Last updated on
4/11/
2006

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Accessories That Are Nice to Have
[Scope Cover] [Observing the Sun] [Dovetail Plate] [A Rich-Field Scope]

Some accessories you will find helpful include:

Equatorial Wedge You must have one for astrophotography, but a wedge is really helpful in other situations. The reason is this: for a computerized scope to track (i.e. counteract the earth’s rotation to keep an object steady in your eyepiece) in alt-azimuth mode - which means tracking along two axes simultaneously - its computer really needs to begin by understanding what direction the scope is pointed towards (this wasn’t possible at all before computers were integrated with scopes because the calculations involved in accurately tracking a rotation around a polar axis without a polar alignment were too difficult).  So for tracking in alt-azimuth mode you need to perform an alignment with two stars, as described in the scope’s operating manual.

While an alignment is fairly easy in dark skies, for a lot of each month the moon is very bright and especially if you live in a light-polluted area it won’t be easy to see many night sky objects during that time.  This means you will have a hard time aligning the scope with two alignment stars for alt-azimuth tracking.  But if you have a wedge properly set for your latitude the scope only needs to track around one axis (the polar axis) which it can do without the alignment-star process .  Just set the tripod so the wedge roughly faces Polaris, turn the equatorial tracking on, and you’re good to go for visual observing of anything you can find visually (i.e. without the GoTo capability).  This won’t be good enough for astrophotography, which really needs an accurate polar alignment, but you can easily observe the moon without a two-star alignment. Also, there’s no convenient way to get an altazimuth-mounted GoTo scope to track the sun during the day because you can’t see any of the alignment stars then, either.  (You would need to align the scope the night before and leave it on so the computer remembers the alignment.)  In my experience a wedge really opens up a lot of observing time - the sun and the moon are great objects in a telescope, and observing the sun allows you to use that great scope during hours you’re normally awake.

You will still need to do the two-star alignment, even with a wedge, if you want to use the scope’s GoTo capability.  The point here is that for a fourth of each month or so (when the moon is bright) you probably won’t be able to see most of the objects that are difficult to find and for which you needed the GoTo feature.  And since you won’t be able to easily do the alignment during that time, you may as well get the wedge and observe the moon or sun. (I presume the new GPS GoTo scopes eliminate this alignment issue and can be used in altazimuth mode at any time, but I haven’t used one so I can’t say for sure.)

By the way, you need to know your lattitude to set the wedge.  If you don’t know what it is, go to the
U.S. Census’ Tiger Map Server, where you can get your location coordinates almost down to your actual street (which is way more accurate than you need for an initial setting of the wedge).

There is a difference between a wedge good enough for visual observing and one sturdy and vibration-free enough for astrophotography. I discuss various wedge options on the Equatorial Wedge page under the Photo Basics section of this site.

Observing the Sun Click on the Observing the Sun button above for accessories you need for solar observing.

Dust Cap for NexStar Scopes My NexStar11GPS scope came with a dust cap for the front cell, that is so tight-fitting that it’s difficult to remove and install. If you also have this problem, consider getting a Kendrick Kwik Focus, which is a commercially-manufactured Hartmann Mask focus aid for scopes. This cap, designed for assisting in achieving focus for astrophotography, fastens to the front cell with three nylon screws and is much easier to install and remove from the front cell than the friction-fit dust cap my scope came with.

Power for the Scope
If you’re observing close to a house you can save the AA batteries in the U2K with an AC adapter.  You can purchase one from Celestron, model #18773. Radio Shack sells a 12 volt AC adapter but be careful using it.  Mine started drifting up in voltage and this is dangerous to the scope’s computer chips (and motors, presumably). I recommend the manufacturer’s adapter rather than the Radio Shack units.
     The power cord from an AC adapter can be annoying - it’s one more thing to keep untangled as the scope slews to find an object you requested. An alternative is rechargable batteries, but you need to be careful about which kind you use.  For example the U2K requires 12 volts - it is designed to tolerate some variation in voltage but it won’t work well if the voltage drops too low because it just won’t get enough power. Now, the electricity delivered by batteries results from a chemical reaction in the battery and the voltage the battery can produce is directly related to the type of chemical reaction used. So if you look carefully at Nickel Metal Hydride (NiMH) cells you will see that they produce 1.2 volts, not the 1.5 volts that alkaline batteries produce. Stacking eight of these in the U2K only produces 9.8 volts, not 12 volts.  NiCad cells are a little better at 1.25 volts but that’s still too low. The U2K’s Manual warns you to only use Alkaline batteries and guess what? Alkalines produce 1.5 volts - eight of them gets you 12 volts.  Fortunately, rechargable alkaline batteries are available - you can order Rayovac rechargable alkaline AA cells (at around $1.00 apiece in quantity) and a charger from Sundance Solar.  This is less expensive in the long run, than constantly replacing regular AA cells.  (The only disadvantage to rechargable alkalines in general is that the number of recharge cycles before they quit taking a charge, is a lot smaller for them than for NiMH batteries.)

