Time is flying by. I confirmed with Shelyak that we are good with the spacing on the imaging camera. Next was the guider set up. For the Lhires spectrograph, the guider is really the critical sensor for acquisition because as we discussed in an earlier post the camera that is recording the spectrum is totally blind to anything beyond the slit. It only can deal with the light coming through the slit. That’s why focusing the lens inside of the spectrograph is so key because the resolution of the spectrum really depends on that more than anything. The guider I am using is a ZWO ASI 174. It has a 1936 x 1216 pixel CMOS chip, field of view 6 x 10 arc min. The chip size is a little bigger than other options for the guider on this setup. The QE (quantum efficiency) is pretty high, 78%, so this should make things easier for plate solving and finding guide stars, at least theoretically. The camera is light weight and there is no cooler because all we are doing is plate solving, finding stars and guiding. Nothing else. Plate solving, by the way, is a frequent function of astronomical optical platforms. What happens is your system takes an image of the field of view and tries to match what it is seeing with known stellar databases. It is one of the most remarkable technological accomplishments of the early part of the 21st century in my opinion because now amateurs can do what the professionals do! Once the image is “matched” to the known stellar database by the software, the image is considered “solved” and the system now “knows” where it is in the sky! Many software programs are capable of doing this now.
The guide port on the spectrograph is tiny! 1/2 inch and not much more. The thread on it is called a “C” mount. I’ve heard of T-threads, M42 and several others but not this one. It just so happened that ZWO had this exact adaptor for the camera so I was in luck there!
I screwed the camera onto the port and it looks like everything is good in terms of focusing the slit, which is the dark straight line. The backfocus on the camera is 6.5mm and the distance from the spectrograph to the camera face was measured with the caliper at around 45mm so it appears that the total distance is correct within a mm or 2 if you look at the diagram below. Now the illuminated field looks weird. I get that, but I’m thinking it may be vignetting or internal reflection or something, maybe in part due to the larger chip size. Bottom line is we have a slit line of a few pixels in width that is centered on the chip shown by the cross hairs below!
What we have so far is a focused spectrograph doublet lens which we accomplished by using the internal calbration lamp Day 89. ,and now (hopefully) a centered slit in the guide camera. We are now ready to go to the telescope!