900 Mount Upgrades and Spare Parts

Since the introduction of the 900 German Equatorial in late 1994, we have offered successive generations with improvements in both the mechanical construction and drive system. Since our design and production staff includes experienced CCD imagers, we are continually pushing the mounts to the limits of performance. Equally important, our customers have offered valuable feedback and suggestions throughout the years.

We are pleased to offer a variety of upgrades and spare parts so that you can obtain maximum performance from the 900 mount that you own. Some of these upgrades are available for all 900s and some are appropriate to particular models, but not others. Please check the product descriptions carefully to determine if a particular upgrade or component can be used with your mount. If you are confused by the HDA, QMD, SMD and GTO designations of the 900 mounts, please refer to a brief history of all 900 models. If you require a spare part not listed below, please do not hesitate to call.


900 Hi-Lo Latitude Wedge (900WDG) and Wedge Assembly (900WDGA) for 0-20 or 68-88 Latitude
If your latitude is below 20 or above 68 degrees, one of these wedges will allow you to place your 900 mount in the proper position for polar alignment. You will use the wedge in place of the Standard Pier Adapter (900SPA) or Precision-Adjust Rotating Pier Adapter (900RPA) shown above. The 900 Wedge Assembly (900WDGA) comes complete with the azimuth adjustor block, the center pivot screw and the four locking knobs with their hardened washers. Those of you who already have a Standard Pier Adapter can order the 900 Wedge by itself (900WDG). Just transfer the azimuth adjustor block, the center pivot screw and the four locking knobs supplied with your original adapter to this wedge and bolt it onto your pier. Both of the wedge pier adapter configurations include six 5/16-18 x 5/8 socket buttonhead screws and six 5/16 x 9/16"OD x 0.060" flat washers, which enable you to attach the pier adapter to your Astro-Physics pier.




Please note that this adapter must fit inside another part and be bolted from the side. It cannot be bolted directly onto a flat surface. For custom installations onto a flat surface, insert the 900WDG into our our 900 Flat Surface Adapter (900FSA), which has been bolted to the flat surface of your pier.

The 900 Hi-Lo Latitude Wedge can be set up for either tropical (0-20 deg.) or polar (68-88 deg.) latitudes. Simply remove the 6 screws that attach the flat plate, rotate the plate so the Azimuth Adjuster Block will either be on the low side (tropical 0-20 deg.) or on the high side (polar 68-88 deg.) and re-assemble.

The 900 Hi-Lo Latitude Wedge can also be used with the 900 Precision Adjust Rotating Pier Adapter (900RPA) to make polar alignment easier for mounts used in the field. The Wedge has been drilled and tapped to accept the 900 Flat Surface Adapter (900FSA). Attach the Flat Surface Adapter to the top of the Wedge with four 1/4 - 20 x 3/4" socket head cap screws (not included). If you have the 900 Wedge Assembly, just remove the four Locking Knobs and the Azimuth Adjuster Block and leave the center pivot screw in place so it won't get lost! Then install the Precision Adjust Rotating Pier Adapter into the Flat Surface Adapter. Note: The photo is of an earlier version and does not show the holes that accept the flat surface adapter.

Mounts: All 900 mount models (HDA, QMD, SMD and GTO) that have been produced.

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900 Flat Surface Adapter (119FSA)




Use this Flat Surface Adapter to attach either the Precision-Adjust Rotating Pier Adapter (900RPA) or the Hi-Lo Latitude Wedge (900WDG) to a flat plate on your custom made pier or tripod. You can also use it with the Standard Pier Adapter (900SPA) to give a more finished appearance.

The 900 Flat Surface Adapter is precision machined aluminum. It has been polished smooth and then anodized black.

The Flat Surface Adapter bolts onto the flat plate on top of your pier or tripod, then the Pier Adapter or Hi-Lo Latitude Wedge simply slips in (just as it fits into our pier) and you fasten from the side with the six screws and washers provided with the Pier Adapter.

The bolt circle for attaching the 900 Flat Surface Adapter to your pier is 7.230" in diameter. This is the same size bolt circle as is found on our Standard Pier Adapter (900SPA), so if you are replacing a Standard Pier Adapter with a Rotating Adapter or Wedge, you can use the same holes. The holes in your flat mounting surface must form a square with sides running east - west and north - south as opposed to a diamond (45° rotation) with diagonals running east - west and north - south.
The holes for mounting the 900FSA to your flat surface are counter-bored to accept the head of a standard ¼" socket head cap screw (13/32" or 0.406" diameter).   This allows the Pier Adapter to sit perfectly in the Flat Surface Adapter.
Because installations will vary, we do not provide the eight screws for mounting the Flat Surface Adapter to your pier or tripod. Your pier's top plate should be a minimum of 7.75" in diameter. The 900 Flat Surface Adapter itself is 8.375" in diameter

Mounts: All 900 mount models (HDA, QMD, SMD and GTO) that have been produced and the 1100GTO mount. Note: although the older 800 mounts had the same diameter base as the 900 series, their bases are too thick for this adapter. DO NOT purchase this adapter for an 800 mount!

