Friday, December 30, 2011

New Scope Arrives

The new scope has arrived. The seller did a nice job packing it, and it's in beautiful condition. The only way to tell that it's used is a few clamp marks on the dovetail.

First impressions of the focuser suggest it's a great improvement over the GSO I've been using on my ST80. Tonight or tomorrow I'm going to reconfigure my imaging set up to have the AT72ED and ST80 side-by-side to see how it all balances out.

Of course, the forecast for the week ahead is clouds, clouds, and even more clouds. So don't expect a star test report, much less an image made using it anytime soon.

Wednesday, December 28, 2011

The Waiting Game: Dollar Days

No, I don't mean "Dollar Days" as in when a store offers a sale where most items are priced in whole numbers of dollars. It's a measure of anticipation and patience required when you've purchased something that must be delivered at a later date. I thought of this in 1978 when I ordered my first real telescope, an 8" f/6 Meade Newtonian on a GEM. It worked like this: The total cost was about $600, and the wait between placing the order and receiving it was two months (60 days). Multiply the two together, 60x600, and you get 36,000 $days. The larger the amount of dollar days, the greater the "anguish" of waiting you must endure.

I recalled this as I was checking the UPS tracking number on my incoming AT72ED. It has a price tag of $316 (including shipping, insurance, and a nudge for PayPal) and the wait is 5 days. That's a paltry 1,580 $days. Hardly even a mild test compared that old Meade.

Oh, and the telescope is black. I would have settled for any color. At night all telescopes are the same color, and all home-made telescopes look like they were made by a master craftsman.

Although I might have haggled a bit over the price if it had been the pink model from Astro-Tech. Pink would be a conversation started at star parties, which is fine. But I have a feeling that the pink models don't sell quite as well. I base this on the fact that some retailers call them purple.

A pink AT72ED
This doesn't look very purple to me, but pictures can be deceptive.  Have you see one of these in real life, and how does it look to you?

Tuesday, December 27, 2011

The decline(?) of Sky and Telescope printed magazine

I was trying to round up my S&Ts from the many places in my home into which they diffuse, and I was struck by how much their thickness varied over the years. With so many magazines failing or on the ropes, it set me thinking. The only thing to do was to enter the number of pages in each issue into a spreadsheet, right? And once having that done, It was logical to graph the time series. Cue Rod Serling--"Submitted for your consideration:"

Sky and Telescope Pages per issue, Feb 1997-  Jan 2011

This shows pages on the vertical axis and time on the horizontal. The dots and black line are for S&T, the green line is the Standard and Poor's 500 index scaled for comparison.
Some observations:
  • The general annual pattern is lots of pages in the December and January issues for obvious reason; a minimum in the spring and an uptick for July/August (graduations, Father's Day, it's warm out), another drop in September/October/November
  • S&T tracks along with the S&P very well through 2004.
  • After 2004 S&T declines rapidly in comparison to the S&P.
  • S&T does not recover along with the S&P in 2005-07 nor 2009-11
  • S&T dips to 82 pages for one issue in the spring of 2009, then holds constant at 86 pages through January 2012
  • The Dec/Jan surge is missing for the holiday seasons of 2009-10,2010-11, 2011-2012
It of looks like S&T is hovering at a size of 86 pages despite a multi-year trend that suggests it should be much smaller at this time. That there were no Holiday surges in the last three years suggests that astro vendors have shifted their advertising bucks elsewhere.

Is the printed version of S&T a lingering relic? Will becoming an e-zine save it? I usually resubscribe in three-year increments, but I think from now on it will be only a year at a time.

Update (January, 2013): The 86-page flatline begun in April 2009 continued another year through the February 2013 issue except for the June issue, in which the page count was only 82. This was matched the record low of May 2009. As you can see from the chart, the spring into summer season has generally had slimmer issues.

The S&P index now stands about 15% above its value during January 2011, so S&T continues to lag in comparison. 

Sunday, December 25, 2011

Yet Another Telescope

Telescope #5 joins the fleet.

For a while I've been looking for a better way to do wide-field astrophotography. I started by trying to use my ST-80 in this role. Its focuser (a replacement GSO model) just wasn't up to carrying the weight of my SBIG ST-8300M, and its optical quality was not very good. As I've mentioned in past posts, I looked at getting an adapter that would let me mount Canon lenses on the SBIG. That seemed a little too single-purpose, given the cost ($280).

Last night (Christmas Eve) someone put an AT72ED up for sale on Cloudy Nights, and I jumped on it. At $285 + shipping I hope it will be the most flexible solution.  It's a bit faster than my TV102 (f/6 vs. f/7)and much shorter in FL (432 vs. 700 mm). Its field of view is a spacious by 2.4 by 1.8 degrees. Questions remain: How robust is the focuser? How well will the Orion field flattener work with it? Commenters have noted that its focuser is much better than the standard 2" focuser by GSO. I've never read about someone pairing the Orion FF with this scope (most people use the AT or WO FF) so I'll have to find out for myself.

