SC-3 Focus Adjustment

I purchased a new AC4040 camera and SC-3-SHORT guider in September, 2021, but am just now installing it in my NM remote observatory due to medical issues. They are replacing an STX-16803 (failing sensor) and STX Guider (intermittent guiding problems, which I found to be due to pickoff mirror adhesive coming loose, after I removed it from the image train).

With the SC-3 focus knob fully CW (against the back stop) it will not focus unless I move the RCOS 20" RC f/8.1 secondary mirror focuser about 1000 counts (0.635 mm) away from the primary mirror. That allows the SC-3 to focus okay and autoguiding is fine. But of course now the main camera is very out of focus.

Accordding to the latest SBIG Backfocus Calculator spreadsheet (V 1.4, 2022/04/11) the optical distance from the AC4040 sensor to the front of the SC-3 (OTA side) is 2.981" (with an FW7-STX with 3mm filters). With the SC-3 pickoff mirror adjusted to just eliminate any shadowing of the AC4040 sensor, I measure its backfocus distance from sensor to the middle of the pickoff mirror, then to the front of the SC-3 housing at 2.69".

There is no way I can reduce the backfocus distance of the AC4040 relative to the SC-3. I was going to add a temporary 0.21" thick spacer between SC-3 and the OTA today, but that will increase the backfocus distance for both cameras, which doesn't seem like a solution.

The complete optical path from the back of the RC OTA is: RCOS PIR rotator with FFC lens, a custom adapter 1.315" long to adjust the total back focus distance required by the scope design for the AC4040 camera's short BFD, an AO-X, the SC-3, FW7-STX with 3mm filters and the AC4040.

Any suggestions on how increase the focus range for the SC-3? It was against order # 0004057. The picking sheet shows Lot # SC1300M-20112506.

I live in NC and will only be in NM until 05/31 when I head back to NC.

The SC-3 also has very poor retention of the AUX DIN style connector. The plug's shell has a dimple in it which appears to be meant to mate with a spring loaded bump inside the receptacle, but there is nothing there. I mounted an adhesive cable tie holder on the edge of the filter wheel and used a cable tie to secure the AUX cable to the AO-X, which works much better now. Just wondering if the AUX receptacle is defective or a poor design??

Steven Ahlbom
Chapel Hill, NC
Observatory: Mayhill, NM

 

Is it possible for me to reinstall my STX Guider after I repair the pickoff mirror adhesive until this gets sorted out?

My DC Extension connector is fine. It is the AUX (to AO-X) connector that won't stay in. The plug doesn't have a retracting hood to release the latch. It looks like there should be some type of simple spring loaded lever inside the receptacle on the SC-3 PCB that engages with the dimple on the shell of the plug that goes into the AUX connector. It is so loose that the AO-X stops working due to the loose cable after just a couple of slews -- until I secured it with a cable tie and adhesive mount (which must be removed to open the filter wheel).

Sorry to hear about your storm damage. New Mexico's disaster is all of the wildfires and smoke.

 

Colin,

I see the updated configuration guide that now shows I need the SC-3-LONG. However the Star Chaser Product Data Sheet dated January 2021 still shows the SC-3-SHORT is used with the Aluma AC cameras, with no caveat about the FW7-STX. That should be corrected.

 

That's fine. I have a friend staying at my NM home who I am training on how to work on the image train. He will be available to receive the package, install the new SC-3-LONG and return the SHORT one. Keep me posted. Maybe we can move this offline at this point to arrange the details.

I also had the stuck shutter problem on the AC4040 last night while taking darks and flats. One blade was only 2/3 open. Per other posts on this topic we adjusted the stop post (its screw was loose already), but that wouldn't prevent the blade from retracting completely. There was zero clearance at the rear of the blades, so we adjusted the top blade on the motor shaft so that there was 0.010" clearance between the top and bottom blades when closed. Hopefully that will fix it. I understand there is an improved shutter design available. Is that something that I should get or are the adjustments adequate? I will test it tonight to see if it seems okay and let you know.

Thanks,
Steve