After the novelty of having a computer driven scope became more commonplace, the next big question for many has been to try photography. The alt-azimuth setup these mounts have can be used for photographing the moon and planets, since they are nice and bright and the exposures are short. But for everything else, the mount will need to have its axis aligned with the earth's to keep the image from rotating. Unfortunately, in the case of the NexStar SE mounts, putting the well made Celestron wedge under the telescope only sets the scene for discovering these mounts simply do not have the ability to track smoothly enough for even a 30 second exposure.
Background
Telescope mounts come in two general configurations: Alt Az (short for Altitude- Azimuth) and Equatorial. An Alt Az mount is a rotating base with an vertical adjustment. A camera tripod is the simplest version of this type of mount, which although intuitively easy to understand, is not very useful for astronomy. The problem is the earth is rotating on its axis once a day, and unless you are at the exact North or South pole, that axis is at an angle to the ground you are standing on. So for example, if you are at 32 degrees North Latitude, you are going to see the stars move across the sky at an angle where if you looked at the eastern horizon, the stars will meet the ground at 58 degrees (or 32 decrees from vertical). So, in order to track them across the sky with an Alt Az mount, you will need to constantly adjust the bearing and the altitude the telescope is pointed at. While a computer controlled mount can do that automatically, it will still mean an object seen through the telescope will rotate 180 degrees as it crosses the sky (watch where the poles of a crescent moon are as it rises, crosses the middle of the sky, and sets to see this).
This isn't a problem for visual observations, which are typically for a few minutes, but is a serious problem for deep space photography, where long exposures are needed to record dim objects. Having the inmage turn during exposures is going to smear the outer part of the image, even if the telescope had tracked the object perfectly.
The time tested answer to this problem has been the equatorial mount, which takes the alt az setup and leans it over so one axis is parallel to the earth's, and the telesope can then follow an object by moving in one direction at a constant rate (a complete circle in a day). The other benefits are the lack of rotation in the image, and the simpler tracking really doesn't require more than a clock timed motor.
In the case of Celestron's NexStar 6SE and NexStar 8 SE, these mounts come as computer driven Alt Az setups with a flat tripod base. In order to track in an equatorial configuration, the base of the telescope needs to be tilted so it is parallel to the earth's equator. This is similar to the larger NexStar CPC telescopes such as the CPC 800 and CPC 1100, which convert into highly capable astrophotography platforms with a tilting baseplate called a Wedge.
The Celestron 93658 wedge is only used by the NexStar 8SE and NexStar 6SE telescopes. The larger twin arm fork telescopes use the larger heavy duty wedge. Note, if you have a wedge and tripod for one of the larger NexStar telescopes, you can mount the NexStar SE telescopes directly do it for an extremely sturdy foundation (and this is the lest expensive way to discover this is unfortunately not needed by these telescopes).
Description and Usage
The Celestron 93658 wedge is a set of die-cast parts and steel hardware with a rough textured black finish. From experience with this on other Celestron equipment, I can assure you keeping it clean looking is impossible, and if you happen to be near a desert (as I am), the fine dust which shows up in the air has a way of getting into this finish so no form of cleaning I am familiar with can remove it. The wedge is made to mount on top of the stainless steel tripod which comes with the NexStar 6 SE and 8 SE telescopes. The mount already has the mount holes in place for the bolts on the wedge, which comes bagged in its box with the user only needing to fold it open to your approximate latitude and attach the locking arms and screws. Unlike my previous experience with the larger HD wedge, this component went together on the first try, and results in what appears to be an extremely refined and compact photography setup.
The problems with this component actually stem from the telescope mounted to it. I took the telescope and parked it with the arm over one of the bolt holes to make mounting simpler. The next step was to setup outside. Since this mount had never been polar aligned before, I took the wedge and tripod out separately and set it up so the plate of the wedge seemed to be pointed towards the North Star. This technique actually works better than it sounds, and on older mounts like the C5+ or the on German Equatorials, will generally leave the mount fairly close enough for polar alignment within a few minutes. I took the telescope out and held it so the drive base was hanging from the arm. Since I had parked it with the arm over one of the pads, the bolt hole was lined up with the notch on the top of the wedge, so the base dropped right in and getting both of the other bolts installed was a quick task.