The seller is someone who just upgraded to an AT111 and apparently decided he could do without the 72. As for me, this will probably take the place of my ST80 as my travel scope. The ST80 now can retire to its role as Autoguider, for which it is well suited.

Thursday, December 22, 2011

Serendipity and a cranky cable

Tonight I thought I'd go back to the Horsehead and redo the H alpha exposures. But when I asked the GoTo to take me there, it instead put me right on the Flame Nebula. I decided to go with the what the mount apparently preferred.

I'm current imaging with my MK-67, an f/12 Mak-Cass. Its focal length is 1800 mm. The ST-8300M's pixel size of 5.4 microns  is about 1/3 the size it should be (about 16 microns for the average poor seeing I get here). I'm therefore binning 3x3 on the camera, which give me about 1.65 arcseconds per pixel. Looking at some of the subs, this seems to work very well.

As I write this I've got 22 subs in for the Flame. Tomorrow night I'll go back (unless the mount has other plans) and shoot some other bands if it holds clear.The odd lines I mentioned last time are gone. I asked on the MAS forum about the noise I was getting the other night. It looks like the best suggestions were about bad or loose cables. Something was acting up tonight when PHD went crazy. The oddest thing was that each exposure caused the PHD window to jump back and forth by about the window frame's width (8 pixels or so). It also couldn't calibrate correctly. Multiple restarts didn't fix this, nor did unplugging the USB cable at the laptop.  However unplugging and reseating the cable at the autoguider did the trick. I begin to suspect that the autoguider's socket is the problem.

The Horsehead image came out okay, but needed more exposures. I think the flame will look much better. I aim to get 30x5min exposures for it.I'll post the Ha images in the next couple of days.

Wednesday, December 21, 2011

A blurry cross and noise

Last night I tried to image an Einstein Cross and the Horsehead nebula. The lensing galaxy was imaged, but 20 or so 2-minute exposures were not adequate.  I'm also fairly certain that a 6" telescope is not adequate for resolving this feature.

The horsehead was interesting. It took a lot of time to compose the image. The focuser and extension tube didn't have a good grip on the CCD, so focus drifted a little. Polar alignment was good, as 5" exposures had round stars. There was what I think is some sort of electrical interference coming from (pick one or more) the mount, laptop, or power strip. This resulted in some random noise and narrow horizontal bands. I'll see if I can pin down the source when I shoot dark frames tonight. Tomorrow night is supposed to be clear again, so I'll retry at least the horsehead. I need a target for earlier in the evening, as the HH doesn't clear the trees until several hours after darkness begins.

Monday, December 19, 2011

Next Telescope?

I enjoy wide field imaging, and it's easier for someone like me who is learning astrophotography. Tracking errors are less evident, and the hardware to image is smaller and less heavy.  The mount has an easier time carrying the load, and the smaller profile means wind presents less of a problem.

What should it be? I had my sights set on an AT65EDQ, a triplet with built-in field flattener:
Astro-Tech AT65EDQ
Typical Moonlight Focuser
But it seems to be no longer for sale, and if it comes back it will probably have a long period on back order.  The AT65's focuser is probably not capable of carrying my ST-8300M and filter wheel. So it almost demands a focuser upgrade, and that is expensive: Something like $400: Above is a high-quality Moonlight focuser that would be perfect.

When all is said and done, it's nearly $1,000.

Another option is to use the two Canon lenses I use with my XTi. This requires an SBIG adapter that costs $280:
SBIG EOS adapter for ST-8300 w/filter wheel
This covers the focal length range from 17 to 70 mm, which is shorter than I want. If one of these were to pop up for sale used, I'd probably go for it for full constellation imaging. Moog adapters used to be available for this purpose, but their business has been scaled back and because of the filter wheel, this would require custom work on their part.

I already have a field flattener for my ST-80, so any short-focus telescope is basically ready for imaging. What I want is something with a better than average 2" dual-speed focuser and focal length less than 430 mm. The model I've read about that seems to have the best focuser is the AT72ED; new this is $380 and used usually round $250-$300. Another scope is the Stellarvue SV70ED, which also gets very good reviews.

Astro-Tech AT72ED
Stellarvue SV70ED
I've put in a "wanted" post on my club's web site, and I'm watching Cloudy Nights and Astromart for used scopes and SBIG-Canon lens adapters.  I wonder how long it will take for something to show up.

IC 410 image

I have collected OII images for IC 410 and created a composite image.  I think it's only passable. The optical alignment of the CCD and telescope was far from perfect, and I was too lazy to get a good polar alignment. On the other hand, the amount of nebulosity that shows up is more than I expected.

