2023-02-27: Bad weather streak, some imaging
The weather in West Texas has been horrendous, with clouds, rain and winds up to 60mph!! Only 3 nights of imaging in 3 weeks. The new moon has basically been wasted! But here are some updates:
- Guiding has improved incredibly with the added mass on the counterweight. I was surprised to see such a measurable improvement. When seeing is good I am now below 1 arc second RMS consistently, on every session. I said it below, but I guess I have to say it again. This was a very welcome surprise.
- Collimation is holding well. I stopped parking the telescope head down: although it is better to avoid collecting dust on the primary, I think the mirror, not being glued, does not like too much being suspended by its edges and possibly its clips. This leads to the mirror moving in its cell and decollimation. I am working on a new cell, made of carbone fiber to better match the thermal expansion of the borosilicate. The mirror will be glued on it.
- Looking at the corner of my images, it is pretty good, but I think it is possible to improve. My guess is that the primary is slightly pinched in its cell.
- Still some slight astigmatism when out of focus, small, but very consistent. Not sure if it is the primary being pinched, or the secondary, or some misalignment.
2023-02-16: Star test and imaging
Clear sky at last! Star tested Moana, resumed imaging. Tested Mico and started imaging. It is great to have 2 NINA instances running and 2 cameras churning images in parallel.
Also managed to run LRGBLRGB… on NINA without triggering autofocus, which is the correct way to image comets.
The added weight to the mount (18lbs) to decrease the inertial moment arm must be working wonders: for the first time (ever!) I started guiding below 1 arc second RMS in both RA and Dec! The low wind and good seeing were also helping.
2023-02-12: Upgrades and collimation
The following updates were performed:
- Added counterweight 18lbs.
- Fixed the light panel PWM power supply.
- Added a second 12v bus for the piggy back imager.
- Routed a new 12v power line through the mount for the piggyback electrical bus. For that I had to remove the scope and disassemble the top of the mount. I also checked that the USB and the main 12v line were not twisted. They were not: after 4 months of remote operations including 1 mount lost + manual remote recovery, I had no twists!
- Turned the camera so the sensor frame is squared with the spider. Also turned the scope in its ring so the camera is closest to the equatorial axis to reduce the moment of inertia.
- Installed a color camera on the piggyback, for dual imaging. The piggy back is a 200mm Borg 55FL Fluorite (some kind of Petzval Refractor from Borg in Japan), with an APSC color camera (ASI 071). The Borg is OK for 4/3 sensors, but exhibits coma for anything bigger, so this system is far from perfect, but since I have the camera and refractor, I may as well use them. I decided to nickname the Mini Color system “MiCo”. The Borg will eventually go and be replaced by a Canon-compatible telephoto lens (maybe the Sigma 180) with an Astromechanic focuser.
- Disassembled secondary and primary, dry cleaned them all (as well as the coma corrector), reassembled, and collimated with Catseyes autocollimator. This should have been easy, quick and smooth. But it was at the end of a very long day after driving 9 hours and working on the scope for another 6 hours, and somehow I botched the collimation. The scope started to exhibit terrible astigmatism. I initially tough the astigmatism was due to a pinched primary, or a pinched secondary or both. This seemed plausible, as I had just tightened the primary cell vigorously. So now I had a bad collimation and a bad diagnostic. I finally went to bed, and the mystery was not resolved until the following morning, with daylight, some coffee and a clear brain. The astigmatism was due to focuser axial error -with the daylight I saw the focuser axial shift clearly in the Telecat. Having found a likely cause I could precisely research my predicament, and found the following, in Vic Menard’s words, talking about Newtonians. “An oblique optical axis may indeed deliver an astigmatic image (ref.: Amateur Astronomer’s Handbook, J.B.Sidgwick). Furthermore, the aberration is more pronounced in larger apertures. (page 79, Astigmatism and Field Curvature). In the field, I’ve noted astigmatic defects caused by errors in both axes (focuser and primary) that smear the entire field of view unless the eye is very carefully centered on the exit pupil. The astigmatism was most pronounced with a significant focuser axial error combined with a minimal primary mirror axial error. The aberrations can be resolved with proper axial collimation, and the sensitivity of the pupil disappears as well.” With this new knowledge, I eventually fixed the problem to a good autocollimator read. Then, of course, the clouds rolled in. Although I was pretty sure the problem was solved, I could not star test. So I had to eventually leave DSO without the certainty I had solved my astigmatism problem.
- Last but not least, I saw one of the wild donkey who inhabits the Davis Mountain. Pretty cool!