Skylight Filter for Dust Protection Celestron and Meade make a skylight (1A) filter that threads on the scope’s rear cell in front of the diagonal.  Although it is said to increase contrast, the main purpose in having one is to prevent dirt and dust from entering the optical tube assembly (OTA), and that seems to me to be an excellent idea (especially since I live in the Southwest which can be very dusty).  I think it’s good insurance since you really don’t want to open the OTA to clean the mirror or the back of the corrector plate unless you really know what you’re doing - alternatively you would need to send the OTA back to the manufacturer for cleaning. The cost of Celestron’s 1A filter is about $40; Meade’s is a little more.  Most of the accessories from Celestron or Meade, that thread on to the rear cell of an 8” catadioptric scope are interchangeable.

Note: There are some advantages in using a JMI NGF-S focus motor on the rear cell of an SCT, and if you decide to use one there is a less expensive alternative for keeping dust out of your scope.  See the discussion of the NGF-S on the Focus Options page for more information on this.

Nebula Filters Nebula filters are formulated to enhance the contrast of many nebulae and can enable you to see nebulae that are not very visible in light-polluted locations, and in fact they can enhance the visibility of some nebulae even at very dark observing sites. They are available from Astronomik (high-quality filters from Germany and sold by Adirondack Video Astronomy), Thousand Oaks, Lumicon, Orion, Celestron, or Meade. They are sized for the back of your eyepiece, either 1.25” or 2”. You can also get one threaded for the rear cell of your SCT but that isn’t a good idea - these filters only work for some nebulae (e.g. none of the reflection nebulae) so adding and removing one mounted to the rear cell can be a real pain when you switch from viewing a nebula to viewing stars or a galaxy - see below for an explanation of this.  (Rear-cell filters can be useful for astrophotography of nebulae with CCD cameras where it may not be possible to mount an eyepiece-threaded filter.) Nebula filters tend to be expensive, starting at $80 to $100 for 1.25” ones and going towards $200 for the 2” filters, but those are not unreasonable prices considering the vacuum-deposition technology involved, which results in coatings that transmit very specific wavelength regions and block the rest of the visible spectrum to increase contrast for nebulae. Depending on the size of your scope you may well find that some nebulae are almost invisible from your observing site without one of these filters.

Narrowband nebula filters, commonly called Ultra High Contrast (UHC) filters, pass the Hydrogen-alpha, Hydrogen-beta, and Oxygen-3 spectral wavelengths (although note that the Orion UHC filters do not include the H-alpha bandpass) and block the rest of the visible spectrum; O-III filters pass only the Oxygen-3 spectral wavelength.  Since stars (and consequently galaxies) are broadband emitters these filters will not help you view stars, star clusters, or galaxies - they will in fact just make those objects more dim (which is why a rear cell mounted UHC filter is not a good idea). Reflection nebulae are merely reflecting starlight so they are not helped by UHC filters either.  But the atoms in emission nebulae and many planetary nebulae have excitation characteristics that cause them to emit light strongly in the H-alpha, H-beta, and/or O-III wavelengths so a UHC or O-III filter can increase the contrast of these objects; they won’t get brighter - only more aperture does that - but they become easier to see. Dave Knisely provides a very good and detailed discussion of these filters in his Useful Filters for Viewing Deep-Sky Objects article.  My experience is consistent with his conclusions: narrowband UHC and O-III filters can really enhance the contrast of some nebulae but the broadband filters don’t seem to help much of anything (though perhaps they do in very light-polluted areas). More specifically, a UHC filter boosts contrast for more of the nebulae out there than does an O-III filter, but for those nebulae whose emission is concentrated in the O-III wavelength an O-III filter works better than a UHC. So most experienced observers recommend that you get both a UHC and an O-III filter but get the UHC filter first.

Finally, a third type of filter called an H-Beta blocks all light but the Hydrogen-beta wavelength and is useful for a few very specific nebulae that strongly emit in the the H-beta wavelength.  The Horsehead nebula is one of these few; since the Horsehead nebula is both a very desirable object to see and one notoriously difficult to see without an H-beta filter, many observers do purchase an H-beta filter to see the Horsehead. (It is also helpful for the California Nebula, the Cocoon Nebula, and a few others.)

Color (Planetary) Filters Celestron, Meade, and Orion all sell color filters that enhance your planetary (and lunar) observing, but they only sell these filters in sets of four. You can check their Web sites for details on which filter is best used for which application, but the only vendor who sells individual 1.25” or 2” color filters is Hands-On Optics as far as I can tell. These filters thread into the bottom of your eyepiece, and usually you would use a 1.25” filter since you wouldn’t use a 2” wide-field eyepiece for lunar or planetary work.  They tend to be very in expensive (from $10 to $20 each in the 1.25” size) and they really do allow you to see different planetary details.  Do get an orange filter for the moon - I was advised to get one when I bought my scope and it really helps provide definition of features on the lunar surface.

Orion also sells some nice multiple-filter cases that allow you to store your filters so that they’re much more accessible during an observing session than individual cases (or filter pouches), but still well-protected during storage. As shown here on the left, they have cases that can store two, four, or six 1.25” filters, and cases for two or four 2” filters.  These are inexpensive - from $2.75 to $4.50. That’s less than the cost of shipping them but they’re easy to add to an order of other items.

