Posted 18 July 2014 - 11:16 AM
Bruce was interested in the process. So... here is goes. This is for science stuff: astro-art of doubles is a completely different skill set and one I am not good at. Also note that this is the way I make observations and not meant to suggest it is the only way or even best or most efficient way.
1. Two important steps. First, complete collimation. I use MetaGuide. Second, polar alignment. If you are not exactly polar aligned it will throw your camera angle off as you move from one region of the sky to the other. Tip: try to re-calibrate when you move the scope to a new region.
2. Stuck at the end of the Crayford focuser is a flip mirror with eyepieces fitted with rings to be parfocal to the video camera and the video camera in line with a 2x-5x barlow. (F10? -- minimum is 3X; F20? you can use 2x) Focus of the video camera via a mask on a close bright star at regular intervals if possible.
3. Eyepiece/flip mirror find the double. I do go-to: this is science not pleasure: get to the target by the faster means possible. Experience shows where the double is relative to the camera (don't think its always at the center shown by the cross hairs!).
4. Flip the mirror, you should see the double integrating at, say, 250ms. If not increase the integration until you acquire. Prepare to be frustrated -- acquiring doubles at F30-F50 on a 1/4" chip is a acquired skill.
5. Begin decreasing the integration time. Do so until you can barely see both the primary and the secondary. As integration time decreases the stars will begin "boiling" or "dancing." We are sampling within the point spread function and seeing "super speckles." If there is a magnitude difference, concentrate on the secondary. This can be anywhere from 1ms to 1000ms depending on the magnitude. Wide pairs do not require fancy techniques, the closer the pair, the faster the integration required. Experimentation is necessary. The main worry is saturation of the primary. When beginning take video stream of different integration times and keep a record of saturation versus integration time. Remember -- this is science not art: as you decrease integration time the pictures start to look really ugly, some of mine are just scatters of pixel hits and others look like boiling masses.
6. Take 1000-4000 frames for close pairs, as few as 100 for wide pairs. If you can take them as fits (Firecapture), so much the better, If not, then you will have to post-process to bmp. (I am assuming a PC, ask Dave about Macs).
7. For an actual session the first pair imaged is a calibration pair. You want something wide, not close and in the region you are working. Rectilinear pairs or wider Grade 1 or grade 2 orbital pairs are good, many of the WDS calibration pairs are more suited for larger scopes. I try for 10-20" separation and 100-200 frames.
8 Finally, program stars. I usually work one region at a time and if I make large move I take another calibration pair. In fact, I take calibration pairs at regular intervals.
9. At the end of the run, take another calibration pair to insure no camera movement.
10. Post-processing and measuring is well covered in the REDUC tutorials by Florent Losse if you use that program.
Now, much depends on those calibration stars as they are used to determine camera angle and plate scale. Best practice is to take several. Check to see how well each agrees with the calculated theta and rho for that night and take the average. Odd balls do pop up, so do not go blindly into the night. The best way for the beginner to begin is to measure only brighter and well separated (>5") rectilinear pairs. Why? Because you can compare your results with the predicted results of professionals. If your measures fall within the errors of an O-C, then you gain confidence that your measures have scientific worth. You can calculate the predicted theta and rho simply, it ain't rocket science.