Lumicon Giant Easy Guider

By Philip Perkins


I purchased my Lumicon Giant Easy Guider from F.C. Meichsner in Boston in November 1996. Since then I have had an opportunity to use it in all of its various modes of operation. The following is a report of my experiences.

The equipment primarily used with the GEG consists of the following:

I have not used the GEG with any other equipment, except that I used it briefly with the Pictor 201XT Autoguider and a Nikon FM2 camera body. It appeared to work successfully with both of these. I have heard that the GEG does not work successfully with all SCTs, such as the LX200 8", however I believe that it is intended to work with the Celestron-11 and Celestron-14. From this it appears that it should also work with the Meade LX200 12", but I have not seen any reports to confirm this.

The Lumicon Giant Easy Guider is both an Off-Axis Guider and a Focal Reducer in one instrument. As it's name implies, it is a large and substantially constructed device which weighs nearly 2-1/2 lbs with the lens installed. It has a large 80mm focal reducing lens which can be used in two different positions, or it can be removed entirely (for the f/10 'straight through' mode). The focal reducing lens yields crisply defined star images to the edge of the 35mm frame. Star images at the edge of the field have noticeably less coma than without the lens installed. As an Off-Axis Guider, it is extremely rigidly constructed and there is no potential for any flexure problems.

Advantages

Disadvantages

The mere fact that the GEG is an OAG and Focal Reducing instrument means that it is bound to be a compromise, to some degree, to what some will consider ideal. For some, a separate guidescope will be considered the only optimum way of guiding, and an intrinsically fast instrument the only way to achieve fast f/ratios. Bearing this in mind, the following points apply:

Mode Report

This is a detailed report on each of the four modes of GEG operation.

Notes:

Mode Report - f/10:

Mode Report - f/6.5:

Mode Report - f/5.5:

Mode Report - f/4:

Vignetting

Any vignetting comparison between different f/ratios is only valid if the telescope setup, area of sky being imaged, sky conditions, and film are identical throughout the comparisons and only the exposure time is adjusted to compensate for the different f/ratios. Thus far it has not been possible to commit the time necessary for such a test. The images presented below, although taken at the indicated f/ratios, fall short of the above criteria in at least one respect, and should therefore only be used as a rough guide.

Although the images below truly represent the GEG in all four modes of operation, they were taken in very different conditions. To try to compensate for the differing negative densities I used entirely the default settings of the Sprintscan 35 Plus for all scans - in other words I tried to let the Sprintscan do the compensation. The raw images were then converted to greyscale in Photoshop - the reason for this was to remove the distraction of colour from the images so as to make the vignetting more evident. Finally I adjusted the brightness of the images so that the un-vignetted part was a consistent 'mid-grey' across all images - this was by eye only, so minor inconsistencies may be seen.

f/10 - an unfiltered three hour exposure on hypered PJM-2 taken in dark sky conditions with very little sky fog. There is no vignetting in this mode.

f/6.5 - an unfiltered 30 minute exposure on hypered PJM-2. This negative is significantly more dense than the f/10 negative because the film was very over-hypered (this was when I was still learning to hyper) and there was much more sky fog owing to light pollution. The vignetting shown in this image is definitely emphasised over what you would normally expect to see, which I think was due to a combination of the above factors. In this mode you would normally expect to see a slight darkening only at the extreme corners of the frame with no detectable vignetting across the rest of the frame.

f/5.5 - an unfiltered 2 hour exposure on hypered PJM-2 taken in dark sky conditions with very little sky fog. This negative is slightly more dense than the f/10 negative. The vignetting shown in this image is very representative of what you would expect to see in average conditions in this mode.

f/4 - a 1 hour exposure on hypered PJM-2 taken in dark sky conditions but with moderate sky fog owing to low elevation. A Lumicon Deep Sky filter was used for this exposure. This negative is very thin apart from the nebulosity. The vignetting shown is not representative because in practice this mode has slightly more vignetting than the f/5.5 mode, yet this image apparently shows the least vignetting of all three focal reduction modes. This was probably due to use of the Lumicon DS which had the effect of virtually removing all sky fog and hence the perceived (but not real) amount of vignetting. This is an interesting image because it illustrates three things:
- How the perceived vignetting may be significantly reduced through use of a deep sky filter.
- How the GEG does indeed illuminate the entire 35mm frame even in this fastest mode of operation.
- How the 10" f/10 LX200 may be used to capture the entire Lagoon nebula on a 35mm negative, with room to spare.
 

f/10

f/6.5

f/5.5

f/4


Vignetting Comparison

It is by no means ideal to have vignetting, however the vignetting produced by the GEG is what I would call 'manageable' - it may be either cropped out or removed by a technique in Photoshop. On the other hand the vignetting produced by the Meade / Celestron f/6.3 reducers is what I would call 'unmanageable' - no amount of digital 'trickery' will remove it and the image has to be cropped to be presentable. The result of such cropping is that the resulting image scale is no greater than would be achieved without the reducer. The sole benefit is focal reduction to f/6.3. I have owned both Meade and Celestron f/6.3 reducers (though not at the same time). The particular image below (scanned in identical fashion to those above) was taken through the Celestron device, though results through the Meade device are identical.
 

Vignetting produced by Meade / Celestron

f/6.3 Reducer / Correctors


Sharpness

Aberrations produced by the GEG lens tend to increase in proportion to the degree of focal reduction and are only evident at the edges of the field in the two fastest modes - 'f/5.5' and 'f/4'. Even in the fastest mode of f/4, aberrations are confined to the outer 25% or so of the field. This high resolution crop from the central portion of IC1318 illustrates the degree of sharpness produced by the GEG lens in the fastest mode of f/4. Considering that the SCT design is not noted for being the being the sharpest available, and that the GEG lens introduces an additional optical system, the sharpness obtained here would appear to be acceptable. The slight red flare visible around some stars is due to use of a Deep Sky filter rather than chromatic aberration.

 

Options

This is a report on the various options available for the GEG. There are many options available and the decision on whether these may or may not be useful can be confusing for the new user.

Summary

As an off axis guider the Lumicon GEG is superlative. As a focal reducing instrument it is a compromise in as much as it is a substitute for the ideal solution of having a collection of telescopes of varying focal lengths and f/ratios. For some this ideal may be prohibitive either on cost grounds, or because of the difficulty of transporting a collection of telescopes to a remote dark sky site. In reality there is no such thing as the Holy Grail of having 'many telescopes in one' but the GEG comes closer than any device I have used or heard about. In the fullness of time those of us who are really obsessed with perfection will expand our collection of telescopes in an attempt to cover the entire astrophotographic range by means of dedicated instruments. In the meantime the GEG is an interesting alternative.

How close to the 'Holy Grail'?

GEG at f/10

Image Scale approximately 13 arc minutes

GEG at f/4

Image Scale approximately 1 degree

Yes, I cheated a bit - the f/10 image is cropped far more than the f/4 one! <G> ... Well, you can easily work out the true image scale difference between f/10 and f/4 - I thought it might be more interesting to show what is actually achievable ...

 

 

 

 

 

All text and images Copyright © 1997-2014 by Philip Perkins. All rights reserved.