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Celestron C-9.25 XLT vs C-11 XLT


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Celestron C-9.25 XLT vs C-11 XLT

By Jerry Wise

The Celestron C-11 and Celestron C-9.25 are two of the most popular SCTs used today. Observers often report a magical performance by the 9.25 OTA. On the other hand, the C-11 has also proven to be a fine performer in the large SCT arena. I was fortunate enough to acquire both OTAs and decided to compare the two. The 9.25 XLT was purchased from an individual and is about 5 months old. The 11 inch XLT was ordered new from an online dealer.





Specifications:

The specifications for the 9.25 XLT:

Aperture - 9.25 inches or 235 mm
Focal Length - 92.52 inches or 2350 mm
Focal Ratio - F10
Highest useful magnification - 555x, Lowest 34x
Limiting magnitude - 14.4
Resolution - 0.59 arc seconds
Resolving power - 0.49 arc seconds
Secondary mirror obstruction - 3.35 in., 13% by area, 36.2% by diameter
Light gathering power - 1127 x eye
Optical tube length - 22in.
Optical tube weight - 20 pounds (9.07kg)

The specifications for the 11 inch XLT:

Aperture - 11 inches or 280 mm
Focal Length - 110.24 inches or 2800 mm
Focal Ratio - F10
Highest useful magnification - 660x, Lowest 40x
Limiting magnitude - 14.7
Resolution - 0.5 arc seconds
Resolving power - 0.42 arc seconds
Secondary mirror obstruction - 3.75 in., 12% by area, 34.1% by diameter
Light gathering power - 1593 x eye
Optical tube length - 24in.
Optical tube weight - 27.5 pounds (12.47kg)


XLT Coatings:

Both OTAs were purchased with Celestron XLT coatings. Previous experience and comparisons with non-XLT optics have shown a marked increase in light throughput with premium coatings both on Celestron and Meade products. Here is a link to Celestron's detailed information and graphs on XLT coatings:

http://www.celestron.com/c2/technology_view.php-TechnologyID=2


Method of Comparison:

Many factors influence a comparison of two telescopes. Sky conditions, EP selection, mounting, viewer opinion, and a host of other factors. In this comparison personal opinion will be reserved till the end. Each of the OTAs were mounted on a Celestron CGE mount. Then several deep sky images were photographed using a Canon 300D at prime focus. The idea behind the images is to give the reader a very good idea of how the scopes performed by comparing the images. The images were loaded into PaintShop Pro 9 for minimal contrast and brightness adjustment. Any change in contrast for one OTA image was applied in exactly the same way and to the same degree to the other OTA image PaintShop Pro stores the contrast and brightness settings and automatically applies them to the next image loaded if you do not instruct it differently.

Tube Construction:

Both of these OTAs are aluminum. While the aluminum cools faster and weighs less, the Carbon fiber construction will hold stable focus longer over extended imaging sessions. The Carbon Fiber is also a bit tougher if you bang a scope around a lot. The Carbon Fiber cost more but really looks classy. If I had my choice I would go with Carbon Fiber and put up with a longer cool down time but it's no big deal.

Mounting:

Both OTAs are very manageable and easily mounted on the CGE using aftermarket Losmandy Dovetails. Both scopes are well balanced (for an SCT) and have handles at the base. Fit and finish on both are what you would expect of high end production telescopes. No flaws or blemishes and excellent appearance. This cannot be said for the diagonal and finder that come with the scopes. Best to put them aside. They do not represent the quality of the telescopes in general.

Collimation:

Before each session, a collimation test was performed. The mounted OTA was slewed to a medium brightness star and defocused. While defocused a photograph was taken of the defocused image. Both are presented below. The 9.25 is on the left, the 11 inch on the right:



Camera:

All images are recorded on a Canon 300D camera. The camera does not have the IR filter removal (non-modified). All images are timed using a Canon programmable remote timer.

Focus:

The following may be a bit boring to some. Suffice to say, the focus was carefully performed and photographed. If you have a desire for more detail, please read on.

Both OTAs have a nice focus knob with good feel and little mirror flop. In order to achieve an optimal focus for this review, a FeatherTouch two speed focuser was used on both OTAs The 10 to 1 reduction of the FeatherTocuh fine focus knob lets you carefully achieve the best focus. A Stiletto focuser was used initially and the focus confirmed by a Hartman Mask. Both tubes were placed in the observatory and the observatory shutter opened late in the afternoon. By test time, both OTAs were cooled down and ready for viewing. The C-11 was placed on the CGE first, focused with the Stiletto and the Hartman Mask placed over the front of the OTA. The resulting focus was then photographed. The same procedure was used on the C-9.25. Here are the photographs of the focus star with Hartman Mask on.