What's neat is that one of the MAS club members who is an advanced imager is going to see what he can do with my narrowband subimages. I bet he can make them look a lot better than the above.

Tonight it's supposed be clear again, and I'm going for a target that's completely different. I'm going to use my 6" Mak-Cass to see if I can image an Einstein Cross. I'll use the luminosity filter for that. No cheating on the polar align for this!

After that, I'll probably target the Horsehead nebula in Orion. The slowness of the Mak-Cass (f/12) will mean long exposures! UPDATE: The stratus deck never moved off, so no imaging. Tonight it's again forecast to be clear.

Next: The Next Telescope? Or camera lenses?

Thursday, December 15, 2011

IC 410

Tonight I'm trying to image IC 410, a large emission nebula in Auriga. With no western stars unblocked by the trees, I was unable to get a good alignment. I therefore skipped the polar alignment and moved right to getting light frames using shorter two minute exposures to minimize drift. I also decided to do 2x2 on-camera binning because the nebulosity is so dim. IC 410 is almost round, so composition was easy. Go-To took me to the star field I had noted would be useful for composition, and sure enough the first frame had the target in it just fine. Right now (2300 CST) I've got 40 light frames. I've got the camera running at -35°C for the first time, so I'll need to shoot some dark frames.

I'm only doing H-alpha tonight, and will post a link to the image if it turns out OK.

Now 0200 CST on 12/16/11, and done. 90x2min lights, 30x2 minutes darks, 30x0.04 bias frames. 2x2 binned on camera.

Here's the result; click on the image to see it at 48% of its original size:

Fat stars, but it catches the structure around the central nebulosity. Portions of IC 417 and 405 are in the upper left and right corners, respectively. If it's clear tomorrow night (12/17) I'll shoot some OIII frames and make a color composite.

Wednesday, December 14, 2011

We Interrupt our Regular Programming

In August of 2010 I had a stroke. It was my own doing. Against my wife's advice I tried to let time heal a bad cough. My coughing was so energetic that I managed to put a rip in my carotid artery and a small clot formed. A bit of the clot broke off and decided to get chummy with my brain.

Apparently if coughing were an Olympic event I could be a medalist.

It was a mild stroke by almost any measure. Mostly it affected my cognitive ability.  The day after the stroke I struggled to put words into alphabetical order, I repeatedly failed to make correct change, and my speech was a little unclear. But the vision problem that had signaled the stroke's beginning was gone, and the weakness on my left side was gone in only a couple of days. Most importantly, my memories were still completely intact. As I said, a mild stroke. The emotional aspects were the worst parts: The shock of knowing my brain was damaged, realizing the pain I'd put my family through.

In five days I was home. In 9 days I attended the state fair, although I only had enough energy to stay for a few hours. As I've learned, stroke recovery demands an insanely large amount of energy. A couple of months of rehab brought my cognitive skills close to what they had been. I was driving again in October, and shoveling snow by December. I've gradually continued my recovery in the months since . I may never be 100% again, but I'm darn close. In some respects, I may be better than I was. (My sense of smell has become rather acute since the stroke.)

Astronomy played a role in my recovery.  I needed to work my mind, stretching it to grow. My body needed strengthening and my spirits some lifting.  I decided to plunge back into astrophotography.  Less than a month after the stroke I resumed. 

If you've read earlier posts you know what goes into imaging.  In my case it's a lot of carrying things from house to back yard. The heaviest piece is the CGEM mount which weighs in at 41 pounds. At first this was a struggle, but it seemed to get a little lighter with each trip.

Then there's imaging itself. With so many ways to mess up, it's a challenge to a healthy person's mental capacity.  I was using a DSLR at that point which is not all that much easier to use than a CCD. Night after night I imaged, taking advantage of the pleasant fall.

It was gratifying to know that I could still make images, and I'm convinced that it helped speed my recovery.

There may be some truth to the advice to get back in the saddle as soon as you can. It seemed to work for me.

Oh, and listen to your wife when she tells you to see the doctor. Don' be a dummy like I was. I got off easy, you may not!

Become aware of stroke symptoms. The sooner you get help (don't hesitate to call 911) the better the damage can be minimized. Read more about strokes here.

Tuesday, December 13, 2011

Logic of Acquisition

I own four telescopes, two of which I seldom use. I think this might be typical in this hobby for anyone who has gone beyond the beginner stage. Some of  this can probably be attributed to the need to have the latest neat thing. But the root cause stems I think from the fact that no one telescope is good at everything. An amateur's interests can change over a span of years and the need to have a telescope to fit new tastes leads to the slow building of a small fleet of telescopes. I'll cite my case as an example of the purchasing logic that can apply.

In 1998 my wife and I went to see a total solar eclipse in Aruba. At the time I had no working telescopes, so I picked up a "grab and go" Short-Tube 80 from Orion. The ST-80 worked well on a simple tripod, for which I purchased a three-axis head. This worked great in Aruba, both then and in 1986 when we returned.

