This is just a brain dump of the various problems I had trying to
get the C11 up and running last night. (Extremely long, boring and
detailed notes.)
It's often said that the SCT is
not the best option for those who want to get into imaging. And last night, I was reminded of why. I've been around the block enough to know that IF you are going to shoot with the SCT, you will be dealing with a litany of potential problems:
1) Dewing over the corrector plate.
2) Tube currents/Heat plumes.
3) Focus shift due to the aluminum tube.
4) Mirror flop during an imaging run AND when you move the telescope.
5) Not enough stars if you choose to use an OAG (off-axis guider).
6) Poor guiding due to separate guide scope attached to the OTA.
7) Pointing not accurate enough with GOTO because your field of view is tiny.
8) Mount not up to guiding accurately resulting in trails and weird shapes.
9) Coma, coma, coma... spherical abberation?
10) Collimation? What?
11) PLUS all the miscellaneous problems associated with imaging in general.
Spoiler alert: I was able to deal with all the problems -- though it took 5-6 hours. (Technically, I didn't even try No. 6 -- it's a fool's errand in my opinion to use a separate guide scope.)
1) I have two dew shields for the C11 -- don't ask me why. One is a plain ABS-plastic type of affair that seals together by velcro. It's basically a rolled-up tube. At the beginning of the evening, after initial cooldown of approx. 1 hours, I secured the new dew shield around the front of the tube. It sags. No matter how much you finagle with it, it sags. If pointed straight up, you can get it to stay pretty co-planar with the tube.
I spent an hour shooting video of the moon. I'm trying to continue my series on large rilles (or rimae) and I got some footage of some rilles around Mare Humorum. As it was getting chilly, I decided to grab some hot chocolate.
When I came back out, I fired up the video camera and pointed the scope toward Orion's belt. Increased exposure from 1/40th to 3.5 seconds -- nothing. Increased to 5 seconds -- again nothing. Changed the gain -- nothing again. Eventually I found a bright star, but only the star was visible, nothing around it. My first ridiculous thought was, 'Maybe I'm pointing at a barren part of the sky and since I'm shooting at F/10 everything is dim...' I looked up and there were clouds moving through Orion. A-ha! I waited 5-10 minutes and then the clouds moved on, but the situation was the same. I increased exposure to 10 seconds. I could only see super bright stars.
I finally looked down the crooked dew shield and the light from my headlamp was a big red bloom of diffuse color. A layer of dew had coated my corrector. Surprised? No.
My other dew shield has a built-in dew heater. The dew shield has a notch that allows it to sit much closer in to the tube. It sagged much less substantially than the basic dew shield. I found my dew controller and plugged in the heater then set the temperature for the max. setting. After 5 minutes, I could barely feel a little warmth from the outside of the shield. For good measure, I found a couple of shoe warmers (similiar to chemical hand warmers) and put them in the bottom of the dew shield for good measure. After waiting for an hour (more hot drinks), about 3/4" of the outer corrector had cleared of dew. Another 30 minutes, it hadn't gotten any better.
I thought of calling it a night, but then I remembered my wife's hair dryer. I grabbed it and used it on the low setting. I kept the dryer at least 18 inches away and moved in a circular motion. After 2 minutes, nothing seemed to change. (Typical.) Then the dew started becoming less dense and it was clearing near the center. After another 2-3 minutes of slow circles, the dew was gone. I immediately placed the heated dew shield back in place and dew was not a problem for the rest of the evening.
Lesson: Give up and never try. Um... I mean no matter what the supposed conditions, place the dew shield and dew heater on the SCT after initial setup. Once dew gets going, it's really hard to remove.
2) When I setup the C11, I usually open the back, point the scope down and let the heat rise out of the tube. Usually, 1 to 1.5 hours is sufficient for this initial cooldown. (It's not a bad idea to keep the diagonal and eyepieces out as well so they can cool to ambient temps. Keep them covered.) Before imaging, I usually spend 15-20 minutes doing a little visual work. And in this case, I was able to check off a few items off of my lunar observing list. The views were surprisingly decent with only a little shimmer through a 14mm EP. As I switched to 8mm (~350x mag), I noticed that conditions were "worsening". In the span of 5-6 minutes, it went from occasional shimmering to a more constant shimmering.
By the time I setup the imaging gear, the moon was boiling in the camera view. It just got worse and worse as time wore on. (I wonder if the camera heat was also contributing.) Temps were still dropping and the tube was dripping from all the dew. The problem is that the aluminum tube can drop below ambient and create all kinds of heat current. When I defocused on a star, you could see the heat plume like a wiggling line perpendicular to the round star.
