Forest Fires, Smoke, and Transparency

Forest fires are raging in the northwestern U.S. and western Canada, taking lives and resulting in the destruction of property. There are a lot of fires:

Active fires on 8/31/15. Map from http://activefiremaps.fs.fed.us/

The effects are not limited to the areas of burning. Smoke is being carried hundreds of miles eastward, leading to occasional serious decreases in air quality that can affect those with respiratory illness.

Another far less serious effect is the greatly diminished transparency of the sky. During the day the sky is a yellow-brown veil and at night dimmer stars are extinguished and the moon starts to look like it's in eclipse:

The full moon at an altitude of 34 degrees. It should be colorless, not yellow!

Most observing and imaging activities are put on hold until either the fires end or the winds shift. This degree of transparency loss has happened before, earlier this year and once last year. Prior to that I think one must go back about seven years or so to see a similar event.

Using the Celestron Field Flattener with an SBIG ST-8300M and FW8-8300

I purchased a used Celestron 9.25" XLT SCT intending to use it primarily for planetary imaging at f/10 or greater. There was no need for a focal reducer/flattener. I did purchase a used Celestron 94175 f/6.3 flattener/reducer so I could, if I wanted to, image deep sky objects. As it turns out, deep sky objects have been the C925's main use.

Reducer/Flatteners like the Celestron 94175 work best when at a specific distance from the sensor. Can this distance be known? Let's use the calculator at  Wilmslow Astro and find out.

For a standard f/10 SCT, the separation to give f/6.3 (where we presume the field will be flattest) should be 105mm.

Two nights ago I imaged NGC 7008, the Fetus Nebula, using my CCD and the focal reducer. The stars were nice and round all the way to the edge:

NGC 7008 (click to enlarge)

Astrometry.net reported that the image scale was 1.58" per pixel, which translates to f/6.0. According to the above calculator this should happen at a reducer/CCD separation of 113mm**. 

If the reducer is flattest at f/6.3 I need to decrease the separation from 113mm to 105mm, or by about 8mm.

Happily, I can do that. The nosepiece I was using was a 2" focuser-to-TeleVue IS adapter + IS to T-ring adapter. I also have a nosepiece that's just 2" focuser to T-ring, and amazingly it will let the CCD get about 8mm closer. Just about perfect!***

The next night I'm out I'll try this configuration and see if it gets the focal ratio correct and also produces round stars.

* This suggests that the performance of the reducer is rather insensitive to the separation. Here I'm 8mm off and the stars look good. This has limits, though. In a previous image a 12.7mm spacer was included and the stars at the corners were plainly distorted. That image had an image scale of 1.65" per pixel, which corresponds to a focal ratio of  f/5.75. The calculator says this happens at a separation of about 123mm. I measured the separation as about 125mm. That image suggests the separation reduction should be 20mm, which is roughly the same as 12.7 + 7.3 (the spacer plus the ~8 mm suggested by the other image).

** One thing that's usually left ambiguous in reducer explanations is the point on the reducer from which the CCD distance is measured. In the case of the system operating at f/6 I measured the CCD to be 125mm from the front of the reducer. which is the same thing as 114mm from the center and about 100mm from the rear thread base. Because the center position in only 1mm different from the formula's value of 113mm, and given the inexactness of my measurement, It's reasonable to conclude separation should be measured from the center of the reducer.

*** Is this an accident? The Antares 50mm long 2" focus tube on the back of the SCT makes this possible, but it predates the ST-8300 + filter wheel and couldn't be designed to work with their backfocus requirement. But it works out nicely, right?

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What does this say about using a 0.5 focal reducer? The model I have is a SmartAstronomy 2", which is probably identical to the GSO 2". GSO says that the focal length is 106mm and the optimal separation is 53mm. The calculator puts it a bit larger at 56mm. Because this reducer screws into 2" nosepieces, the closest I can get it to my CCD is about 69mm (measuring from the center of the reducer). That puts it at about f/4. I'd guess that's too far from ideal to produce pleasing images, but it's worth a try. A Lumicon low profile nosepiece could trim 9mm off that, getting it close: f/4.7. Trying that will cost about $35.

What's Correctable in this Image?

How do you assess an image's quality? Here are some factors I use to evaluate my images:

Stars:

• Are they round?
• Are they focused?
• Do they have pleasantly fuzzy edges?
• Are they free of color fringing due to optics or processing?
• Do they have color that's pleasing in both hue and saturation? 

Background:

• Is it a neutral gray?
• Is it as smooth as might be expected from the data?
• Is it free from any substantial gradients?

Nebulosity:

• Does it fade smoothly into the background?
• Is it the right color (both hue and saturation)?
• Has the data been processed correctly to reveal fainter portions?

 Defects:

• Is the image free from dust shadows and other unwanted diffraction effects?
• Have "rogue" pixels been cleaned up?
• Have the effects of vignetting been corrected?
• Is the image flat?

Here's my latest image, and let's see how it measures up.

Planetary nebula Jones 1 in Pegasus
(cropped but full scale, 105 minutes of RGB at f/6.3, ST-8300M binned 2x2)

Focus is decent but the stars are not round, probably indicating a tracking issue. The stars are reasonably fuzzy and show some color--not a lot, but enough for my taste.

Background color and intensity is good. The histogram isn't clipped, and the nebulosity fades smoothly into the background. So far as I can tell by looking at other images the object's color is fairly captured, as is the amount of structure given the imaging system.

Software was used to reduce the effects of light pollution and vignetting.

There weren't many bad pixels to clean up in this cropped image thanks to fresh dark and bias frames. The full field was not at all flat in a way that suggests that the sensor to flattener distance was significantly off.

Add that all up and I conclude this is an acceptable image but nothing special. A big improvement would be  to improve the tracking. Adjusting the sensor spacing would reduce the need for cropping. And collecting more data would help--it almost always does.

Regarding the tracking, I'm beginning to wonder if the Orion MiniGuider is adequate for guiding a 1480mm imaging scope. Some experimentation is in order along with trying different spacers to correct the flatness issue.

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.