The ST-80 is a great little telescope, but not very good at showing the planets or double stars. By the next year I was finding myself getting interested in the Astronomical League's observing lists, starting with the Telescopic Messier and Double Star lists. I needed a little more aperture and a longer focal length. Another refractor would do, but the flood of good, economical overseas glass was yet to happen.  INTES Maksutovs were getting great reviews, though, so I picked up an MK-67: A fixed-mirror Maksutov-Cassegrain with 150 mm of aperture and 1800 mm focal length. I used it to do those two lists.  The MK-67 was so good I was able to see Pluto (while it still was a planet, so now I've seen all 9).

The next list I went after was the Herschel 400. This list is more demanding than the Messier list, and needs larger aperture than 150 mm.  Always budget-minded, I opted for a 250 mm Newtonian/Dobsonian telescope from Orion. I finished the H400 list with that, and did most of the Caldwell list too.

I had the misfortune in 2001 to look through a Tele-vue 102 at a dark-sky star party.  I was greatly impressed with the views it provided. Several years later the same telescope was put up for sale and in 2004 it became mine. My conversion from a "reflector guy" ended. I considered the TV-102 as a versatile lifetime scope, which in part justified the cost.

In 2007 and 2008 I became interested in astrophotography.  I started with Solar System webcam imaging that put the MK-67 to good use.  Then I noticed what people were doing with DSLRs, so I got a consumer Canon DSLR and put it to work, first with the ST-80 and then the TV-102. No additional telescopes were needed, but I did get a new mount in 2010 that was better able to carry the combined weight of the two telescopes. (usually the TV-102 is used for imaging and the ST-80 for guiding.)

In early 2011 I purchased a used CCD camera and have been learning CCD imaging since then.  The DSLR is being used mainly as a vacation camera. The MK-67 I hope to put to use for small-field objects, despite its slowness at f/12. The Dob sits around gathering dust, as it's basically worthless for anything other than visual observing. I plan to replace the mirror eventually, remount the scope with rings and use it for imaging, but that remains to be seen.

Each new telescope fulfilled a developing need in my hobby.  Next up is to move to wide-field narrowband imaging either by buying an adapter so that I can use the DSLR's lenses, or--if I win  the lottery--purchase a small refractor like the Astro-Tech AT65EDQ.

Next time: We Interrupt Our Regular Programming

Sunday, December 11, 2011

Plans are good except when they're not

Last night I had plans to re-image the Soul Nebula in H-alpha, but I was too tired to set up my gear. When I looked out at 10:30 the full moon was surrounded by a nice Corona, so the evening wasn't as clear as predicted.

If it's not one thing, it's another. And oftentimes several.

Saturday, December 10, 2011

Narrowband Imaging V: Image Processing

There's not much I can write here that hasn't been written better by others with vastly more experience than I have. Here are a few observations:
  • Calibration, aligning and stacking of the collected light frames is important, but the processing that follows is where the art enters in. Most software packages dedicated to astrophotography will do the calibrations, alignment and stacking in an automated fashion. All you need to do is set a few preferences and wait.
  • Whatever software you use for postprocessing, try to become familiar with its many features.
  • Experimentation is good. Try to jot down the steps you take when in the final stages of postprocessing. This will help you replicate your methodology.
  • Don't delete your original images.  As you learn more you may wish to reprocess them.
  • These are your images.  You're the ultimate audience, and the only person you're trying to please.
When posting images for others to view:
  • Don't post large full-scale images to forums. Reduce your images to a maximum dimension of about 800 pixels. Provide a link to a full-scale image housed on a web-based photo service if you want to people to see it. 
  • When posting .jpg images, don't overdo the compression.  Overcompressing produces distracting artifacts, particularly around stars.
  • Placing copyright text on an image is basically a waste of time. The only way to prevent people from using your images without your permission is to not post them. (This is a personal thing with me; I think copyright notices look pretentious.)
Speaking of matters of personal taste; here are some of mine:
  • I don't like backgrounds that are absolutely black; a dark neutral gray is perfectly fine and more realistic.
  • Stretching an image is easy to overdo and can cause hard edges on nebulae. I usually prefer a softer-looking image that is a bit understretched.
  • If you want diffraction spikes on your stars, don't use software to create them. Get a telescope with a spider-supported secondary, or create a mask that holds fine threads across the optical path.  Better yet, learn to like images without spikes. 
  • Don't overdo filtering. Unsharp masks and other sharpening algorithms can introduce artifacts that detract from image quality. Toggle between before and after to watch what an enhancement does to nebular texture. If it introduces features that are barely there in the "before" image weaken the enhancement.

Thursday, December 8, 2011

Narrowband Imaging IV: Setting Up

To give you an idea of what is involved in performing NI, here's what an NI newbie (me) does to set up for an imaging evening.

