Sloppy SCT Alignment and How to Correct It

Optical alignment is important, and particularly so when imaging. Stars are unforgiving when it comes to illustrating every little imperfection in your imaging system.

Sure, everyone knows that fast Newtonians need careful collimation. And most refractor users know that collimation is something that they almost never have to do. Then there's me...

I bought a used Celestron 9.25" SCT a while ago, and it was in perfect alignment at that time. It was easy for me to pretend it was much like a refractor and that the alignment would stay perfect. When it eventually went out of whack I started to tweak it as if I was working with my f/5 Dob. The fact that it came with Bob's Knobs encouraged me to do my tweaking in the field, leading to dreadful alignment. That was no fault of the knobs, I just didn't know what I was doing.

It took a couple of nights of dreadful images to convince me that I need to do a careful alignment of the scope. If you want to see a good explanation of how to go about this, look at Thierry Legault's instructions. He basically suggests a three step approach:

1. Course Alignment

This is the traditional centering of the secondary shadow within the defocused star. While this can be done by eye, I found it was useful to employ an imaging device to display the star. (An Orion StarShoot Autoguider works great for this.) Using this method lets you employ a nice program by Gilbert Grillot that overlays red concentric circles on whatever you're using for imaging.

2. Higher Magnification Alignment

Repeat the first step with higher magnification and using a dimmer star. You're once again centering the shadow. You'll use a shorter focal length eyepiece or a Barlow. I used a 4X Barlow with the SSAG.

3. Diffraction Ring Alignment

With the star focused you examine the diffraction rings. They should be concentric circles, and you have to carefully adjust the alignment until they are.

I did steps 1 and 2 indoors using the Hubble Optics Artificial Star. Step 3 required that I have the Hubble "star" a greater distance from the telescope than I could attain.

Here's the change in images:

Before (L) and After (R) the two-step alignment (click to enlarge)

These are from different nights and are of different targets, but it's abundantly clear that the Before image is awful--and it's worth noting these images are unscaled crops from the center of the image. I won't show you the field edge stars of  the Before, they're that bad. Here are a couple of stars from the above crops to emphasize the improvement:

Before and After (as above), 5X actual size

That blob on the left is actually a star. The elongation of the After star is due mainly to tracking error on a breezy night. Doing steps 1 and 2 led to much better stars across the entire image.

HELPFUL HINTS for Indoor Collimation:

Put your scope on a controllable mount. Every alignment adjustment will shift the star's position, and it's much easier to reacquire the "star" if you can use the mount's hand control.

Disable Tracking. The "star" isn't moving like a real star, so your scope shouldn't be trying to track it. Some mounts may allow you to turn off tracking, but my CGEM doesn't have that capability. However, it does have hibernate mode. If you have a CGEM, point your scope at the "star," activate hibernation, and leave the power on. Your hand control's RA and Dec motion buttons will still operate. 

You may have to use extension tubes. Start with the largest diagonal you've got, and then add extensions as needed. Remember, for steps 1 and 2 you don't need to reach focus, you just have to get the defocused star to fit into the field of view.

Moonlight Imaging

I don't mean yesterday's blue moon but a pair of Sharpless objects. I've started working on the AL Arp Galaxy program, and it suffers from a seasonal bias--external galaxies tend to be seen best when placed away from the plane of our own galaxy. The Bright nebula program had an opposite bias in that most nebulae are in or close to the plane of the Milky Way. When the Milky way is well placed in the sky you're kept busy imaging nebulae; when it's not, you have almost nothing to do. The opposite holds for galaxies.

Two examples: The BN list has 15 objects in Cygnus, while the Arp has only one; The BN has one object in Ursa major while the Arp has 34!

The way to avoid the "hurry up and wait" problem is to do multiple programs at once, choosing programs that have complementary biases. So I think I'll also do the Planetary Nebula program (concentrated in the galactic plane) along with the Arp (concentrated at the poles).

Here's an image for the PN Program. It's not my first; for that I'll have to look back through my old images. This one is Sh 2-71, very bright and sitting just west of a much larger and dimmer Sh 2-72.

Sh 2-71 at right, the much more extended and dimmer Sh 2-72 at center left.

