Sunday, August 5, 2018

#152 - Saturday, August 4, 2018 - Testing the Lumicon Off-Axis Guider

It's been a few weeks!  Of course, you  know how it goes - you get a shiny new astronomy toy, and it's guaranteed to be cloudy for weeks!  And boy did we get rained on and stormed on.  That's all my fault, folks - I got an off-axis guider to test from Lumicon!

The week before I gave my talk on The Astro Imaging Channel, I logged on to watch Cary Chleborad's talk on his company Optical Structures and their ownership of Lumicon, JMI Telescopes, Astrodon, and Farpoint.  Cary asked after my talk if I'd be willing to test one of a series of off-axis guiders that is made to already have the spacings correct for specific astro-camera manufacturers - in my case, my ZWO, plus a Gosky filter wheel.  This was my first time using an off-axis guider; up till now, I've been using a guide scope.  There are some big advantages to off-axis guiding, especially for Schmidt-Cassegrain telescopes, like my Celestron 8-inch:
1. Weight.  I've been testing using my C8 on my Celestron Advanced VX mount, which has a payload capacity of 30 lbs (not including the counterweights).  With the C8 and my Borg refractor acting as guide scope, I was sitting at about 19 lbs or so.  The rule of thumb for astrophotography is you want to stay at half or less of the payload capacity of the mount in order for the guiding and tracking to work well.  With the off-axis guider (OAG), I can take off the Borg, and just have this very lightweight accessory hanging off the back of the scope.
2. Flexure.  When the guide scope is mounted on top, or on the side, or wherever, as the scope moves around from one side of the meridian to the other, the weight distribution changes, and gravity pulls on different parts of your rig.  This can cause the guide scope to point ever so slightly off of where you calibrated it at the beginning of the night, which will make guiding less effective over the course of the night.  With an OAG, its optical path is the exact same as your primary camera, so there is no more possible difference between the guide camera and the primary camera.
3. Mirror flop.  Schmidt-Cassegrain telescopes have big primary mirrors that are not attached to the back of the telescope - rather, they are held in the center by a baffle (the tube where the light cone from the secondary mirror is directed down to your eyepiece), and move up and down when you turn the focuser.  Again because of gravity, as you cross the meridian and head down toward the horizon, the mirror will tend to shift, since it's not bolted into place.  This will not only cause you to lose focus sometimes, but it is also a source of flexure, since the primary camera will now be pointed to a slightly different angle than you calibrated the guide scope to at the beginning of the night.  With an OAG, when the mirror moves slightly, what the guide camera sees will move with it, along with the primary camera.

However, there are a few drawbacks, namely being able to find a guide star.  OAGs work by having a prism on the edge that picks off an area from just off the camera's field of view.  It's a pretty small field, so you may not have a whole lot of choices of stars to guide on.

Looking down inside.  At the bottom is the ZWO camera's giant sensor, and then you can see the pick-off mirror on the right, and the QHY5's sensor reflected back at you.  

The directions say to line up the mirror with the long end of the primary camera's chip, but the filter wheel ended up being too close to get enough clearance to rotate it around, since the guide camera barrel gets in the way of the filter wheel.

Not only that, but the screws that attach the OAG to the 2-inch-barrel-sized connector that screws onto the filter wheel are hard to turn because they're so close to the filter wheel.

I got out to the observatory around 8:30 PM and got my telescope rig put together, and then I added on the OAG and cameras.

Lumicon's off-axis guider attached to my Celestron C8, with my ZWO ASI1600MM Pro as the primary camera and my QHY5 mono as the guide camera.

Once it got dark enough, I slewed over to Arcturus to get into focus.  I had two very lucky things happen: the first was that my finderscope was actually pretty close (I've been having issues with it lately), and the second is that the telescope happened to already be nearly in focus!  So I didn't have to turn up the exposure time to try and see a huge white donut, and I didn't have to take off the camera and throw on an eyepiece to make sure that there was actually even a star in the field.  So that saved some time!