During dusk, haul things out of the house and do other simple tasks:
  1. Carry out Mount (put it on the pier, check level. I have a cement pier, which spares me having to lug out the field tripod.)
  2. Carry out Work table (unfold and set up)
  3. Carry out Counterweight and counterweight shaft (attach to mount)
  4. Carry out Telescope assembly (put it on the mount)
  5. Carry out Two deep cycle batteries (one for laptop and dew preventers, the other for the mount and CCD. I choose to run from batteries in preparation for setting up at remote sites without AC power.)
  6. Carry out Tackle box with cables, etc.
  7. connect Autoguider with Mount
  8. connect Mount to Battery
  9. Temporarily put CCD on telescope
  10. Balance the load
  11. Replace CCD with diagonal and illuminated eyepiece
  12. If using dew preventers, put them on the telescopes and connect to battery
When Polaris becomes visible:
  1. Adjust mount to bring Polaris into view in Polar Axis bore.
  2. Align finder with imaging scope
  3. Turn on mount and do two-star align, augmented with two more calibration stars
  4. Do software Polar align (If I were patient, I would do a drift align here; at this point I should repeat the two-star align with calibration stars, but usually I don't--goto is reasonably accurate.)
  5. Turn on dew preventers (if needed)
  6. Connect laptop to battery and turn on
  7. Swap out diagonal and illuminated eyepiece used during alignment for CCD
  8. Connect CCD to battery
  9. Connect CCD to laptop
  10. Start software for CCD control, turn on cooling
  11. Slew to bright star (be careful of CCD-to-laptop cable)
  12. Connect Autoguider to laptop
  13. Focus CCD with narrowband filter
  14. Slew to target (be careful of CCD-to-laptop cable)
  15. Compose the image frame using luminosity filter (do NOT refocus)
  16. Connect Autoguider to laptop
  17. Start PHD
  18. Select guide star and calibrate PHD
  19. While PHD is calibrating, switch to narrowband filter and expose a test frame for desired exposure time
  20. Check that CCD cooling is correct.
  21. If test frame is OK (composition is correct, etc.) aside from star streaking caused by PHD calibration and PHD is done and locked on the guide star, begin shooting light frames.
  22. Periodically recheck progress, watching for any irregularities in the latest light frame. Also check PHD for uncorrected drift
When changing filters, with new filter in place repeat step 13 and resume shooting light frames, periodically rechecking the system as in step 22.

If you don't have dark and bias frames for this temperature and exposure time, shoot them at the end of the evening. If I shoot flat frames, it's usually the next day. So far I've not seen the need to shoot sky flats. I would were I using LRGB filters.

When all imaging is completed:
  1. Return mount to home position
  2. Power off mount
  3. Turn off CCD cooling and disconnect from software
  4. Disconnect CCD and Autoguider from laptop
  5. Disconnect Autoguider from mount
  6. Shut down laptop, disconnect from battery
  7. Carry in laptop
  8. Disconnect CCD from battery
  9. Stow cables in tackle box and carry in
  10. Carry in batteries and telescope assembly
  11. Carry in telescope assembly
  12. If the weather will be OK, leave the mount on the pier and cover it.
  13. If bad weather is on the way, remove the counterweight and counterweight shaft and carry in. Take down the work table and carry in 
It all looks so simple this way. I'm sure I've left out some steps, but I'll add them as they come to mind.

The most difficult part for me is the two-star alignment with calibration stars followed by polar alignment, because I can't see the western part of the sky.

I also tend to spend quite a bit of time on image composition, trying to get it "just right."

Also time-consuming is focusing because of how stars are dimmed by a narrowband filter.  I have tried using Jupiter for focusing to good effect. I've also used a Bahtinov mask a few tunes, but so far it doesn't save that much time or improve image quality any over what I can do with my camera control software's focus utility. I need to work with the mask more.

If you have an observatory you can pretty much skip the entire "During dusk..." section.  That's where I hope to be by the end of next summer!

Next Time: Image Processing

Narrowband Imaging Esthetics

I left out an important distinction between NI and one-shot color imaging: What the end product looks like. DSLRs and one-shot color cameras are very good at reproducing the actual colors of an object. Reflection nebula and galaxies shine by scattered or direct starlight. Both of these are broadband, and so not well captured by NI. NI maps emission bands to the RGB portions of the final image, resulting in colors that are far from realistic. (Think of Hubble nebular pictures, with their bright greens and blues.) This is why you often see hybrid images of galaxies taken through H-alpha and RGB filters. The former accentuates the emission nebulae within the galaxy, while the broadband filters reveal the stellar portion of the galaxy.