This image is in H-alpha and is another illustration of the power of narrowband imaging. These objects were in Aquila and the full moon was in Aquarius, not very far away. The moon was up for the entire data acquisition time and I was imaging under a red zone sky as well. In other words, the sky was bright.

(Imaged using a TV-102, ST-8300M; 6 x 600s autoguided exposures.)

Last night the unbeatable factor was clouds; a lovely clear night slowly went overcast as I was collecting O-III data. I had to settle for an hour of H-alpha.

It's nice to know that even though we have no remedy for clouds, we can image deep despite the moon.

The Next Imaging List: Arp Galaxies

A quick glance at the AL list of observing programs suggested to me that the next agenda item for me is the Arp Peculiar Galaxy Northern list. This lets you image any 100 out of 338 objects in the list, which is a lot nicer than the bright nebula list. The ALBN required 100 objects out of the 132 on the list that are visible from my latitude (i.e. having a declination northward of -45 degrees). There wasn't a lot of wiggle room for the ALBN!

The submitting requirements are much saner than the BN list. Apparently all that's wanted are the images themselves. There is this strange statement:

Also, photo enhancing of your image is not required, but, again, if you choose to do so then that will be accepted.

I'm not clear about what they mean by "photo enhancing," but everyone should do at least a simple stretch. I plan on doing the usual image processing to reduce noise and get as much as I can out of the images.

Most of these objects are fairly small. The largest visible from my latitude is M101; it nicely fits into the field of  my T2i when imaged through my C 925 @ f/6.3.

My plan is to image all 100 using the same scope and imaging device. Right now I'm leaning toward using the above T2i, just for the ease of getting color.

I'd also like to use the same exposures for all the objects. There will have to be some experimentation to see what that exposure might be. If the total exposure time turns out to be short enough (10 minutes or so), I might use in-camera long exposure noise reduction (LENR) instead of dark frames.

The Last Image from the July Road Trip

This is an image I've wanted to take for some time. Unfortunately clouds kept the total exposure time at only 63 minutes. The nebulae in the image require no introduction.

200mm Zuiko lens @ f/5.6, 63m total exposure LRGB. Taken at Jeffers Petroglyphs

Can the Summer be going by so quickly? No more travel for me (except to Cherry Grove or perhaps one of the county parks I've scouted out) until September or October. Boo!

Three More for the ALBN and What was Lost is Now Found

LBN 140 was something I unsuccessfully tried to image from my red zone back yard using Ha. At NSP I gave it a try because the last night it was sitting in an area that still had good transparency. The first luminance frame showed some hints of nebulosity, so I continued on and collected more.

LBN 140

Getting this was a real bonus as it has a brightness of 6 on Lynds scale. A big thank you to the dark sky of NSP!

Two objects that I tried to image at the 2015 Nebraska Star Party didn't come out thanks to a really annoying line of clouds that slowly slid over them. Luckily, the one clear night at Jeffers allowed me to go after them again. These were IC 4812 and NGC 6729, luminous parts of a larger patch of nebulosity:

IC 4812 and NGC 6729

This would have been better in the AT65, but I decided to stay with the 200mm lens.

With these two I end up at 103, three more than needed.

Still to be processed is a 200mm field LRGB image of the Lagoon/Trifid area.

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Now for something strange. At NSP 2014 I lost my mount's counterweight shaft end bolt, the thing that keeps counterweights from accidentally sliding off and crushing your foot. You know, this thing:

The lost bolt, now found

The bolt can easily be replaced by a bolt and washer (google and you'll see a lot of images of replacements), but it has the same specs as the bolt on my old CG5 mount, so I used that one as a replacement. However, knowing that I'd be going back to NSP again this year I began to entertain a notion that I might find my lost bolt. I know, NSP is a big place in which to find such a small thing, they don't mow the observing fields all that short, and there's been a year for it to get trampled, corroded, covered with dirt, or otherwise abused and knocked around. A bird might have decided to carry off the shiny thing. Realistically, chances of finding it were slim.

