Stephen Hamilton's Astronomy Blog

My thoughts on Astronomy, Imaging, and just about anything else I may come up with.

<January 2007>



Meade 6.3 Focal Reducer Test and Setup

Why Write this article:

There has been a lot of discussion concerning the use of standard Meade (and other's) focal reducers with the DSI cameras including questions of optimal distance, expected performance, etc.  I conducted the test below in February of 2006 along with testing several other focal reducers.  Since many people have the Meade 6.3 focal reducer and are copnsidering using it with thier cameras and various scopes, I though it was appropriate to post my findings on the use of this focal reducer and the results.  While I attemtped to be very precise and careful in my measurements, you may find that your particular setup may differ as may your results.  This is not a conclusive article on the use of the reducers in all situations but should give yo a good idea of what you may expect when using them with a DSI camera.


This article documents tests conducted using the Meade 6.3 focal reducer.  Where applicable, images and photometric data will be supplied along with detailed measurements to support my findings.  Overall, a few tests were conducted utilizing the focal reducer in various configurations.  The details of each test primary test are documented here

Test Conditions:

These tests were conducted on the night of Friday, February 24, 2006 between 8:00 and 11:00 PM.  During the tests, the sky conditions were generally very good with seeing around 8/10ths and transparency about 7-8/10ths.  Outside temperature range was between 3.8C at 8:00 PM and -0.9C at 11:00 PM.  DSI-Pro II chip temperature ranged from 9.0C to 7.5C throughout the evening.  Dark Frames were captured at 8.5C and used for all images utilizing the auto-dark subtract sub-routines in Envisage.

Equipment Setup:

Scope:      Meade 8” LX90 UHTC
Mount:      Equatorially mounted on the Meade Super Wedge
Camera:   DSI-Pro II
Filters:     ATIK Filter Wheel used during 6.3 tests, no filters (or wheel) during 3.3 tests
Guiding:   DSI-Pro thru ETX-90 mounted on the LX90 – Envisage controlled guiding
Alignment:Polar aligned using Kochab’s Clock method, no drift or iterative alignment required
PEC:        Off

Focal Length Measurements:

In order to accurately measure the focal length and hence the focal reduction of each image, the following formulas were utilized:

Distance between known stars – 
            D = Cos (A) = sin(d1)*sin(d2) + cos (d1)*cos(d2)*cos (ra1-ra2)
            (where d= Declination and ra = Right Ascension)

Pixel Distance between known stars – 
            S = SQR{(x1-x2)2 + (y1-y2)2}
            (where x and y are the coordinates of the stars in the image)

Pixels to mm – 
            L = S * .0083 
            (8.3 microns per pixel in DSI Pro II)

Focal length = (L)/(D) * (180)/P

Stars selected for measurements in all images do not necessarily equate the best stars to be used for optimal measurements based purely on their location in the image but instead are based on available information for any given star in the image.   Standard SAO catalogued stars were used for all measurements and thus, at times, may not be as far from each other in the image as other available stars.   With this in mind, stars utilized for measurements were based on their available data and then selected based on their distance from each other to ensure accuracy.

Star Measurements:

In order to effectively measure the actual shape and characteristic of the stars, I chose to use the Star Image Tool in AIP4WIN v2.  This tool delivers detailed information on each star selected including Size, Star Profile, elongation, diagonal and other characteristics.  The information is provided as textual data and a graph as can be seen in the images below:


The image on the left shows a star that is nearly perfectly round, having no perceptible elongation and the graph indicates the pixel values vs. the radius from the center of the star being very smooth to the outer boundaries. 

The image on the right depicts what would be seen for a very elongated star.  It also depicts what would be expected in a star that is flared strongly due to vignetting at the edge of an image as the pixel values disperse near the outer points of the star with no smooth transitions.

Meade 6.3 Focal Reducer Test

The test conducted was with the Meade 6.3 focal reducer and was mounted as seen in the diagram below:

As seen in the image, this setup places the CCD chip 62mm behind the glass of the focal reducer.  Based on past usage of these focal reducers, it is my belief that this is a very effective length that provides the best results.

While the ATiK filter wheel was used for this test, the filter wheel is very narrow at 19mm and closely replicates the same setup using the standard DSI-Pro II filter slide.

The object selected for this test was NGC1977, the Running Man Nebula in Orion, located approximately 1 degree north of M42.  This object was selected due to its size being nearly perfect for the expected focal length.  The image below is the Luminance layer taken thru the Meade IR blocking filter.  This image is composed of 30 x 1 minute exposures with no processing.

As can be seen from this image, the stars are nearly perfectly round edge to edge with only minimal vignetting apparent in the far corners.  Note:  The “Lens Flare” that can be seen around the 3 brightest stars in the lower section of the image is common in images of this object and is no reflection on the equipment being utilized.

The stars noted as S1 and S2 were selected for measuring the focal length.  Using the equations stated earlier in this article, the following measurements were calculated:

Angular Distance between the two stars:  0° 12” 3.52’

Pixel Distance between the two stars:  440.07 pixels or 3.652581 mm on the chip

Calculated Focal Length (L/D)*180/P) = 1041.30 mm with a focal ratio of 5.12

These results were very surprising and several measurements were made using different star sets to ensure the accuracy of the findings.  The average of all measurements was within statistical standards and differed by no more then 10mm in focal length or .1 in focal ratio.  It should be noted that even when using the exact same stars for multiple measurements, deviation in the determination of the centeroid of a given star will often times cause the results to differ by as much as 5 pixels at any time though generally less depending on the size of the star selected.

Star Measurements for 6.3 Focal Reducer

The image below depicts the stars that were utilized for measurements and their results:

As can be seen, the stars are virtually identical in each test with only a slightly higher diagonal measurement at the edges, a nearly perfect flat field.

Final Analysis of the 6.3 Reducer

Focal Reduction – This was the biggest surprise in the measurements but those measurements bear out that this is an excellent focal reducer going well beyond the manufacturers indicated values

Flat Field – The stars from the center and edges of the image were close to perfect in their measurements and visually can not be distinguishes in their elongation or diagonal.  This focal reducer in this configuration provides excellent field flattening.

Vignetting - The corners of the image show only the slightest amount of vignetting.  In a highly stretched image, this is more evident and the user would need to take that into consideration when processing. 

Overall, this focal reducer in this configuration provides excellent results and versatility.  The focal reduction achieved with this setup, while probably near the edge of the capabilities of this reducer, is outstanding and should provide similar or even better results when used closer to its 6.3 stated resolution. 


Published Wednesday, January 10, 2007 11:30 AM by E2Pilot


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