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Right-Angle Polar Alignment Scope Adapter for 900GTO (RAP900)

This adapter fastens to the polar axis rear plate on the 900 mount, allowing use of our Right-Angle Polar Alignment Scope (RAPAS). The RAPAS can be used with all 900 mounts. This adapter is push/pull adjustable for precise alignment. Once adjusted, it will retain its precision, allowing the polar alignment scope to be removed and re-inserted as needed. Includes three replacement (longer) socket head cap screws. Please see this link for additional information.

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Pier Adapter Knob Kit for 900

9KBKIT Knobs with Washers

This kit contains four machined and black anodized Pier Adapter Knobs and 1/4" ID x 1" OD flat washers. If you do not purchase one of our pier adapters shown above, for instance, if you purchase the Monolith Pier from Particle Wave Technologies, you will need these knobs to attach your mount to the Monolith.

The kit can also be purchased as spares if you lose or damage the knobs that came with your Standard Pier Adapter (900SPA) or Precision-Adjust Rotating Pier Adapter (900RPA). Please note, a complete set is included with your pier adapter when you purchase it.

Mounts: All 900 mount models (HDA, QMD, SMD and GTO) that have been produced.

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900 Altitude Adjuster Assembly (9ALKIT)

Individuals who own the very earliest 900 mounts may want to consider upgrading to this more substantial altitude assembly. You may also consider purchasing just the Altitude Adjustor Brace (9ALBKIT) described below if you want to reuse the original Altitude Knob, Threaded Rod and other components, which are common to both designs. Installation instructions.

Includes: Our part # S900AA-A, four 1/4-20 x 5/8" flathead socket cap screws.

Mounts: All 900 mounts (HDA, QMD, SMD and GTO) that have been produced.
Note: This Heavy Duty Altitude Adjuster Upgrade is now included on all new 900GTO Mounts and does not need to be added to a new 900GTO purchase.

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900 Altitude Adjuster Brace (9ALBKIT)

If you do not wish to replace the entire Altitude Adjuster Assembly with 9ALKIT above, you may wish to consider purchasing just the brace and installing the other original parts yourself. Installation instructions.

Includes: Our part # M9060-A and four 1/4-20 x 5/8" flathead socket cap screws.

Mounts: All 900 models (HDA, QMD, SMD and GTO) that have been produced.
Note: This Heavy Duty Altitude Adjuster Brace Upgrade is now included on all new 900GTO Mounts and does not need to be added to a new 900GTO purchase.

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900 R.A. Clutch Set Screws (F0021), recommend two

If you operate your mount remotely and are concerned about cables snagging on the RA clutch knobs, you may wish to remove them and substitute our modified set screws. Please do not attempt to install standard set screws, since they must be a particular length and profile to work properly. If you install the modified screws, you will have to use the 5/32" allen-head wrench from the set included with your mount to loosen and tighten them. Please tighten evenly with respect to the opposing clutch knob and do not over tighten. These set screws are only necessary for the side of the mount that has the cable. The normal clutch knobs will remain on the other side. Please note that each mount requires only 2 modified clutch set screws.

Mounts: All 900 mount models (DA, QMD, SMD and GTO) that have been produced.
Note: This is an optional item and is not included with new 900GTO purchases.

Please note that shipping on this item is extra.

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Replacement Assembly for 900/1200 Dovetail Knob (DOVEKBREP)

To reduce the possibility of a cable snag while operating your mount remotely, you may wish to remove one of the dovetail knobs that secure the Dec axis to the R.A. axis, and substitute it with our replacement assembly.  The assembly would only be needed for the side of the mount that has the cable.  The standard dovetail knob will remain on the other side.  You will only need to purchase one per mount.

Use the 3/16" allen-head wrench included with your mount to install and adjust tightness. Tighten to approximately the same tension as the corresponding knob on the other side.

WARNING: Do not over-tighten the knob replacement assembly!

Note that a small portion of the assembly will protrude from the side of the mount. This is normal and should not interfere with the mount cable during remote operation.

Please refer to the image at the right.

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900GTO Name Plate (M12611E9G)

Engraving on the name plate has been changed to "Astro-Physics 900GTO Servo Drive". The original SMD (servo motor drive, but not go-to) and GTO (servo motor drive with go-to) versions of the mount shared the same name plate "Astro-Physics 900 Servo Drive". Note that the GTO designation was not there. We decided to more clearly identify the mount and offer the new name plates to prior GTO owners, as well. You will need two name plates, one for either side of the Dec. axis.

