NexStar Resource Site

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Mounting Other OTAs on NexStar Mounts

The combination of GoTo, tracking, light weight, and low price (particularly the SLT models) make Celestron's computerized alt-az mounts very attractive as potential mounts for a wide variety of optical tube assemblies (OTAs). General concerns to keep in mind include:

  • a longer OTA may affect your ability to point at objects high overhead
  • a longer OTA may cause balance problems and thus stress the motors or impair tracking accuracy
  • an OTA weighing much more than the original may strain the motors, shortening their life

See this article for a list of recommended weight limits for Celestron's alt-az mounts.

The NexStar SE, Evolution, SLT, LCM, SkyProdigy and Astro Fi mounts use a standard Vixen/CG-5 dovetail bracket to hold the optical tube. Some of the mounts have a plastic cowling around the dovetail bracket as a cosmetic cover to mate closely with the provided optical tube. This cowling may need to be removed if you attempt to mount a larger diameter optical tube. Vixen/CG-5 dovetail rails/bars are available in a wide variety of lengths from most astronomy equipment vendors. For many optical tubes you will also need tube rings to attach the tube to the dovetail bar.

Mounting an additional OTA piggyback on the main tube is quite common for the CPC, CPC Deluxe HD and NexStar GPS series. The piggyback OTA is often a short-tube refractor used to provide wide-field views or for guiding during imaging. The main optical tube provides the high-resolution and light-gathering capabilities required for most objects while the short-tube refractor provides stunning wide-field views of open clusters and other extended objects. The main optical tube has mounting screws for rails along the top and bottom. Rails and brackets for mounting are available from many equipment vendors. ADM in particular has a wide variety available ( Be sure to get rails that are specifically designed for your specific model to ensure they line up with the mounting screws.

For the NexStar 5/5i/8/8i mount (they are the same), the best option would be "Ray's Brackets".  Unfortunately they are no longer available new so if you don't find a set available used, you will need to fashion something yourself.  You would start by disassembling the alt axis (see here).  You would then detach the existing rail from the optical tube and mount and attach some sort of dovetail bracket (like this) to the side away from the drive gear.  Then reattach the rail/dovetail bracket combo to the fork arm. This would then allow you to mount any suitable OTA with a dovetail rail, including the original OTA.

For the original NexStar 60/76/80/102/114/130 GT, you could mount another OTA in the existing tube ring.  The inside diameter of the N130 tube ring is 150mm, the N114 tube ring is 140mm, the N80 tube ring is 90mm and the N60 tube ring is 63mm.  OTAs slightly larger than this would fit just fine.  A more flexible option would be the Baader Bracket.  Unfortunately, this bracket is also no longer available and hard to find on the used market.  You can fashion something yourself.  Start by disassembling the alt axis (see here).  You would then place some sort of dovetail bracket (like this) in place of the large ring (keep the red friction disk between the gear and the dovetail bracket).  If you find the bracket does not allow the OTA's rail to clear the fork arm frame, you might need a spacer similar to figure 5 here.   This would then allow you to mount any suitable OTA with a dovetail rail, including the original OTA.

The original NexStar 4 GT can be adapted similarly to the other NexStar GTs but you will find removal of the existing OTA to be a bit of a challenge. Here is an outline of the removal process.  Basically you must first remove the hand control hook located in the fork arm.  Be careful not to loose the screws into the fork arm.  After removing the hook, you will see a nut attached to the altitude axis. Remove this nut while supporting the optical tube.  At this point, the optical tube, a large gear, and the shaft they both ride on will come free from the fork arm. To remove the gear from the optical tube, you must rotate the gear to align the three holes with the screws found below. The best way to do this is to put the assembly back in the fork arm and push down a bit on the optical tube.  Once you can access the screws, remove them and the OTA will finally come off of the bracket attached to the gear

Please note that the diameter of a scope is almost never the same as its aperture, so be sure to check before ordering that OTA of your dreams if you intend to mount it in any specific tube ring.

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Michael Swanson
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