Next, I slewed the scope to move the star to one side of the image and then the other to try and get it onto the guide camera's field-of-view, but I didn't see anything.  So I detached the QHY5 and put a 25 mm eyepiece there instead, which happened to be in perfect focus with the ZWO camera.  After slewing around a bit, I found it, put it in the middle, and put the QHY camera back on.  What I got - and forgot to take a screenshot of - was a crescent-shaped blob, which was the star very out-of-focus (so a big donut) but cut off on the edges since the prism only grabs part of the light cone.  I pulled the QHY camera out to try and find the focal point, but I actually needed it to be closer, which wasn't going to work.  Hmm.  Maybe I was mis-remembring the schematics Cary had sent me and the filter wheel was supposed to go before the OAG?  

I took the cameras off to try and figure out whether I had all of the necessary adapters to put the filter wheel before the OAG, but I didn't.  I needed something that would accept the 2-inch eyepiece barrel size of the telescope-end of the OAG that I could attach to my filter wheel, but I didn't have anything like that, even among all of my various adapters I've collected over the last three years.  So I resigned myself to imaging monochrome without the filter wheel, at least for testing purposes.  I attached the ZWO camera directly to the OAG and brought it into focus, but then I still couldn't get close enough with the QHY camera.  Then I asked myself, did I need the primary camera to be closer, or farther away, to work?  I found a star with an eyepiece attached to the guide camera slot, and then focused the telescope so that the guide camera was in focus when it was about halfway inserted.  Then I detached the ZWO camera and held it and pulled it away from the telescope to see if the focus was getting better or worse.  It got better, and I could see how much extra space I needed to have both the ZWO and the QHY in focus at the same time.  Being farther back is good - it's easy to add extenders.  My ZWO camera came with a few, including a 21mm-wide screw-on extension tube.  I attached that to the camera, and then I attached the filter wheel, and then the OAG got attached to the telescope.  I forgot to get a picture last night, so here's one I took today off-telescope.

Once I put the filter wheel back on and the 21mm spacer, I was at last able to bring the guide camera into focus!  It was hard to get it perfect because it would adjust a bit once I tightened the screws, and it was hard to hold steady enough to tell whether the star was really in focus.  I did get a parfocal ring on there though, and I'm going to buy several of those 1.25" connectors and parfocal rings so I can have one for each of my configurations where the guide system doesn't include a focuser, such as when I use my Orion 50mm mini-guider with my Borg.

All right, so I was finally ready to start trying to guide.  It was after 1 AM.  Some serious clouds had been rolling through, even though none of the five forecasts I use had predicted any, so good thing I wasn't really trying to image anything.  It was starting to clear up, but it was the most clear to the east, so I tried globular cluster M15 first.  However, my gotos were not great, even after I polar aligned using SharpCap (which is amazing by the way, and totally worth the $15/year for the subscription to be able to use it - basically turns your camera into a Polemaster), and my finderscope was dewing up, making it hard to find whatever star the Precise Goto routine picked, especially from my light-polluted area.  I didn't see M15 in the primary camera.  So I tried the Dumbbell instead - still no dice.  I probably wasn't using the right star for the precise goto.  Then I tried M16, the Eagle Nebula, since I could see one of the precise goto stars, Nunki, pretty well, but I couldn't find any guide stars, even in star-filled Sagittarius.  I thought I saw one or two, but they were kind of dim and smudgy, so I slewed to Vega to try and re-focus the guide scope again.  Since I was already up there, I decided to try the Ring Nebula, M57.  I could see a dim star or two in the FOV of the guide camera in PHD, but not with enough SNR (signal-to-noise ratio) - PHD kept losing it during calibration.  Taking darks probably would have helped, but for some reason every time I try to do that, when I turn on the dark subtraction, the image turns pretty much all white.  Hopefully there's an update out there for PHD that fixes this problem.  (And when I say PHD, I really mean PHD2).

Finally, after attempting to calibrate a few more times, a bright star drifted into view of the guide camera!  I edged M57 over a bit to bring it far enough into the guide camera's image to be able to use, and then I was finally able to calibrate.  It was bright enough that I could use 3s frames.  I don't think it was Vega, since I had looked at that in PHD as well and it was much brighter, but it was probably one of the other constellation stars of Lyra.  The calibration was still a little wonky though, but I knew this was more of a problem with the declination backlash I was getting rather than the OAG, since I'd seen that backlash while I was aligning the scope earlier.  The AVX is hard to balance because its two axes are not nearly as "loose" as other mounts I've used, so you kind of have to guess as to whether you're close to balanced, and you can't make it a very precise balance, in either axis.  I'll have to work on that.  I'm also going to need a front counterweight for that scope, or a longer dovetail bar.

