon.

Quote

"Certainly so when compared to any reducer using a *non-optimized* lens, like the simple 'binocular' objectives employed in the Lumicon Easy Guiders"

 

So I have a GEG and I have a 6.3 reducer as well.

So I took out the 80mm lens from the GEG and compared it to the 6.3.

 

Unless the finder scope and binocular have a large convex surface at the front and a large concave surface on the rear and look to be pretty much the same or similar shape to a Celestron reducer but a hell of a lot bigger my guess is it has nothing to do with a binocular lens or a finder scope and in fact is just what it is a very big and wide reducer corrector.

From using it, it has change my c11 from a small view f 6.3 to a f5 with very flat and wide field of view that the celetron corrector has no hope in matching. It fact I don't even use it any more.

Octan,

Please use a soft straight edge ( paper or similar ) and hold the straight edge against each side of the lens.

Side #1 is Strongly Convex

Side #2 is Weakly Convex

In your photo, it may only appear that one side is concave.



#55 GlennLeDrew

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Posted 07 September 2017 - 10:23 AM

If the rear surface is indeed weakly convex, this would accord with the lens installed in my friend's guider, which seems very much to be an 80mm binocular objective. The 2" Lumicon guider I once owned had what appeared to be a 50mm bino objective. Commonly available 1.25" 0.5X reducers most certainly use what are effectively 25mm bino objectives. The old Rich Field Adapters for SCTs available in the '80s and '90s used similar lenses.

 

These standard cemented achromats having conjugates at infinity/one focal length and aperture ratios of f/3.7-4 have been widely used for decades as reducers. If not stated as correcting certain aberrations or flattening the focal surface, or specified as being for use with a particular type or brand of telescope, a reducer can almost safely be assumed to contain such an off the shelf 'bino objective.'



#56 mvas

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Posted 07 September 2017 - 11:12 PM

If the rear surface is indeed weakly convex, this would accord with the lens installed in my friend's guider, which seems very much to be an 80mm binocular objective. The 2" Lumicon guider I once owned had what appeared to be a 50mm bino objective. Commonly available 1.25" 0.5X reducers most certainly use what are effectively 25mm bino objectives. The old Rich Field Adapters for SCTs available in the '80s and '90s used similar lenses.

 

These standard cemented achromats having conjugates at infinity/one focal length and aperture ratios of f/3.7-4 have been widely used for decades as reducers. If not stated as correcting certain aberrations or flattening the focal surface, or specified as being for use with a particular type or brand of telescope, a reducer can almost safely be assumed to contain such an off the shelf 'bino objective.'

Using a straight edge, I checked for myself and this is what I see  ...

Side #1 is strongly convex

Side #2 is weakly convex

It is not a meniscus.

It is an asymmetric convex lens - front convex & rear convex have two different radius of curvature.

The previous photo showing "concave" is really tricking the eyes.

I only see two reflections - one large and one small.

If there is a 3rd reflection ( cemented ? ) I cannot see it. 


Edited by mvas, 08 September 2017 - 07:06 AM.


#57 GlennLeDrew

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Posted 08 September 2017 - 11:40 AM

To check if a lens is your basic "bino objective" type, hold a ~15mm f.l. eyepiece behind it. If it delivers an image like that of a bino or finder, with no more spherical (or chromatic) aberration than expected, chances are the lens is just that. If damaged, you could replace it with another of the same diameter taken from a bino. There might be some small difference in focal length and hence reduction factor (at given separation), but otherwise it would be pretty much the same.



#58 mvas

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Posted 10 September 2017 - 02:51 PM

I am looking to buy a lumicon giant easy guider and i have found 2, one has no lense and the other does not have one at all. I have searched the world over and cannot find one to buy, and suggestions on where to find one would be greatly appreciated, thanks Jason

Did you see this ad ?

" ...  You don't have to buy the focal reducing lens separately -- it's included ! ..."

https://www.sbscient...nt-easy-guider/


Edited by mvas, 10 September 2017 - 02:52 PM.


#59 aaronmkii

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Posted 18 March 2018 - 04:39 PM

Hi - Could someone please send me a copy of the instructions. The two jpgs posted were very useful. Thanks for that, but I could like to see what it says about shimming the prism in the f/5.5 and f/4 position. I have just moved my lens to that position.

 

Please shoot me a message to get my email.

 

Thanks,

 

Aaron



#60 mvas

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Posted 23 March 2018 - 05:15 PM

Hi - Could someone please send me a copy of the instructions. The two jpgs posted were very useful. Thanks for that, but I could like to see what it says about shimming the prism in the f/5.5 and f/4 position. I have just moved my lens to that position.

 

Please shoot me a message to get my email.

 

Thanks,

 

Aaron

 

If you put the lens in the front position ( telescope side ) of the Focal Reducer then ...

  • Remove the tube that holds the Guiding Eyepiece.
  • Once removed, you will expose to two 6-32 flat head screws holding the Pick-Off mirror.
  • Remove those two flat head screws.
  • Lift out the bracket, which is attached to the Pick-Off mirror & set aside.
  • Now, add 1 or 2 pieces of tape ( 0.010" - 0.020" shim ) in front ( telescope side) by those two screw holes
  • This will add backward "TILT" to the Pick-Off bracket = 1° - 3°
  • Re-install the Pick-Off mirror
  • Re-attach the Eyepiece Tube.
  • Done

During the day you can look thru the Guide Tube, without an eyepiece to verify if ...

The image of the section of corrector plate ( tube edge to central obstruction ) is well centered in the Pick-Off mirror.

Below is a photo from the instruction pages showing where to add the "spacer" - in two locations - front-left side & front-right side of recessed area

(click on photo to zoom-in)

 


Edited by mvas, 23 March 2018 - 10:55 PM.



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