Mounts: 900GTO only. We will request the serial number of your mount when you order to ensure that this is the proper name plate for your mount.

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900GTO Quick Release Control Box Adapter Kit (9CBKIT)

If you are upgrading your 900GTO from the GTOCP1 Control Box and plan to attach it to the RA axis, you will need this adapter kit. It features a dovetail attachment so that the GTOCP4 (with the CBA912KIT attached) or GTOCP3 control box can be easily removed. Simply loosen the two thumbscrews and lift the control box from its position. We also provide two set screws if you prefer to use them.

If you wish to mount the new control box on the side of the pier, consider the Pier/Tripod Control Box Adapter (CBAPT).

Includes: Our part #s M9396-A, E0262, two 8-32 x 1/4" socket set screws, two 8-32 x 3/8" thumbscrews (to use in place of the set screws, if desired) and three 10-32 x 3/8" socket buttonhead screws.

Mounts: 900GTO only.

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R.A. Sight Hole Cover - for 900GTO mounts shipped since October 2005 (M9403)

This sight hole cover threads onto the back of all 900GTO mounts shipped since October 2005. One of these aluminum covers is included with all new mount purchases, but now they are available separately for anyone who needs one.

The rear of the 900GTO R.A. axis was redesigned in October 2005 to be slightly larger than the 1200s, 400s and earlier versions of 900s. This change resulted in the need for a larger diameter cover.

If you are uncertain when your 900GTO shipped, please use the photos (right) as a reference for where to measure on your mount to determine the correct cover size. We used a 1200GTO since it was readily available; however, the measurement points are identical on older 900 mounts.

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900 Declination Axis Upgrade (900DU)

For all previous model 900 mounts, we can upgrade the bearing system in the Dec axis to the present standard. This will allow full tracking and guiding with excellent precision in all mount orientations including upside down operation well past the zenith. In addition, we will add a spring-loaded bracket, modify the worm bearing support for more precise worm reversals and check out all guiding functions with our test equipment. List of things we will install:

  • Add new thrust bearing to the Dec worm wheel
  • Add preload system to shaft bearings
  • Add spring-loaded motor box bracket
  • Modify the worm bearing support and adjust preload
  • Set all preloads to factory specifications
  • Re-grease worm wheel and motor reduction gears
  • Test the fully assembled Dec. axis for precise reversal motion

Should I upgrade my mount?: It is only when you are trying to control the mount to arc second position accuracies in declination that it becomes important to have this axis modified to the latest configuration. If you don't fit this profile, the upgrade is not necessary. This is a very expensive upgrade procedure and will only be done to mounts that exhibit a problem in the Declination Backlash Test.

Mounts: All 900 mounts with serial number 900450 and earlier numbers, as well as 900501. Mount models include 900HDA, 900QMD, 900SMD and 900GTO models. All mounts with later serial numbers were built with these upgrade features at the time of production.

This upgrade is very time-consuming, so several mounts must be done at one time in order to be cost effective. Since we are providing this upgrade with no profit, we ask that you arrange to send your mount in according to our upgrade schedule. If you require that your mount be upgraded at a different time, there will be a surcharge.

Procedure for upgrade:

  • Find the serial number of your mount. It is stamped on a metal plate that may be located on the inside of the polar forks or in one of the honeycomb cutouts where the RA and Dec axes connect (you have to take the mount apart).
  • Determine that there is a problem. You must run the Declination Backlash Test. Download and install PulseGuide from this link.
  • If this test shows a problem, send the results to If we determine that the 900DU can improve performance, we will add your name to the list of people requesting the upgrade. Once the upgrade is scheduled, we will issue a Return Authorization number.
  • Send the dec axis to arrive between the dates specified below. We will begin the repair right after these dates, so it is important that all parts be here on time. If your dec axis is late, there may be a surcharge (to cover additional setup costs) or it may have to wait for another time. Be sure that you pack it well. We suggest that you wrap it in a thick layer of bubble wrap, particularly around the motor box area and put it in a box with packing material. Then, put that box in another larger box, again with packing material between the boxes. That will assure safe transport.
  • Assuming that the process goes smoothly, we should be able to ship it in a week to 10 days (allow additional time for shipping). Please note that shipping and insurance charges are additional. We normally ship ground, so please let us know if you want it shipped by a faster service.

Upgrade Schedule: June 26, 2015

The Dec axis MUST arrive between the dates below so that we can update your mount during the full moon time, for your convenience. Please read the procedure above.

Dec axis MUST ARRIVE between June 22-25, 2015
Upgrade begins June exceptions!

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This page was last modified: July 30, 2018

Astro-Physics, Inc.
11250 Forest Hills Road, Machesney Park, IL  61115, U.S.A.
Phone: 815-282-1513   Fax: 815-282-9847