There clearly needs to be a way to get the hot air out of the tube throughout the night -- I am considering drilling 2 holes and installing fans on the back. Also, I'm considering a small fan on the top near the corrector. (I wonder what the smallest fan is? 20mm?)
By 3 am, the scope seemed to have equilibrated as the frames coming down from the 6D were decent.
3) I used a Bhatinov mask to help focus. This is a pretty decent way to focus. But as I did a series of shots over the course of an hour, it was clear that the stars were becoming larger and more bloated. I was loosing critical focus but since I was really just testing, it was okay.
I think an upgraded focuser with a motor drive will solve this issue down the road. I think it needs to be refocused probably every 20-30 minutes in the early evening, then maybe every 45 minutes. To really nail this focus issue, one could try the
temperature compensating focuser from Optec or the
ONAG from Innovations Foresight.
4) Later in the evening, I was able to get some longer shots with the 6D through the C11. Two things stood out: a) I had obvious hard vignetting in all the corners. The vignette was curved so the edge of the OAG was obviously in the way. (More on this later.) b) Outside of the central 25% of the image there was obvious star bloating. It got so bad in the corners that the stars looked like warped donuts. Much to my surprise the central 10-15% was really sharp. This made me rethink selling this OTA. The bloating was much more pronounced on the "left" side of the image and it seemed the central area (the best area in terms of sharpness) was actually right of center.
I think what's going on is that the nature of the focuser pushes the "right" side of the mirror closer to the corrector plate resulting in a "tilt" that causes the non-symmetric coma pattern. This is one of the reasons I'd like to do a
Roth Ritter mod so that I not only correct focus shift, but also remove the tilt.
5) So I used the
TSOAG9 off-axis guider from TS telescopes to connect the 6D and the guide camera. I had to remove the off-axis prism to connect everything together. The TSOAG9 is a pretty clever OAG since it only takes up the thickness of a regular T-adapter. That means that you can incorporate off-axis guiding and still be able to connect to accessories like a coma corrector or a flattener. Though it's a wide (48mm width vs 42mm) t-type adapter, it's NOT wide enough to let the sensor of the full-frame Canon to see through.
All the corners had strong vignetting.
No matter where I placed the height of the guide camera, I couldn't see any stars! I spent a good 30 minutes trying every conceivable position -- really far out and really close in. A couple times I tapped the surface of the CMOS chamber glass. I lengthened the exposures to 10 seconds -- nothing.
But at some point, the image screen froze (not the computer) and no images would come down. Aha! I thought it was buggy camera drivers so I restarted the computer. Just for kicks I took another image through the 6D -- about 30 seconds. A weird set of prismatic streaks were evident in the bottom of the frame. What!?!!?!! It was like a spectrum... from an optical prism!
The OAG prism was facing the wrong way -- the guide camera wasn't collecting any light because it was facing the sensor. :o
I quickly turned the pick-off prism around and was in business. The stars were not smears or streaks but they were all comatic -- with a bright center and a trailing bloom of light.
5a) Immediately, I tried calibrating with the new setup. It would fail calibration after two or three steps to the west. I would get the dreaded "star lost" message over and over again. I could see the star on the screen... why was it lost? After trying 5-6 times, I noticed that
PHD was trying to figure out where the star was before it ended it's command to move the mount! So it was only seeing a smear of a star. I was puzzled.
...and a bit depressed so I took another 30 second exposure through the 6D. There were streaks! I tried another one and there were two globs of concentrated light for every star! I felt stupid like a complete newbie trying imaging for the first time. More puzzlement. The evening was feeling like it was going to be a complete waste.
5b) Then I started looking at all the cables -- everything seemed fine and familiar, no snags around the RA axis. Then I tried the power cable for the dew shield, it felt heavy, it pulled the shield down.
The scope had moved in a way that it was essentially dragging the dew controller which I had left on top of the (now soaked) deep-cycle battery. I moved the dew controller up to a bungee cord I had tied to the pier. Now everything seemed fine.
Before I tried recalibrating, I had a vague deja vu moment because I've had guiding issues with my 8" f/6 reflector. I wanted to double-check that PHD knew that I was guiding at 2800mm focal length. When I checked the settings in Advanced Setup, it did NOT know my focal length. I corrected that info. Also, PHD has a setting that allows it to "care less" about the shape of the star. It's under the
guiding tab in Advanced Setup. The default setting for Tolerance is 50, but if you set it high like at 95, it doesn't care what shape the star is. After I set it, it calibrated very quickly.