Collimation had started to drift slightly in the previous months, so I knew something in the optical train was not stable. Some time ago, tracking low on the Western horizon I experienced a defocusing and decollimating event, which I strongly suspect was due to the primary starting to have some play in its cell: while tracking low, the primary is on its side, and not supported by the whiffle tree. The event felt akin to an SCT mirror flop. I remotely moved the telescope vertical (looking at the zenith) then leaning Eastward, and managed to unflop the mirror, but from hereon I knew I was due to a road trip. The only question was: could I keep it together a couple weeks until I received the new camera I have to install and do only 1 road trip for all the fixes and upgrades or … was it game over already?
Well, today I had another flop event, and from this one I did not recover. I managed to refocus (and therefore measure the mirror movement on axis by converting focus steps to microns), but I did not managed to unflop the mirror and the scope is out of collimation. I did not try too hard to unflop either: the right thing to do is to go to the observatory, tighten the mirror cell, carefully collimate and do the other upgrades.
Upgrades todo list. Those are my notes for the first trip back to the observatory since installation 4 months ago. I added a few comments & explanation as some people (considering remote imaging) may find it useful.
- Mark focus position on focuser, mark scope position in its rings.
- Remove the primary, clean it, secure it in the cell. Possibly glue it to the pads.
- Clean the secondary.
- Reassemble the system, recollimate. Follow the Vic Menard collimation book. Use the Glatter laser to get in the ballpark quickly, but finely collimate with the catseye tools (autocollimator). If time allows during day, investigate why the Takahashi collimating scope gives results so different from the other tools for centering the center of the primary. At night, star test & fine tune collimatin with the x4 Powermate. Once the scope is collimated and the Powermate still on, indulge some planetary imaging if the night is long enough. Put the coma corrector back and check the camera tilt with NINA, ASTAP and CDD inspector – should all be the same, but who knows!
- See if coma corrector and filters need blowing.
- Mark focus position. Calibrate the zero position of the feather touch focuser to the almost fully racked position. This will allow large defocus for the Pixinsight wave front analyzer (or the Innovation Foresight wave front analyzer). If time allows record the defocused images and check collimation with the wave front analyzer.
- Connect the secondary heater and add a an electrical fuse to protect it.
- Remove the Guide Scope piggy back: I never use it, as 100% of operations have been Off Axis Guided. That will reduce weight on the scope side, reduce overall inertial moment arm and reduce electrical load on the 12v bus. Possibly add a weight to preserve Dec balance. If rebalancing Dec by moving the scope in its ring redo the meridian limit procedure. Here and here.
- Add a new 18lbs counterweight to the shaft. The goal is to increase weight on the counterweight side, so the weights can ride closer to the polar axis and thus decrease inertial moment arm. You can read this if you need the full explanation. The second goal is to increase the frequency of the mount’s harmonic oscillations. This should help guiding in high wind (above 12mph). Follow AP’s recommendation: “we recommend a slightly heavy RA balance towards the counterweights and a slightly heavy Dec balance toward the back end of the scope when imaging.”
- Rise the flat panel.
- Change the flat panel power supply to the Pegasus Pocket box to have functional variable illumination of the panel using PWM.
- Change the power supply of the exhaust fan of the electronic cabinet to the Pegasus Box, so I can remote control the fan according to temperature inside the electronic cabinet, rather than having a thermostat-only activated fan.
- Add a new 12v power supply and power bus, route additional 12v cable through mount. Check any twisting of cables already in the mount. This is installed in prevision of a future wide field piggy back.
- Remove the network switch and direct connect to computer instead.
- Install the new camera if I get it by then. If not, redo the dark and flat library for current camera. Add 3mn darks. Also add higher gain.
- Redo the flats as collimation has changed.
- Square camera sensor to diffraction pattern for esthetic reasons. Achieve by rotating the coma corrector (and the rest of the imaging train attached to it). Recalibrate guider after rotation.
I checked the images from the telescope icing night, and they look just fine. Maybe just a bit soft at the coldest point of the night, just before dawn. But I do not think I had any significant icing on the primary, secondary, corrector or sensor. That’s good as I imagine it could not be positive for the coating of those various surfaces!
This morning I check the observatory cameras before doing the usual mount park, with the instrument head down (to protect from dust accumulation in the tube & mirrors). I found the telescope was marbled with frost , at least on the outside. Low temperature combined with 80% humidity resulted in the formation of a thin ice layer on the carbon fiber tube. The electronic, camera and mount still worked perfectly. I will look at the pictures and see if the ice affected the optics.