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!
This is the guide port on the Lhires spectrograph. The opening is maybe 1/2-3/4 inch. The threads are male “C-mount” threads and actually are at the end of a small lens which is used to bring the spectrograph slit into focus
This is the lens which fits into the guide port sleeve and serves to focus the slit onto the guider chip
Technical drawing for Lhires showing the optimal distance to the guider ccd is 53mm. Subtract backfocus of ASI 174 camera which is 6.5 and that leaves 46.5mm distance between the spectrograph base and the camera face. I am at about 45mm for that so I think we’re good!
Recall the guider is seeing reflected light from the slit. It is the only camera that can actually see the optical field of view so in essence it is the eye of the spectrograph!
This is a 0.2 sec unbinned exposure in a room with what I thought was pretty dim lighting! I rotated the guider until the slit (black line) was dead center in the cross hair. Orientation of the slit is not critical as long as it is centered. The illumination is strange and not sure why it appears asymmetric like this but we will soon find out if this is correct or not!
2 camera configuration with guider on top, imager on the bottom
Well certainly there is no argument that automated imaging is the coolest thing ever! But..it’s not easy to get consistently error free runs. I think I have had maybe 3-5 sessions that went totally error free in the last month. It really is remarkable that this can work at all! I mean, think about it for a second: the scope slews to a star with, for example, your red filter in place since you are starting with red filtered images. Then it focuses on that star, changes to the clear filter, slews to your target to within often less than 1 arc sec, changes back to the red filter, finds the right guide star, starts your red images, refocuses after 2 hours (each time it focuses it moves off of the target to focus, then back to the target to resume images!), dithers each image after it is captured (dithering is a routine where images are offset by 2-3 pixels to improve noise reduction when they are combined during processing), flips to the west side of the meridian when the object transits, refocuses and finds the target again! Then after your light frames, it will park your scope, maybe take dark frames if you want and then at dawn slews to the neutral point in the sky for sky flats if you want those! So there are a lot of things that have to happen and hence a lot of things can screw up ! Luckily for me I just need to go outside maybe 25 feet away and don’t have to troubleshoot from a 1000 miles or so!
For those of you thinking about trying this, I would say that the number one thing that you need to get straight on your system is this thing called “plate solving”. This is the way your system knows where it is in space. For example in the planetarium program “The Sky” which is the one I have used for years and that is the one I am familiar with, your camera takes an image, copies it into the planetarium program (the sky) and uses one of several stellar databases to match what the image shows to what “The Sky” shows. If it is able to match them, this is a successful “plate solve” and allows the system to then move exactly where you direct it to. If it can’t match the two and the plate solve fails, image automation is not possible. There are several ways to optimize this process (hopefully will demonstrate on a subsequent video :), but even if you are able to do that there are unforeseen issues that crop up. Here are a few of those:
1) Your target happens to be in a star-poor region of sky. Obviously if there are only a few stars there is less of a chance for a match. If you increase exposure it may not necessarily help if noise is increased.
2) Your target is in a region of sky glow. Similar problem as number 1. Some people will use a light pollution filter for plate solving. Haven’t tried that yet but an interesting thought.
3) Trying to plate solve with a waxing Moon. Very difficult. I would limit attempts to when the Moon is out, to no brighter than quarter phase
4) Clouds roll in! Not sure what to do about this. I think in that case you’re out of luck. The automation program I am using now (CCD Autopilot) does not have a routine (yet) for delaying the imaging until clouds pass.