DSI Pro Mods

Astronomical Photometry with a DSI Pro HPO Software Learning Desk Hopkins Phoenix Observatory Phoenix, AZ 5 September 2006 Abstract The color Deep Sky Imager (DSI) cannot be used for astronomical filter photometry, The DSI Pro can. To do serious work requires some simple modifications to the camera.

These modifications include replacing the filter slide with a filter wheel and adding a simple thermoelectric cooler (TEC). With these modifications the DSI Pro (and DSI Pro II) provide a means to do professional astronomical photometry with a modest telescope even in a light polluted suburban location. This paper describes these modifications and what is needed to do BVRI CCD photometry with the camera. Figure 1 Modified DSI Pro on Meade 12” LX200 GPS Telescope While CCD cameras for astronomical use have been around for more than a decade, it has only been recently that affordable and easy to use CCD devices have been made available to the amateur market.

Some of the first of these were web cams. Some folks figured them out and made modifications that produced surprisingly high quality results. Web cams work best with brighter objects such as the planets, Moon and solar imaging.

For deep sky imaging the web cams need to be modified. These modifications are beyond many amateurs. When Meade introduced the color Deep Sky Imager (DSI), that changed.

Now one could do deep sky imaging without needing to make modifications to the camera. In addition, the AutoStar software that comes with the DSI series is excellent.

It’s worth the price of the camera alone. Learning the software requires some dedication as the quality of the software far exceeds the documentation. Two problems prevent the color DSI use for filter photometry. Because the color DSI CCD chip has built-in RGB filters, it would be hard or impossible to find proper filters to match the camera to the standard photometric system.

The other problem, while mainly a problem during the warmer months, is that the camera is uncooled and operates at ambient temperature. Shortly after the color DSI was released, Meade released a monochrome version, the DSI Pro.

The DSI Pro provides a good starting point for a serious CCD photometry. Because the camera is monochrome a 4-filter slide for RGB filters is included. The filters are extra. The advantages of using the DSI Pro with external filters are the monochrome camera is more sensitive and provides better resolution.

While the included filter slide can be used to hold photometric filters, it quickly becomes apparent that is not a good idea. The photometric filters are expensive and the filter slide is open exposing the filters to dust, dew, fingerprints and scratches. A filter wheel is a much better way to hold and protect the filters.

The first step is to replace the filter slide holder with a low profile adapter plate similar to that used on the color DSI, see figure 2. This gets rid of the large openings where the slide goes and reduce the distance to the CCD chip. These adapters can be purchased from ScopeStuff at http://www.scopestuff.com/ for $37.

Note: The original 1.25” nose piece can be used with the new adapter, but it’s better to mount the filter wheel closer to the camera than allowed by the nose piece. Replacing the original slide holder adapter with the low profile adapter plate is very simple.

There are 4 small Phillips-head screws that hold the adapter in place. Those screws and the original adapter are removed and replaced with the low profile adapter and new shorter screws. Note: There are 4 hex-head screws at the corners of the camera. The hex-head screws hold the camera together.

Do not remove those screws for this modification. Figure 2 Original DSI Pro with filter slide and low profile/original adapters With the low profile adapter installed on the DSI Pro the next step is to add a filter wheel.

There are several on the market that range for well under $100 to close to $1,000. An excellent compromise is the ATIK Manual Filter Wheel. This can also be purchased from ScopeStuff for $199. The filter wheel is well-made and can hold up to 5 - 1.25” screw-in filters.

Figure 3 shows the camera with the ATIK filter wheel and F/3.3 focal reducer. The reason for the focal reducer is to provide a larger field of view for the imager. This makes finding the stars and getting comparison stars in the same view much easier.

Figure 3 Modified DSI Pro with filter wheel and F3.3 Focal Reducer For most photometry special filters must be used. The RGB filters for the DSI Pro and DSI Pro II cannot be used as photometric filters. For CCD photometry 5 photometric filters are available.

While there are other filters, these are the most common used. They are ultraviolet, blue, visual, red and infrared (UBVRI). Unless you are planning on using a meter-sized telescope, the U filter will not be of much value. as the DSI Pro CCD is not sensitive in that band.

Plan on using just the BVRI filters. There are several places you can buy filters. Astrodon has the least expensive yet good quality filters at $309 for a set of BVRI filters. See Schuler UBVRI Photometric Filters from Astrodon at http://www.astrodon.com/products/product.cfm?CatID=4 The DSI series cameras use ambient cooling.

With cool night time temperatures these units perform well. Dark frames must still be created and subtracted from each image to get rid of hot pixels, however. During the summer months in Phoenix, Arizona observatory temperatures can approach 100 degrees Fahrenheit at midnight.

With these temperatures, even subtracting dark frames does not work well. Adding cooling to the DSI Pro started as an experiment to see what could be done at a minimum cost and amount of modifications. Note: This cooling modification can be used on all the DSI series cameras.

In addition to reducing dark current cooling increases the camera’s sensitivity. Perhaps the easiest way to cool the CCD is using a Thermoelectric Cooler (TEC). TECs are fascinating devices.

TECs consist of a sandwich of semiconductors between two metal plates. By applying a DC voltage across the plates, one plate gets hot while the other one is cooled. It can actually be surprising the amount of heat and cooling produced.

Care must be taken when experimenting so that the unit doesn’t overheat. A heat sink is very important on the hot side. Thermal grease is needed on both plate surfaces to provide effective transfer of the heat and cooling.

Some will note that there is no temperature regulation or sensor. These could be added, but add to the complexity. The lack of temperature regulation is not a problem. If the camera is allowed to run with power on and cooling for 15 to 30 minutes, the temperature will stabilize.

The exact temperature is not important. Once the unit has stabilized new dark frames for that evening should be taken. This procedure works well. For parts I recommend the following: 40 mm TEC, part number PJT-7 @ $14.75 12 VDC 3.5 A power supply part number PS-1231 @ $15.85 Heat sink with 12 VDC fan, part