The upshot of this is that if you expect realistic colors, NI may not be for you, or you should resign yourself to occasional trips to dark sky sites for some objects. If monochrome images or false colors don't bother you, then NI will deliver what you want from your own urban backyard

Wednesday, December 7, 2011

Narrowband Imaging III: Guiding

Narrowband filters pass light only near discrete wavelengths. This makes them effective at reducing light pollution, which is mainly spread across a broad range of wavelengths. The downside is that faint nebula don't generate all that much radiation even at the passed wavelengths.  This means that you will probably want to have a total exposure times of several hours using each filter. (Narrowband imaging is not for those who need instant gratification!) You can do this by combining a large number of short exposures or a small number of long exposures.  Generally, the latter is preferred.

Let's assume you want to use modestly long five minute exposures (also known as subs).  It's not unusual for skilled imagers to use exposures as long as 20 minutes to an hour.  There are a number of things that can happen during five minutes to degrade your images; wind, poor polar alignment, and imperfections in your mount's mechanics can all lead to misshapen stars instead of nice round ones.

Your first line of defense is polar alignment. A good but probably not adequate alignment can be obtained using a polar axis scope. A better alignments results from using a software method that is built into many modern mounts.  The best alignment results from using the declination drift method. The drift and software methods require you to use an eyepiece with illuminated cross hairs. The drift method demands time and patience, and can be confusing the first few times you try it. But it's the only real way to get a nearly perfect alignment. With a near-perfect polar alignment you can take exposures of five to ten minutes with minimal defects. The longer the focal length of your imaging telescope the greater the need for an accurate polar alignment.

A great polar alignment virtually eliminates field rotation when using a GEM mount or wedge-mounted fork. There's still a problem of small motions in right ascension and declination caused by imperfections in your mount's mechanics. These can result in distorted star shapes. There are several ways of dealing with this: autoguiding, periodic error correction (PEC), and adaptive optics. The latter remains very expensive at this time, so let's concentrate on the first two. Autoguiding uses a separate telescope and camera to track the motion of a guide star. As mount imperfections and wind cause the guide star to move, software sends pointing corrections to the mount.  Used alone, autoguiding does a good job of eliminating drift.  When used with PEC, autoguiding reduces tracking errors even more.

Some imaging CCD cameras come with a built-in autoguiding sensor and use a guide star from the edge of the field of view. In some models this places the autoguider sensor behind the narrowband filter, which is a disadvantage. Many people prefer a separate autoguiding camera and telescope  for this reason, and for the reduced cost.

Thanks to modern software, your autoguiding telescope's focal length can be much shorter than that of your imaging telescope.  A favorite autoguiding telescope is the ubiquitous f/5 80mm achromat. These are generally easy to buy used. The autoguiding camera can be something designed specially for guiding, or an older CCD camera such as a Meade DSI.

If you decide to use an autoguiding telescope, you must mount it securely.  Flexure is what happens when the orientations of the optical axes of the guiding and imaging telescopes change relative to each other. Usually this happens when the mounting hardware bends or shifts. You don't need the two optical axes parallel, but you do need to keep them from changing their relative orientations.

The most popular software for autoguiding is called Push Here, Dummy (PHD). It's freeware and excellent. PHD is simple to use, but it has some parameters you can adjust to make it work better with your setup. Experimentation is encouraged.

Some autoguiders incorporate the guiding software within them. This convenience raises their price.

None of the above methods will do you much good if you don't balance your imaging system properly. it's important to balance the mount's load around both the polar and declination axes. In my experience, the nights I've had  the most trouble are those when I didn't do a good job of balancing. Usually I balance with the imaging telescope in the orientation it will have during imaging.

Next Time: Setting up

Tuesday, December 6, 2011

Narrowband Imaging II: Components

Last time I wrote about reasons for why you might want to give narrowband imaging a try.  To summarize, try it out if you enjoy astrophotography, live under significant light pollution, and enjoy a challenge to your learning skills and budget.

Here's what you might need in addition to what you already have:
  • A monchrome CCD camera (includes adapter for prime focus imaging, power supply, software for camera control)
  • A laptop computer
  • A mount that is capable of carrying the weight of your telescope and camera
  • Narrowband filters for Hydrogen-alpha, OIII and SII wavelengths
  • A filter wheel
Let's consider each in turn, in each case looking.

CCD cameras come in a bewildering variety.  If you've been using a DSLR with a nice big APS-C multi-megapixel sensor, don't automatically assume bigger is better. A one megapixel CCD is really all you need for imaging.  Smaller sensors don't extend as far off axis where distortions such as field flatness and coma can become apparent. The also make smaller images that are less demanding in terms of memory, download time, and processing time. Smaller CCDs also allow you to use 1.25" filters, which are much less expensive than 36 mm or 2" filters. The downsides of smaller sensors are that their field of view is smaller and target acquisition and composing can be more difficult..

If you can afford it, get a CCD camera with temperature regulation. An example is the ATIK 314L+. An unregulated CCD is less expensive, but you'll probably want to shoot dark frames before and after each imaging session. (Some CCDs are said to generate so little thermal noise that dark frames are unnecessary. My opinion is that dark frames are always worth using.)