Maybe it was the small chance of finding the bolt that compelled me to look for it. Or maybe I just really hate losing things. So I set up camp as close to where I was last year as I could remember, put up my tent and set out my mount and observing table in the same relative positions. Of course I immediately searched the area under the mount and table with great expectation. Okay, that was unrealistic.

I began a not very methodical search extending outward from the mount and within a minute saw the bolt gleaming up at me, oriented as in the above picture, shining like a tiny mirror reflecting sunlight. It was only about ten feet away from this year's placement of the mount.

Had it been cloudy or I had been farther off in my positioning I might not have seen it.

As you can see in the image above, it picked up a little rust on the threads during its year of idle time in the wilds of Nebraska. Otherwise it's fine.

This year--so far as I know--I left nothing behind at NSP. But I think I'll go back again next year anyway.

AL Bright Nebula Observing Program DONE!

Finally! I've now imaged 100 of the "Bright" nebula objects.

There were only a handful of hours with clear (if not very transparent) skies at this year's Nebraska Star Party, but it was all I needed. I shot groups of objects in Scorpius and Sagittarius/Serpens to collect the six objects I needed.

Here's the first group, NGC 6357 (the Lobster), Sh 2-12, and Sh 2-13.

 

And here's the second group, IC 4701, LBN 53, and LBN 70

Still to process are a possible three objects, one from NSP and the other two from Jeffers Petroglyphs. The total integration time to reach 100 objects was 180.6 hours. That's a bit over 7.5 DAYS of exposure time!

Nebraska Star Party, 2015

Just back from NSP by way of Jeffers Petroglyphs (in Minnesota).

The Bad: This NSP did not have cooperative skies. I arrived Monday, and that night had about two hours of decent sky for imaging my southern targets--visual observers who can dodge clouds had a better evening. Tuesday night was cloudy with a threat of severe weather; many people fled their tents on hilltops and spent the night in low areas. As it turned out the bad weather never materialized. Wednesday night was cloudy, with a distant thunderstorm lighting up the sky around 2:30 A.M. Thursday night had promise, but a high cirrus layer with some opaque streaks cut the transparency to poor or worse.

NSP 2015 typical Sunset sky, a preview of the evening's coming lack of transparency

A big storm passing to the south at sunset on Tuesday.

Word was that the nights leading up to the "official" NSP week were clouded out as well.

The Good: This NSP had an attendance of about 275, reported to be the largest in ten years. (David Knisely reports attendance was 286--see Cloudy Nights.) The Wednesday speakers were excellent, and the program included a live stream of a New Horizons press conference.

I discovered the Valentine Public Library and spent a couple of mornings there in air conditioned comfort catching up on email and the news.

Imaging went only so-so. I think I finished my 100 AL Bright Nebula objects. It's not official yet as I haven't had time to process the images. I did have time to get a few extra objects, so I may end up with a total of 103. It's all luminance because of the conditions, though. I'll be posting the processed images tomorrow.

And how was the weather? Let's compare with the averages...
             
              Averages  Records   --- 2015 Observations ---
Date          High Low  High Low  High Low Departure Precip
July 12 (Sun) 89   60   109  42    94  68     +6     trace
July 13 (Mon) 89   60   107  41    93  57      0     none
July 14 (Tue) 89   61   105  44    90  60      0     none
July 15 (Wed) 89   61   110  40    79  66     -2     trace
July 16 (Thu) 89   61   113  45    88  60     -1     none


Very normal, really. A little warm at first, then cooler. It rained on Sunday night and unfortunately also on Friday, the public night. Overall the period was  more humid than normal with dew point temperatures in the upper 60s sliding down to the upper 50s.

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The poor weather continued into my Friday at Jeffers Petroglyphs, where it was hot and humid (90F with a 73F dew point) and rain and severe weather was possible. I decided to return home, then drove back to Jeffers for Saturday. Once again it turned out that there was no rain, much less severe storms. Saturday night varied between episodes of sparkling clarity and hazy lack of transparancy that resembled NSP's weather. I collected 200mm LRGB frames of the Lagoon/Trifid area and reimaged one of the "bonus" objects that had been partially obscured at NSP.

Sad to say  that 2015 may be the last of the annual Jeffers gatherings once known as the Prairie Grass Stargaze.