Arrakis Observatory - Alternative STV Focuser and Mounting Methods

STV Focuser Improvements

The STV camera can be used with a telescope where focusing is done in the usual manner with the scopes native focuser. However, the STV is often used as a stand alone camera or autoguider with the SBIG Focal Reducer Lens and eFinder assembly. The eFinder is a thick-walled tube that screws into the female T-thread in the STV head. The Focal Reducer Lens, attached to a short extension tube, fits into the skyward end of the eFinder. Focusing this device consists of sliding the lens/extension tube combination in and out of the eFinder tube and locking it in place with two thumb screws once focus is achieved. Achieving accurate focus with this setup is challenging. A replacement for the stock SBIG eFinder is described below.

An alternative focuser for the Reducer Lens can be assembled from off-the-shelf parts. This consists of four parts, listed in order from the direction of the incoming light to the STV camera head:

SBIG FR237 Focal Reducer Lens (31.7mm male threaded base)
SBIG CFW8 Filter Cell Holder (31.7 female to 31.7mm male) (or any 1.25" filter)
Borg 7316 Adapter (31.7 female to 36.4mm P1.0 female thread)
Borg 7315 Helical Focuser (31.7 P1.0 male to 42.0mm P0.75 male T-thread)
STV camera head (42.0mm P0.75 female T-thread)

This combination of parts will focus the Reducer Lens at infinity at about the 2.2 mark on the Borg focuser scale and allows for closer focusing with additional extension of the focuser to its maximum count/travel of 10.

Focuser parts.
From left to right, the parts stack from sky to camera head.

Assembled focuser.

Assembled focuser mounted on the STV.

The dovetail bracket on the camera body replaces the stock SBIG Tripod Adapter.

It attaches to a female dovetail bracket on the guide scope's focuser housing.

The Borg parts can be sourced at http://www.sciencecenter.net/hutech/borg/focuser.htm

The SBIG parts can be sourced at http://www.sbig.com

A note of caution regarding the 1.25" filter cell. This piece is used as a spacer and just happens to have the right height and threads but was not designed for that use. Filter cells typically have a very short run of threads on the male end so they don't bottom out on an eyepiece or protrude out of the filter carousal they are usually used in. This can result in a poor connection when threaded into the 7216 adapter. I have one of these that works well in this setup but have seen another that had a somewhat tenuous connection and tended to "snap" out of the 7216's female threads when unscrewing it rather than rotating on its threads like it should. A nice solution would be to have a spacer made up with a deeper male thread. If the spacer was made just a little shorter, it would place the Borg focuser in the midrange of its travel when at infinity. If you are thinking that you might be able to come to focus without this piece as a spacer, it won't reach focus.

Using a filter cell as a spacer in this setup has the benefit of allowing imaging with any filters you might wish to use, including tri-color imaging, light pollution filters and IR blocking filters. The imaging train is also fully threaded which is always preferable over slip-fit thumb screw secured connections which are described below.

There are several other configurations that will also work. The filter cell holder could be replaced with a T-thread male to T-thread female spacer on the face of the camera head to achieve the correct CCD to lens distance. In addition, Hutech has posted an alternative which doesn't use a filter cell as a spacer. In that configuration, the standard 1.25" nosepiece that comes with the STV is threaded onto the camera and the Borg focuser slides over that and is secured with two thumb screws. That configuration should work well but is not fully threaded. It may be possible to use filters with that setup as well by screwing them into the nosepiece before sliding the helical focuser over the nosepiece and securing it with the thumb screws. A description of that solution can be found here.

The scheme for using the nosepiece rather than the filter cell is as follows, listed in order from the direction of the incoming light to the STV camera head:

SBIG FR237 Focal Reducer Lens (31.7mm male threaded base)
Borg 7316 Adapter (31.7 female to 36.4mm P1.0 female thread)
Borg 7315 Helical Focuser (31.7 P1.0 male to 1.25" ID female slip-fit tube with two thumb screws)
SBIG 1.25" Nosepiece (1.25" OD male cylinder to 42.0mm P0.75 male T-thread)
STV camera head (42.0mm P0.75 female T-thread)

A third alternative parts array is possible using a different adapter and reversing the helical focuser. Listed in order from lens to camera head:

SBIG FR237 Focal Reducer Lens (31.7mm male threaded base)
SBIG 1.25" extension tube (31.7mm female to 1.25" OD male cylinder)
Borg 7315 Helical Focuser (1.25" ID female slip-fit tube with two thumb screws to 36.4mm P1.0 male thread) (position reversed, end for end, from the above pictures)
Borg 7mm adapter with 36.4mm P1.0 female thread to 42.0mm P0.75 male T-thread (available from Borg, but no part number)
STV camera head (42.0mm P0.75 female T-thread)

