There are a number of issue that can arise if any extension cables are inserted between the SBIG power supply and the camera. Anything that decreases the voltage applied to the camera itself can be a potential problem. Of course in some cases, such extensions are necessary to work with the a specific mount/OTA etc. I spent an inordinate amount of time uncovering and resolving these issues and thought I would document the various causes and effects you might see as an aid in diagnosing your specific situation.
The AO-7 seems to be the first level of sensitivity to voltage drop problems caused by extension cables. This will show up as the guide star moving diagonally in increasing amplitude from corner to corner of the guide chip until it is essentially hitting the limits. Successful AO-7 guiding is impossible in this situation. This can sometimes happen even if no extension cables are used. See the SBIG fix below. In my case, this was resolved by applying the SBIG fix to my ST-8E camera, replacing the power cable in the Paramount ME with an 18 gauge one, and modifying the SBIG power supply to increase the 5 volt output by 0.2 volts.
Even with the above changes, I had noticed a noise pattern in images after extreme stretches of the data, when using an ST-10XME. This turned out to be most visible in a stretched and magnified image of a 10-minute 1x1 dark field. I believe this is a good, sensitive indicator of voltage drop problems or even interference issues from other sources. The approach consists of taking a 10-minute dark field with the camera connected directly to your computer and the power supply plugged directly into the camera. No extension cables on either the USB cable or the power cable. Critically examine various parts of the resultant image by stretching the image to reveal the lowest level pixels and magnifying the image so that the individual pixels are visible. This can be done in most of your image acquisition programs. Once this is done, locate the camera on your scope, using whatever extensions you plan on using and power up the scope. Take another dark field and critically examine it with the same stretch and magnification. If it looks like the first image, you are home free and good to go. If not, remove additions one at a time to identify the culprit. If it turns out to be power-related, see the Fixes section, below. If the problem is still there with the camera powered directly from the SBIG power supply, you may have a bad USB cable, noisy USB extender, etc.
Shown below are some typical issues I ran into as I investigated these problems. The images were taken with an ST-10XME and each frame represents the lower left 76 x 48 pixel area of a 10 minute, 1x1 dark frame at -10°C. You should examine all areas of your dark field to see if there are any localized problems.
||Dark field Image Result|
|Image 1 was taken with a direct
connection to the PC in my office with no extensions on the power supply
cable. Note how sharp the hot pixels are. This image was
unchanged as I tried different cables an extenders. I saw no change
between the Icron Ranger and a daisy-chained combination of 3 16-foot
Image 4 was taken with the camera mounted on the OTA and powered through the mount with 18 gauge internal wiring. The mount was not powered up. Note the vertical smearing. In a reduced image, this shows up as increased noise as the image is stretched to reveal low level detail. For me the smearing was even worse when the camera temperature was lowered to -20°C.
Image 5 was taken in the same scenario as Image 4 but with all the mount electronics powered - ME, TCC, RoboFocus, etc. It may not be apparent in the small images but there is a very slight increase in smearing. I was able to impact the levels of this noise by changes in grounding of the camera and mount.
It should also be noted that there are many situations where you can be "on the edge". For example, there is anecdotal evidence that operating at cooler duty cycles over 90% "adds noise". The voltage drop issue is made worse by the added current of the cooler being fully on giving more voltage drop and perhaps pushing you over the edge. For example, using the 18 gauge internal wiring of the ME with the SBIG AC power supply, the critical +5 volt line measured at the camera changes from 4.85 volts (cooler on) to 5.3 volts (cooler off). That is a huge change! With a stiff power supply located close to the camera, I suspect one could operate at 100% duty cycle without issue. For example, with my dc-dc converter described below, the voltage change with the same camera went from 5.3 volts (cooler off) to 5.2 volts (cooler off). Another situation could be with a poor connector, i.e., not fully seated or dirty contacts. This could give some intermittent performance, sometimes good, sometimes bad, depending on even the temperature. Hopefully with this diagnostic approach, you can nail these issues down.
No Extensions: This is the obvious solution. One ingenious approach to this problem was done by Mike Rice and Ron Wodaski at Ron's remote telescope in New Mexico. Mike was able to feed the standard length SBIG power cable through the Paramount ME mount and still have enough left over to be usable with his camera.
SBIG Camera Modification: SBIG has a fix that will improve this situation on the new or reworked USB cameras. I don't know when this change was cut in to production. The change is to the digital board. This is the board you see through the slots in the rear of the camera and is not the CCD sealed area. You can remove the back plate easily enough and see if the fix is made by comparing your board around the AO-7 connector to the picture below. The change consists of shorting out two ferrite filters in the power lines to the AO-7 and has been shown to be effective in most cases. I did this myself but if you have any doubts about your ability, I suggest you return the camera to SBIG for them to do it. Here is a before and after image showing how Jeff Hapeman solved the problem.
The circled area shows the two parts, FB1 and FB2 shorted out. SBIG recommends shorting wires but Jeff's approach worked for him. If you have any concerns about warranty, you should contact SBIG and/or have them make the change for you.
Paramount ME Internal Cable Replacement: If you are using a Paramount ME and plan to use through-the-mount wiring, be sure the internal wire gauge is 18 gauge. Early production units were done with 22 gauge. This had worked acceptably on the earlier version of the Paramount's but as cameras begin to require more power and/or less voltage drop, 22 gauge is no longer acceptable. A replacement internal cable is available from Software Bisque. Be warned that even this may not solve the ST-10XME issue. Check your dark field to be sure.
Adjustable AC Power Supply: Mike Rice at New Mexico Skies does a modification to the standard AC power supply that allows adjustable output voltage. This can compensate effectively for any voltage drops in cabling.
SBIG 12 volt power supply: Obtain the 12 volt power supply camera from SBIG and mount this power supply on the OTA. It puts the regulation point close to the camera so no extensions are needed. You then run 12 volts DC to the OTA, which is less sensitive to voltage drop. Thanks to Bill McLaughlin for this idea. Most observers seem to have 12 volts there any how so this may be a matter of slightly increasing the wire gauge. This approach or Ron's approach are the most sure fire way to avoid these problems.
DC - DC Converter: I have built a dc-dc converter that is a bit smaller than the SBIG 12 volt supply. It measures 2" x 2.5" x 5" and weighs less than a pound. This is what it looks like mounted on a 10RC truss. It is built in the ubiquitous Radio Shack project box and has an accessory 12 volt jack to power anything that may need it. It could be even smaller but the Radio Shack box was handy. It seems to be the ultimate solution for what I want. Regulation is right at the camera, efficiency is around 80% and it is light enough to mount to the OTA via a single 10-32 screw.
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