Lunar Observing The moon is quite bright much of each month, and when an 8” scope collects the light from the moon the view through your eyepiece can be blinding. A variable polarizing filter is a pair of polarizing filters set atop each other.  It acts to reduce the light coming through your eyepiece and its density (darkness) can be varied by rotating one filter relative to the second.  Many variable polarizing filters are designed to screw into the bottom of an eyepiece like all other filters; the problem with this arrangement is that you can’t readily change the density of the filter without removing the eyepiece from the diagonal.  A much better design is the Meade variable polarizer. It comes in a housing that looks like a Barlow, into which you place a 1.25” eyepiece, and it has a small protruding knob that is attached to one of the polarizers and can be rotated along a slot in the housing. This allows you to vary the darkness of the filter while you look through the eyepiece.  At $50 it is only a little more expensive than other variable polarizers but it is a lot easier to use.  Note that Celestron also sells a variable polarizer but theirs seems less convenient to me. With the Celestron unit you screw one of the polarizers into the bottom of your eyepiece and the other is in a separate housing into which the eyepiece fits. You can vary the density by rotating the eyepiece in the second housing, but you can’t as easily switch eyepieces as with the Meade filter because you would need to switch the one polarizer filter from eyepiece to eyepiece.  The Meade unit, being self-contained, allows you to switch eyepieces more easily.
     A 90° “correct-image” prism diagonal is also helpful for lunar observing (as it is for most all observing). See the Diagonals page on this site for more information on correct-image diagonals.

A Better Focusing Capability There are ways to improve an SCT’s ability to achieve fine focus on an object.  Although this is critical for astrophotography it is helpful for visual observing as well.  For more information see the Focus Options page (in the Astrophotography Accessories section).

A Telescope Cover This is a cover for your scope that allows you to leave it set up for a few days at a time.  This adds convenience in observing and also allows you to preserve the scope’s star alignment from night to night. Click on the Scope Cover button above for more information. 

Quick-Release Finder Bracket for Celestron SCTs This is helpful for easily removing a Celestron finder scope when you want to store one of those scopes in a fitted hard case (the Meade LX SCT finder scopes are provided with a quick-release bracket). Click here for an explanation about the value of replacing Celestron’s standard finderscope mount with a Quick-Release Finder Bracket.

A Dovetail Plate This is used to interchangeably mount a camera or small scope atop your SCT. Click on the Dovetail Plate link on the top of this page for more information.

A Rich-Field Scope Most telescopes, especially SCTs, have a narrow field of view since they are primarily designed for observing (and magnifying) very small things, whether planets or most deep-sky objects. Ironically, this means that there are deep-sky objects (many star clusters and nebulae) that can’t be really appreciated in a normal scope because they are just too large to fit in the scope’s field of view regardless of how wide-angle an eyepiece you use. Although the list of these large objects is shorter than the list of small deep-sky objects, they do include some of the most beautiful objects in the sky. Therefore experienced amateur astronomers include either a good pair of binoculars in their tool kit, or a small rich-field (short focal-length) telescope mounted atop their primary scope. Of course, compared to binoculars the advantage to mounting a rich-field scope on your SCT if it is a GoTo scope, is that the GoTo capability conveys to the rich-field scope as well. Also, a rich-field scope makes a great finder scope when you need to star-hop to find an object (which, even with a GoTo scope, eventually you will do because many interesting objects aren’t in a GoTo scope’s database).  Click on the Rich-Field Scope button at the top of this page, for more information on some of the relatively inexpensive rich-field scopes available.

Camping Table   Orion sells a “Roll-a-Table”, a 32”-square table that rolls up for storage or transport but can be assembled easily to give you a work surface next to your scope. (This table can probably be purchased at a good camping store as well - which is where I got mine many years ago.) You will find that when observing at remote sites (or even in your backyard) you’ll find a small table for all the stuff involved (eyepiece case, star charts, etc.) to be handy, and this table is a good investment. (Coleman makes another similar table; a Google search on “camping table” will locate many sources for this type of table.)

Projector Stand If you need a small stand to hold a laptop computer or an observing notebook, a projector stand is very handy and is more easily moved around your scope, than a camping table. A company named Stanrite makes two such stands that are well-built and allow the legs to be stored underneath the top so it is easy to store.  The Model SP-2 is available from a lot of audio-video vendors on the Internet, but its height is adjustable from 32” to 56” and even that minimum height is too high for convenient use next to an observing chair for most tripod-mounted SCTs when they’re aimed at the zenith.  The Stanrite Model SP-1 adjusts from 24” to 41” and that’s much more convenient. However, the SP-1 is less useful for its intended use as a projector stand so it’s a lot harder to find a vendor that sells it. One of the only vendors I have found is Dick Blick Art Materials; their stock number for it is #51118-2441 and it sells for about $110. I have one of these and I find it really useful for observing (and also as a laptop stand inside your house - the inexpensive plastic portable laptop stands that are becoming common from mail-order vendors are flimsy and wiggle too much to be useful for laptops - caveat emptor).

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