The first major problem occurred when I did the initial alignment for equatorial north, and then selected the NexStar "Wedge Align" option under utilities. The telescope drove to where it apparently thought polaris was, but was far from the actual star. I had trouble believing my initial guess was off by more than 15 degrees, but went ahead and adjusted the mount, and found I was close to one end of the wedge's range, and the azimuth on the wedge was showing over 10 degrees north of my location by the time I was done. This was intended to be a shortcut to getting the wedge closer to alignment. I then did the follow-up step, which is to repeat the initial alignment, where you go to a couple of stars. To my chagrin, the alignment was now so far off, the telescope tracking was unable to center on objects.
At this point, I had only planned to perform a drift alignment test with a reticle eyepiece to check the NexStar SE mount's performance. Instead, I ended up using the eyepiece for alignment of the wedge using the method shown on Starizona's web site, www.starizona.com.
It soon became apparent the mount was hopelessly far off, but there did turn out to be a shortcut to getting it closer to a true alignment. The NexStar asks for the mount to be moved into a reference position with the telescope pointed at the meridian and aligned 90 degrees from the north pole. It then guesses at where a star is at and drives to it. I noticed how far the scope was away from the real object, and guessed the error was probably close to the remaining error in the wedge. I moved the wedge instead of the mount on the first star, and sure enough, this turned out to be the bulk of the error in the wedge. This time, the drift alignment put the wedge in true, so it was time to see if the mount could take photos.
I set up the camera on the four stars of the trapexium in the heart of the Orion Nebula and started a series of test photos. To my surprise, the stars were all streaks. I tried moving the scope around to make sure the drive was engaged in the direction it had to move to follow the earth and tried again. The stars were still streaks on 3 second exposures. I decided to get a better understanding of what was going on, and took a series of photos with the mount tracking on its own. The results showed the mount wasn't losing or gaining on the stars consistently. Instead, it would advance for a few minutes, then it would start to fall behind for a few minutes, so if you looked at the field about every 10 minutes, it would seem the mount had been tracking perfectly, when it had done anything but. It occurred to me the problem could be something known to happen to clock drives, where a mount with perfect balance can oscillate on the drive. I slewed over to the Plieades to see if purposely putting a small imbalance on the drive will even it out. The results were indistinguishable from the first trial, and the stars were streaks, and flipping through the stack of images showed how the drive slowly outran the earth, then abruptly started falling behind, and a few minutes later started the process over again. In short, the current version of the single arm NexStar mount is simply unsuitable for long exposure astrophotography. Unlike the older C5+ single arm mount, this mount can't produce the even tracking rate needed for photography. To say the least, this is disappointing. Even stranger is the question of why this wedge exists, since the only mount which can go on it can't make effective use of it.
The overall assembly is still easily light enough to pick up and move in one piece, but the poor tracking of the mount makes this largely irrelevant. Taking photos of bright objects like the moon, planets, or the sun with a front end solar filter involves short exposures taken over a short period of time, and under these conditions the wedge is not needed.
Conclusion and Summary
The Celestron 93658 wedge is a reasonably well executed mechanical device for one of the more tedious parts of astronomy- polar alignment. Unfortunately, the mount it is designed to go with has serious tracking issues for long exposure astrophotography. Unless a redesign is undertaken on the NexStar SE series mount, there simply is no reason to buy or use the matching wedge.
Upgrade your Celestron NexStar 5i or 8i telescope with the 93658 equatorial wedge for long-exposure astrophotography. Built-in azimuth and altitude ad...More at eBay
With built-in azimuth and altitude adjustment controls for easy polar alignment, this wedge has a 25-90 degree latitude range. Used on either NexStar ...More at Buy.com Marketplaces
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