Options: Most CCD cameras come with an adapter, either 2" or 1.25" for use in a telescope focuser. This allows you to do prime focus imaging. Most manufaturers either include an AC power supply or sell one separately. Unless you plan on running off a battery, be sure to get a power supply.   Software to control your camera during imaging is usually included with the camera, but ease of use may vary.  If you're not happy with the included software you can opt for a commercial product or choose freeware for this task.

A laptop computer is the central nervous system of imaging.  Your camera control software will run on it, passing instructions to the camera via USB cable. The acquired images will be sent to the laptop for storage by the same cable. After gathering images you may use the laptop for processing.  Processing images can be time comsuming, so if you need to buy a laptop make sure it has enough speed and resources.  Software for Windows laptops is a little more common than for Macs. Avoid any version of Windows released prior to XP. If you want to use a Mac, check first for software supports it, and confirm that software will support the CCD you intend to purchase.

The mount is the most important component of imaging. Add up the weights of your telescope, camera, filter wheel, and autoguiding telescope (if any) to see how heavy a mount you need. It's typical for the weight-carrying ability of mounts to be described for visual observing. Imaging is more demanding.  Generally speaking—and this is very general—any mount that sells for less than $1,000 can carry imaging gear equal to about half of its stated carrying ability. More expensive mounts have carrying weights that are closer to those suited for imaging. If in doubt, consult the Cloudy Nights forums for the opinions of actual users. Even if you don't intend to use autoguiding, I would strongly suggest obtaining a mount that accepts autoguiding commands.

Narrowband filters are a source of continual debate. Are inexpensive ones as good as those that are priced dearly?  Again, Cloudy Nights forums can be a useful guide.

Filter wheels are more than a convenience because they help keep filters clean while in use. If you already have a filter wheel loaded with LRGB filters, you may wish to obtain a new carousel for swapping your NI filters in and out. Consider an 8-filter wheel, so that you can house both narrowband and broadband filters in it, in the event you go to a dark sky location for imaging. Some filter wheels can be controlled by commands passed by USB.  This is a very convenient feature well worth considering.

Imaging Telescope. This isn't all that important. All you need is an achromat with a good focuser. NI does not require an apo, as chromatic abberation is largely eliminated by using narrowband filters.  (Remember to refocus when changing filters!) You may want to upgrade your focuser if it's not two-speed, it slips when carrying the weight of your camera, or it won't keep the camera squared up to the optical axis. You can also use a camera lens as a telescope, but that can get into messy issues of adapters and spacing rings. (I plan on doing this for wide field imaging in 2012.)

My personal NI setup is what might be considered an example of an high entry-level system: If it seems a little pricey, it's because I was sure I would take to NI and be doing it for years.

Camera: SBIG ST-8300M (yes, I went for the megapixels and big sensor size; if you are more frugal than I am, do as I say, not as I do)
Mount: Celestron CGEM (my total imaging weight is about 20 pounds) There are a number of mounts in this price range, and all get good reviews
Narrowband filters: Baader 7nm (not the cheapest, but certainly not the most expensive.  They have worked well for me)
Filter Wheel: SBIG FW8-8300 (USB controlled)

If you really get into NI, you can easily spend many thousands of dollars on premium equipment. You have been warned!

Next Time: Guiding during long exposures.

Narrowband Imaging I: Motivation

(First of a series)

Disclaimer: I do not presume to be anything other than a beginner at narrowband imaging.  All I've done so far is spend one summer and fall getting started. So what I present here should be taken as one person's opinions based upon reading and limited experience.

Why get into CCD narrowband imaging (NI)?
  • You've been DSLR or one-shot color or broadband CCD imaging, but have become dissatisfied with the limits it imposes
  • You live under light-polluted skies and don't want to confine your imaging to trips to dark sky sites.
  • You want a challenge.
  • You have some disposable income.
DSLR cameras are relatively easy to use and provide high-resolution images. An amazing amount of hardware and freeware exists for using them in astrophotography. When modified they can provide fair sensitivity to the wavelengths of light emitted by many nebulae.

The central problem with DSLRs is that they are not temperature-regulated.  The optical sensor within a DSLR operates at a temperature near the ambient.  The warmer the temperature, the more noise generated by the sensor.  This noise shows up in images as a random graininess that grows brighter with exposure time.  This accumulated noise can be somewhat compensated for by shooting dark frames (exposures where the shutter remains closed) and subtracting them from the light images (the pictures of what you want to image). This compensation is limited because it's hard to match the temperature of the dark frames to the temperatures at which the light frames were exposed.  Ambient temperature usually changes during the course of an evening, so you're forced to create dark frames at a temperature only approximating the temperature at which the light frames were taken. If the dark frames are cooler than the light frames you get undercompensation; if too warm, overcompensation results; in either case you lose some of the detail in your image.

