Fiber Mapping

Tip 1: Make sure you have flux!

No flux = no fringes. Map often. If you notice the fringe contrast dropping, check the flux trend. In addition to keeping an eye on fringes on PSTrend on rtd, also keep an eye CutRow + FlxTr to keep an eye on the flux.

If you notice it dropping, remap or ask if the beacons have drifted. Avoid recording data with fluxes that have dropped significantly. Make sure to remap before shutters, too. Page 13, 35.

Tip 2: Take backgrounds while slewing instead of after the star is locked

Backgrounds [TAKE BG] should be done while slewing to avoid losing observing time once a star is locked and ready to fiber map. You can also slew while the backgrounds of the shutter sequence are being done to save time. Pages 16 and 35.

Tip 3: Use an appropriate size fiber map after the flux has been centered.

Use a larger map (Nstep 7-11) to find the flux, but once you do, use the smallest map you can (Nstep 5-7) to keep it centered. Maps take time and often small maps can keep flux well centered and keep you observing efficiently. STST alignments will keep the flux well centered and avoid the need for large maps. Page 24.

Tip 5: Avoid mapping only one or two beams when several others have dropping fluxes.

If you are not recording or actively searching for fringes, consider mapping all beams to get the peak flux on all beams rather than doing them one at a time. If you are recording data and flux begins to drop, consider doing small 3×3 maps (NSTEP=3) with step size of 3 um and “Goto bary X” to move to barycenter of light (page 35).

Tip 6: Center the fluxes in the fiber map when all are present, even when some do not have a Gaussian fit.

A Gaussian fit is not needed to center the flux. If the starlight is clearly present, use Goto max + and then map smaller to get it well centered. Doing a larger map to try to get a Gaussian fit on all beams will take extra time in this situation. Page 24.

Fringe searching and locking

Tip 7: Lock or center fringes as soon as stars are locked or fiber maps are done.

Fringes often appear before you are ready to search for them. Do not let them get away. You can center them, note the offsets, or click the x in the matrix so you do not loop the carts away from the correct offset once you are ready. The PS Trend is important to see these fringes. Have it open when the stars are being locked if you can. Pages 26 and 27.

Tip 8: Adjust cart speed based on fringe SNR.

If the fringes you already found are strong you can use a larger step size when looping the carts (10 µm steps for Prism50) to cover more area quickly. For low snr fringes, make the step sizes smaller 2 µm. In high spectral resolution, the cart offset can be jumped instead of looped to cover the area more quickly. Steps of 1/3 the spectral resolution work well, i.e., 80-100 microns for Mystic Prism 278. Use the < > and « » buttons for large jumps of the cart. Pages 30-32

Tip 9: Do not loop the carts several millimeters from zero without a good reason.

Unless the baseline solution is old, the offsets will be near zero. If you did not find fringes +/- 1 to 2 mm from zero, make sure your fluxes are good and that there are no other issues before looping far away from zero. Page 29.

Tip 10: Do not loop carts far away from known offsets.

If you were already locked on fringes and they have gone away, they are likely very close by. Looping away from the offset by hundreds of microns will not usually find the fringes. Check your fluxes first and then cart tracking (cart rms error) before deciding the fringe offset is far away from where it was recently. Manually stepping the cart up or down 20 microns often finds lost fringes. Page 30.

Tip 11: Do not leave Fringe Lock on while slewing, taking shutters, resetting the ref cart position or fiber mapping.

If there are no fringes or flux, it is good to turn the Fringe Lock off to avoid the servo chasing noise and losing the offsets. Small fiber maps are ok to leave the Fringe Lock on.

Tip 12: Use the GDT offsets matrix to save your offsets.

The matrix saves time by saving any offset for a fringe you see. Click the X to keep it recorded and use it later if you lose the fringe. To read the matrix, the offset of the scope above plus the offset in the matrix equals the offset of the cart on the left. Once you have all the fringes and have clicked the Xs in the W2 column, those are the offsets you should write in your log. Page 27.

Tip 13: Note the trend in offset drift vs. Hour Angle

Offsets drift positive or negative while observing. This is often predictable and is helpful when deciding whether to loop up or down. Noting offsets at the start and end of the data set with the Hour Angle will give an idea which way the trend is going. Slewing west will add to the trend, going east will go against the trend. It is not random where the offsets are and the sooner you understand the trend, the sooner you will find the fringes. Pages 27 and 29.

Tip 14: Watch carefully for weak fringes in the PS Trend plots.

Weak fringes will not trigger the looping servo to stop so fringes must be recognized visually in the PS Trends and the carts stopped manually to acquire the fringes. Learn to recognize the faint diagonal lines that indicate you have passed through the correct offset. Stop the cart and go back if your fringe has crossed the center of the window. Use the <> buttons to manual center fringes when they appear in the window. If the carts are missing fringes often, then consider using smaller step sizes in the search loop or lower the SearchThreshold on gdt. Pages 30-31.

Tip 15: Pay attention to fringes on the Secondary combiner while searching on the Primary.

If the combiners are cophased, either one can guide you to the fringes. Have RTD's open for both, so you can have all fringe information available to get you to the right cart offsets. Use whichever fringe is stronger to get the cart to the right offset on the Primary combiner. High spectral resolution on the secondary combiner can make fringe searching very easy. Make sure combiners are cophased and saved when doing STS so that the fringe finding is easier and more intuitive. Page 31 and 32.

Tip 16: Use the 0 button to loop carts in a new search area.

If you have not found fringes after looping away from zero for a while, you can return to zero and go the other direction. There is no need to loop back through an area unless you know the fringe should be there.

Tip 17: Do not lock fringes when they are on the edge of the PS Trend window.

If fringes are on the edge of the PS Trend window, the fringe tracking servo can get confused and push the fringe away rather than centering it. If the fringe is on the edge, then click the< or > buttons a few times in the same direction as the cart had been looping. Keep clicking until the fringe moves closer to the center (the fringe should get stronger if you are moving in the right direction; it will get fainter and disappear if moving in the wrong direction). Sometimes the fringe will need to go out the top/bottom of the PS Trend plot and back in the other side before you can lock on the real fringe. After the fringe is safely off the edge, then click to lock the fringe.