9.25 to the left, 11 inch to the right:



Targets:

Deepsky targets were used for the comparison. However, many will use these OTAs for planetary and lunar viewing too. Viewing these planetary objects is largely seeing limited and both OTAs do as fine a job as seeing allows on these celestial objects. Photographing these objects also requires a ÒWebCamÓ and some good luck for excellent results. Deepsky photography in this comparison was deemed more repeatable and less subject to short term atmospheric effects.

To show the raw resolving power in a somewhat equal manner, all photos are 2 minutes with no autoguiding. There are also no dark frames subtracted and no ÒstackingÓ to smooth the images. If a contrast and brightness adjustment is made, it is done on composite images after placing the images together. The changes on the images would be identical to both. The C-11 will have greater light collection but the 9.25 seems to hold up well on these commonly photographed objects. (The stars in these images may appear a bit elongated for the 9.25 images. In changing the OTAs the weight may not have been adjusted properly.)

First Object, Globular Cluster M13:

M13 is a 6.2 mag. Globular Cluster 33,600 light years away. To show comparable image scale, the 9.25 image on the left was magnified to near the same size as the C-11. Many will question this method. On review of the raw images there appears to be no visual difference between both images and their original raw images. This image is a fair representation of the original raw images (raw image file size is much too large to be included here).

C 9.25 C 11





Second Object, Dumbbell Nebula M27:

This 7.4 mag. Nebula is 1,250 light years away. Both OTAs pick up the central star well in only a 2 minute, non-stacked exposure.

C 9.25 C 11





Extreme Magnification of area near Star Triangle in M25:

While not a reliable indicator on of resolution, an extreme magnification of the images shows the triangle of stars often used to locate the central star is a bit more apparent in the C-11. However, the slight elongation of the stars in some of the 9.25 images may account for this.


C 9.25 C 11




Overall, both scopes did a fine job on M27. Longer exposures, dark frame subtraction and adding of sub frames would yield some outstanding images.

Raw Images of M27 showing scale of the C-9.25 vs the C-11.

The C-11 obviously has more light collecting capability.

C 9.25 C 11



Third Object, the Ring Nebula M57:

The Ring Nebula is an 8.8 mag. prototype of a planetary nebula 2,300 light years away. A lot of discussion has taken place on exactly how this object is viewed from Earth and what its shape really is. It could be a shell, ring, cylinder, etc. However, both of these OTAs present fine images of M57 with just two minutes of exposure. Even at two minutes both show a hint of the central star.

C 9.25 C 11




Fourth Object, our neighbor the Andromeda Galaxy core, M31:

Andromeda is 2,700 light years away and glows at 3.4 mag. Reliable estimates of stars in the Andromeda galaxy put the count between 800 million and 800 billion.




Both scopes have a bit too much focal length to acquire an all encompassing view of Andromeda and companion galaxies. Piggybacking an Orion ED80 or similar scope on either would make an ideal setup for both deepsky and wide field astrophotography.

Visual Comparison:

Visually the scopes compare well to the above images. A William Optic Dielectric diagonal is used on both OTAs. The scopes have had extensive use over many months with the following EPs:

1. Nagler 17, 16, 13, 11 and 3-6 zoom, Panoptic 19 and 35mm.
2. Meade 6.7, 8.8, 14 UWA series 4000 and 24mm UWA 5000
3. Pentax 10, 20 XW

On planets and the Moon the 9.25 seems to have a slight edge in contrast using a top quality Eyepiece. The C-11 is not far behind, if at all, on faint detail on the planets and moon. My choice would be the C-11 for the extra light gathering on very faint deepsky objects. But that's my viewing preference.

I've done a shootout between the C-11 and a very good example of a Meade 127ED. The Meade is fantastic on planets and the Moon. In a side by side testing over a number of nights the C-11 stayed right with the ED/APO refractor on minute detail. But this can be expected at most seeing limited sites.

Conclusion:

These images will help you decide the merits of the two popular OTAs. I like them both but always seem to go with the largest aperture I can get. The Celestron build quality is excellent. I've never had a problem with a Celestron product. The included diagonal and finder are not usable for most astronomers. Celestron would do better leaving the diagonal and finder at the plant. Both come with a very good 40mm ELux Eyepiece.

Both OTAs are lightweight and perform well. Neither size is fussy about collimation once it is carefully adjusted. Either OTA will make a serious SCT astronomer happy.








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