Rig #2 is working BK Lyn again tonight. The previous session I was using the Ted Agos focal reducer/adapter with the “new” ST8 camera. Unfortunately I do not currently have another compression ring holder like the Astro-Physics one on Rig #1. I did have one that has a 2-inch adapter with compression ring that attaches to the tail but with the Agos adapter it holds the camera a good 6 inches or so off from the back of the telescope. Well, seeing as the camera is not well-supported in that setup it’s not surprising the images were awful, showing off-axis coma across the entire field. Tonight I’m using the Celestron 0.63x reducer directly coupled to the telescope tail stock, along with a couple of odd adapters which hold the camera very rigidly, but not at the optimal distance. Optimal distance from reducer to chip is supposed to be 85mm; the current setup is around 92mm. It may be necessary to order a custom-made adapter. It will be interesting to see how the images change across the field. The only reason I’m using the field flattener/reducer is to get more consistent images across the entire field. Measurements of FWHM taken in the past have shown that the FWHM varies as a function of position – by as much as 10% across the field of the ST8 chip. The first and most important assumption about using small and identical apertures for all stars in a given field is that the PSF is the same all over. In order to maximize S/N I generally use small (1.5xFWHM or smaller) apertures which are most severely affected by changes in the PSF as a function of location on the chip. Time for a 14-inch Edge HD or a Meade ACF.
Rig #1 is taking spectra of RY Tau, using the 1200 line/mm grating. Issues here include the difficulty focusing. Despite getting a good focus prior to taking the first image (maximizing highest pixel value) I still get very asymmetrical line spread functions. I keep going out and turning the focus knob CCW (pushing mirror up) and it does improve the focus, but yet to get it really on-point. One more trip out coming up.
I’ve had lots of trouble getting proper focus on the guide camera on the LHiRes. The images look very comatic – like the optics are out of collimation. Getting an object in focus on the guide camera seems to make it out of focus for the spectrograph. This can be seen by taking a vertical cut across a spectrum trace. Figure 1 shows such a spectrum.
After finishing with RY Tau I decided to try to figure out what was going on. I first tried to maximize the pixel values and minimize the FWHM as measured by MaxIm. Doing so resulted in the image shown. But to actually get an image so that a vertical cut through the spectrum shows a normal “symetrical” profile I need to focus inwards (focus knob turned CCW) quit a long ways. The image as seen in the guide camera goes through a bunch of different shapes, none of them looking like a star image, mostly like what might be produced by badly aberrated optics.
The next thing I tried was to focus using the colimator lens in the LHiRes and while that worked it made the comparison lines out of focus. So I returned the colimator so that the comparison spectrum lines were in focus and tried to collimate the C14 optics. They were not far out of collimation to begin with, but I did center things up a bit better. But again, imaging through the guider following collimation showed off-center secondary shadows when out of focus, and flares radiating from the images when “in” focus.
I decided to mine the posts on the Spectro-L Yahoo group and found that the cause is likely a mis-alignment of the “foldback” mirror that feeds the camera lens for the guide camera. So I’ll be taking the spectrograph off of the telescope to more accurately adjust that mirror. To be continued……