Revised 11/8/02


	main tech notes page
STV Focuser Improvements The STV camera can be used with a telescope where focusing is done in the usual manner with the scopes native focuser. However, the STV is often used as a stand alone camera or autoguider with the SBIG Focal Reducer Lens and eFinder assembly. The eFinder is a thick-walled tube that screws into the female T-thread in the STV head. The Focal Reducer Lens, attached to a short extension tube, fits into the skyward end of the eFinder. Focusing this device consists of sliding the lens/extension tube combination in and out of the eFinder tube and locking it in place with two thumb screws once focus is achieved. Achieving accurate focus with this setup is challenging. A replacement for the stock SBIG eFinder is described below. An alternative focuser for the Reducer Lens can be assembled from off-the-shelf parts. This consists of four parts, listed in order from the direction of the incoming light to the STV camera head: SBIG FR237 Focal Reducer Lens (31.7mm male threaded base) SBIG CFW8 Filter Cell Holder (31.7 female to 31.7mm male) (or any 1.25" filter) Borg 7316 Adapter (31.7 female to 36.4mm P1.0 female thread) Borg 7315 Helical Focuser (31.7 P1.0 male to 42.0mm P0.75 male T-thread) STV camera head (42.0mm P0.75 female T-thread) This combination of parts will focus the Reducer Lens at infinity at about the 2.2 mark on the Borg focuser scale and allows for closer focusing with additional extension of the focuser to its maximum count/travel of 10. Focuser parts. From left to right, the parts stack from sky to camera head. Assembled focuser. Assembled focuser mounted on the STV. The dovetail bracket on the camera body replaces the stock SBIG Tripod Adapter. It attaches to a female dovetail bracket on the guide scope's focuser housing. The Borg parts can be sourced at http://www.sciencecenter.net/hutech/borg/focuser.htm The SBIG parts can be sourced at http://www.sbig.com A note of caution regarding the 1.25" filter cell. This piece is used as a spacer and just happens to have the right height and threads but was not designed for that use. Filter cells typically have a very short run of threads on the male end so they don't bottom out on an eyepiece or protrude out of the filter carousal they are usually used in. This can result in a poor connection when threaded into the 7216 adapter. I have one of these that works well in this setup but have seen another that had a somewhat tenuous connection and tended to "snap" out of the 7216's female threads when unscrewing it rather than rotating on its threads like it should. A nice solution would be to have a spacer made up with a deeper male thread. If the spacer was made just a little shorter, it would place the Borg focuser in the midrange of its travel when at infinity. If you are thinking that you might be able to come to focus without this piece as a spacer, it won't reach focus. Using a filter cell as a spacer in this setup has the benefit of allowing imaging with any filters you might wish to use, including tri-color imaging, light pollution filters and IR blocking filters. The imaging train is also fully threaded which is always preferable over slip-fit thumb screw secured connections which are described below. There are several other configurations that will also work. The filter cell holder could be replaced with a T-thread male to T-thread female spacer on the face of the camera head to achieve the correct CCD to lens distance. In addition, Hutech has posted an alternative which doesn't use a filter cell as a spacer. In that configuration, the standard 1.25" nosepiece that comes with the STV is threaded onto the camera and the Borg focuser slides over that and is secured with two thumb screws. That configuration should work well but is not fully threaded. It may be possible to use filters with that setup as well by screwing them into the nosepiece before sliding the helical focuser over the nosepiece and securing it with the thumb screws. A description of that solution can be found here. The scheme for using the nosepiece rather than the filter cell is as follows, listed in order from the direction of the incoming light to the STV camera head: SBIG FR237 Focal Reducer Lens (31.7mm male threaded base) Borg 7316 Adapter (31.7 female to 36.4mm P1.0 female thread) Borg 7315 Helical Focuser (31.7 P1.0 male to 1.25" ID female slip-fit tube with two thumb screws) SBIG 1.25" Nosepiece (1.25" OD male cylinder to 42.0mm P0.75 male T-thread) STV camera head (42.0mm P0.75 female T-thread) A third alternative parts array is possible using a different adapter and reversing the helical focuser. Listed in order from lens to camera head: SBIG FR237 Focal Reducer Lens (31.7mm male threaded base) SBIG 1.25" extension tube (31.7mm female to 1.25" OD male cylinder) Borg 7315 Helical Focuser (1.25" ID female slip-fit tube with two thumb screws to 36.4mm P1.0 male thread) (position reversed, end for end, from the above pictures) Borg 7mm adapter with 36.4mm P1.0 female thread to 42.0mm P0.75 male T-thread (available from Borg, but no part number) STV camera head (42.0mm P0.75 female T-thread) Revised 11/8/02