One-shot color CCDs are a common alternative to DSLRs. When equipped with a regulated cooling system they can hold the CCD sensor at a fixed temperature, allowing better use of dark frames. In fact, the regulation allows you to preshoot dark frames so that you are spared the need to shoot them during each imaging session. The light sensitivity of color CCDs is also better than that of DSLRs.

Broadband CCD imaging employs a monochrome sensor that is very sensitive to wavelengths of visible and near-infrared wavelengths. As its name implies, it produces gray-shaded images.  Exposures through red, green and blue filters are combined to produce a color image.

A serious shortcoming of DSLR, one-shot and broadband CCD imaging is how poorly they cope with light pollution. Light pollution is generally a broadband phenomenon, filling the entire visible spectrum with noise (i.e., undesirable brightness). Under reasonably dark skies, all  three methods work well. In a suburban or urban location, light pollution produces a serious decrease in signal-to-noise ratio as it overwhelms the photons you're trying to gather from your target object.

NI addresses this problem by employing special filters that pass light only at the wavelengths at which emission nebulae radiate. Light pollution is largely filtered out, whether it comes from urban glow, neighbor's yard lights, or even the Moon. For nebular targets the urban imager can be just as successful as one at a dark site.

There are prices to be paid for this ability.  Narrowband filters can be expensive. Exposures must be very long, requiring a good mount and guiding.  Image acquisition and processing are more difficult. Non-nebular targets such as star clusters and galaxies generally don't turn out as well. But the fact is that NI offers the urban or suburban imager a wide range of targets that would otherwise be impossible.

Next time: A basic NI setup

Monday, December 5, 2011

Whining about the Weather

Ah, Minnesota in Winter. Some things that make observing during Winter special:
  • Looking through an eyepiece only to have your eye begin to tear. That water sloshing around on your eye hardly helps you see clearly.  Usually it leads to frost on the eyepiece lens, and sometimes the joy of having a drop fall onto your eyepiece lens and freeze solid. Heaven help you if your eye bumps into an eyepiece that's at a temperature of -6° F.
  • Snow amplifies sky brighness. Even dark-sky friendly lights can turn against you when their light bounces upward off the white stuff.
  • Cold fingers. Which are marginally better than fingers coated with plastic-dissolving DEET needed to fend off mosquitoes.
  • Cold feet. They say that the way to keep the extremeties warm is to keep the torso toasty. There's a limit to how many layers one can wear and still have use of one's arms and legs.
  • Stiff mounts. The pasty goop that's used to lubricate most mounts can get very grippy in the cold, making drive motors labor duing slews. And they can get noisy too. I had a mount that whined when the temperature fell below zero. (Much like myself.)
  • Feeble hand control displays. Some of these can become almost useless as the cold increases the display latency.
  • Dead batteries.  Most batteries don't like being used in very cold temperatures, and give up the ghost far sooner than you might expect.
  • Deep snow. Planting a tripod or work table in the deep snow can be difficult.
  • The neighbors' backyard lit Christmas decorations. Not to wax humbuggy, but really, twinking lights seem much less festive when you're trying to hunt down something that's at the limits of your telescope and observing skills, your eyes are full of tears, your fingers are stiff with cold and the mount is screeching in your ears.  Just keep telling yourself, "Astronomy is a fun hobby."
So what to do, curl up next to the furnace until March?  Aside from earning the derision of the good folks up in Canada, sitting indoors will make you miss the splendor of the Winter sky. Get yourself some hand warmers for your hands and the hand controller. Get some really good winter wear. It's OK if you look like an escaped balloon from a Thanksgiving day parade so long as you're warm. Regrease your mount.

DO NOT use your .22 rifle to shoot out the neighbors' lights. That's both illegal and ineffective, as anyone who has ever watched How the Grinch Stole Christmas knows. (The cartoon version, of course.)

Best advice: Move someplace else. The worst thing about winter in Minnesota isn't the cold, it's the clouds. Take a look at this from the excellent web site

This shows the percent of time the sky isn't clear enough for looking through anything better than "sucker holes," breaks in the clouds that optimists take as a sign that the sky is clearing.  During those long winter nights we generally spend more than half our time under cloud cover. What fun!

OK, enough whining.  The next clear night I'll be in the back yard humming something from the Grinch. Why just humming? Because my face will be frozen.

Sunday, December 4, 2011

Getting started blogging.

It seems like almost everyone blogs. Maybe this is the reason? Could be, I suppose. I already have been blogging in a rather disorganized, low tech way. But now that I'm on Facebook it's time to step up to the 21st Century.

So what will I be writing about here?  Astronomy, mainly. But I do have a tendency to digress in the classroom, so you can expect some side trips from time to time here. Good luck to you, Readers.