All of the tiny white dots are noisy pixels.  That smudgy thing in the lower third of the center of the image is another star, but the SNR wasn't quite high enough to use.

I'll point out here real quick that I was using a 0.63x focal reducer on my C8, turning its 2032mm of focal length into 1280mm.  Shorter focal lengths are more forgiving of guiding errors, plus the faster focal ratio of f/6.3 rather than f/10 makes for brighter, higher SNR images.

All right.  So did it work?  When I re-selected the star in PHD every 10 minutes or so to kind of re-set the guiding since the dec backlash was so bad, it did pretty all right!  Below are JPGs of the raw FITS files at various exposure times.

2-minute exposure

5-minute exposure

7-minute exposure

Not bad!  I wasn't able to get much more than 3 minutes reliably using my Borg 76mm refractor as a guide scope, probably in large part due to the weight.  I think if I can get the declination axis properly balanced, these results would look even better - you can see a slight elongation of the stars if you zoom in.  This is really exciting!  I'll try it again on my next night out and see.

Before I packed up, Mars was burning bright in the sky, and the third-quarter moon was just beginning to rise, so I took off the OAG and the focal reducer.  For planetary bodies, you definitely don't want the focal reducer, since you want as much resolution and magnification as possible.  While the air had been pretty steady earlier in the evening according to my fellow club members (I was too busy to actually look through the scope), I could see the edges roiling while I was taking the data.  But it didn't look too bad.  And it appears as though that dust storm is clearing up!
Date: 4 August 2018
Object: Mars
Camera: ZWO ASI1600MM Pro
Telescope: Celestron C8
Accessories: Astronomik LRGB filters Type 2c 1.25" 
Mount: Celestron AVX
Frames: L: 1087/2005
    R: 1388/2004
G: 810/2002
B: 436/2001
FPS: 30-60 fps
Exposure: L: 15 ms
  R: 30 ms
  G: 50 ms
  B: 150 ms
ISO/Gain: 50
Stacking program: RegiStax 6

By the way, the "frames" metric is how many frames made it past the quality cutoff I chose for each dataset out of the total.

The aberration around the edges isn't real - it's a result of the fact that when I align the color channels by hand in Photoshop, I can only move in intervals of whole pixels.  It's hit-or-miss as to whether I can get RegiStax to align them for me (and it can do sub-pixel, I think), and when it can't for some reason, I have to do it myself in Photoshop.  Although that is a real polar ice cap on the bottom. :)

The third-quarter moon was rising from behind the trees after I finished imaging Mars, at around 2:30 AM.  I knew I should be going to bed, but I don't often get to image the side lit during this phase, since it's up so late.  It was still low on the horizon, so the atmosphere was really boiling, but these still came out neat!  I also got a tip from the venerable Robert Reeves (whose got a Celestron telescope named after him now, and whom I got to meet at the Texas Star Party - and nab a Meade DSI Pro camera from) to try out a filter in Photoshop CC called Shake Reduction.  It's no substitute for good atmosphere, but it helped a bit!

Date: 4 August 2018
Object: Moon, Third Quarter
Camera: ZWO ASI1600MM Pro
Telescope: Celestron C8
Accessories: Astronomik LRGB Type 2c 1.25" filters
Mount: Celestron AVX
Frames: 816/2006
Exposure: 25 ms
Gain: 50
I didn't apply the filter to this one - it didn't do much for it

Frames: 476/2002
Exposure: 40 ms
Gain: 139

I call this one "On the Shores of Silver Lake" 😁
Frames: 534/2007
Exposure: 30 ms
Gain: 139

Frames: 973/2007
Exposure: 20 ms
Gain: 139

I finally packed it in at 3 AM, and hit the sack at 4 AM!  Late night, but at least I got some images out of it.  And I got to test that Lumicon off-axis guider!  It was an exciting night.  Can't wait to try it again.

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