The guiding was very good at 2800mm focal length. I was quite relieved.
6) No comment.
7) So originally, I was going to shoot the Horsehead. But I wanted to get a couple of guided test shots on the Orion Nebula. I used goto to slew to M42. Nothing. The C11 has a super crappy finder scope that you can't center your target in the crosshairs of the finder. I could tell that I was close to M42, but even after spending 20 minutes trying little steps to the left, right, up, and down, I couldn't get it on the 6D sensor. Then the stars started disappearing and when I looked up, it was clouded over to the south. Yay. I waited about 20 minutes and 80% of the sky was covered by clouds. There was a persistent hole to the Northwest so I goto'd to M31 and used the crappy finder to get it close and took an exposure. I could see the edge of the humongous galaxy. After five minutes of adjusting/moving the mount in a random direction-taking a picture-moving the mount-taking a picture-etc, etc I finally got the camera centered on M31, then synced it on the keypad.
I noticed a problem. Whenever the goto finished, the mount had a tendency to continue moving eastward for 10-30 seconds. I could click "W" on the keypad to stop it. But it was a persistent issue through the night.
After getting the mount calibrated in that area, I entered
NGC 925 on the keypad and it slewed to the object. It was pretty closely centered. So, the key here is the go to a large known object (if possible) near your target, then sync the mount to that. Afterwards your goto should be fine.
I used ISO 25600 and 10 second shots to see what I could see.
At some point, I need to try plate solving again to fix this pointing nonsense.
8) I was pretty amazed at the mount's performance. I've never shot at this long focal length and my initial tests showed tons of promise. I was able to do both a clean 5 and 10 minute guided test with very round stars. There was a hint of lack of eccentricity (fancy way of saying roundness) which pointed to a slight colllimation issue. (The last time I had collimated was about 2 weeks ago.)
The C11 gets bundled with a CGX and CGX-L mounts these days. I'm 90% sure that my results would have been less impressive with a CGX mount. Always overmount.
9) As I mentioned earlier, in my full-frame shots, there is evident coma outside of the central 25% of the image.
While I didn't have any kind of corrector, I have been looking at some possible choices. The traditional one is the Celestron/Meade f/6.3 focal reducer. I tried this device back in the day with my old Meade 8 SCT. While it reduced "coma" by about 25-35%, it created strong vignetting in my film images.
There are a couple of more choices these days:
a)
The Starizona Full-Frame SCT reducer/corrector. Expensive. Probably 1k with adapters.
b) Optec makes something which has identical specs but which is more expensive:
The NexGen NGC 316 f/7.5 reducer. Haven't seen many results with this reducer.
c) There was a 2 inch ASA corrector which seems to have been discontinued. However, their website seems to have
a 4 inch version! (This may only apply to the pure Cassegrain, not the Schmidt-Cassegrain.)
Now there are other possiblities:
Astro-Physics makes a telecompressor that should reduce some coma.
27TVPH. The problem with this is that I can't find a SINGLE example of the 27TVPH and a regular schmidt-cassegrain telescope. But AP makes adapters that connect it to the SCT. Hm...
I stumbled a guy who tried several correctors with
his 8 inch SCT and strangely, the best one was the TV .8x reducer/corrector for fast refrators.
Another one of this results pages.
The question here is: Should I try the other Televue reducer/corrector ? It would be awesome if I could get a moderate improvement of coma/spherical abberation with the Televue RFL-4087 which expects 800-1000mm focal length. The TRF-2008 expects a 400-600mm focal length scope.
I also have the TS telescope 2.5 inch flattener. The guy did have a result with this but it didn't seem to show any difference a bare scope and the 2.5 inch flattener. One thing to note is that he didn't seem to have experimented with spacings.
Here's a reference to a doublet corrector for the SCT that no one seems to have made:
https://www.telescope-optics.net/field_flattener.htm#flattener
10) It had been two weeks since I had collimated. Based on the star shapes in the images, I think collimation could have been better. Here is a link to a DIY collimation method: The Duncan mask is mentioned
here and
here.
11) I'm kind of exhuasted at this point. Since this was a test run the cabling was a mess. If I'm doing this for certain (meaning going full hog on the C11), then I need to add a top Losmandy plate so I can velcro USB and power hubs, a mini-PC, dew controller, etc.
Some pics to follow...
