[MtnGoat]

It's due to a combination of things. One factor is the size of the pixels on the sensor compared to the size of the star image at the focal plane of the lens. Ideally of course a star is a point source, but when focused by a lens it has a finite size which generally covers more than one pixel. You can find sensors with very tiny pixels, but then you lose sensitivity because your little tiny light bucket catching photons is smaller, and you inevitably lose a few at pixel boundaries too. So what many folks do is 'bin' their sensors with tiny pixels, meaning they sum them during readout into 4x4 blocks. Of course then you're back to a larger effective pixel, but at the expense of resolution. And of course at the native size with tinier pixels, the apparent disks are even larger because they're composed of more pixels. Argument rage endlessly about what pixel size is best, large ones are more sensitive but less resolution, though the stars appear smaller in pixel size. (These discussions are like the unleashed dog or gun threads of the astrohead forum world, along with which eyepieces are 'best', refractors vs reflectors, etc.) Secondly, as the star twinkles due to air movement and as tiny irregularities in tracking motion occur, it spreads the star image around to adjacent pixels. The more gain you apply in post processing, the more the stars 'spread' as an adjacent pixel which only got a single taste of the star for an instant, now becomes brighter. The third is the effect of the native image size, which is actually only 510x480, being resized when it's viewed. This of course turns a star a couple pixels across into even more pixels on the monitor. Brighter stars tend to spill into more pixels, thus you see in the images that the fainter ones tend to be nice and small and the bright ones are larger. You can get minimize this to a degree but you are still limited by the math and statistical functions of discretizing what is an essentially an analog process, but it takes very careful matching of scope focal length and sensor size, and even then it's nearly impossible to get away from. Note that on film in old school mode, the same thing happened too. Adjacent emulsion grains change due to light spread around the focal plane a tiny bit, and your stars get bigger looking the brighter they are. Shots taken with huge sensors such as a DSLR tend to present smaller looking stars until you blow them up to about the scale you're seeing here, then they wind up looking similar. There are all kinds of books on matching pixel sampling to focal length and image scale and all kinds of detail depending on how far you want to go and what you consider acceptable. With me taking a garden variety entry level imager and slapping it on whatever scope I can aim in the general direction of up, i'm not even close to that level of perfection.

[Backpacker Joe]

Thanks for the info. MtnHubble. I might try some of that when I get my 400 F5.6 L lens. Should be some fun.

[MtnGoat]

Here are examples of what you can accomplish with a basic DSLR, a wide angle lens..and a plain old fixed tripod. the nice thing about short focal lengths is that it takes a while for the stars to trail, so you can get some longish exposures, then you just mooch around for SW that will allow you to stack frames together. Basic Astrophotography from a fixed tripod

[MtnGoat]

Took this one while wating for Comet Lulin to clear the trees. M64, the Blackeye Galazy. 17 million light years distant. 30 minutes of 15 second subframes.

M64 the "Black Eye" Galaxy, 30 minutes of 15 second subframes Orion Short tube 90mm

[MtnGoat]

Two of the three galaxies in the "Leo Triplet". M65 and M66, at roughly 30-35 million light years. The third member is slightly too far away to capture in the same shot with this setup.

M65 and M66, two spiral galaxies at about 35 million light years 25 minutes with the ST90mm/EQ2/DSI-C 15 second subframes

[MtnGoat]

Here's some other stuff

Not bad for a first try, Zhumell 12" F5 Newtonian Reflector

The Moon..duh. Zhumell 12"F5 Newtonian Reflector

NCG 3628 is 35,000,000 light years distant.The galaxy is seen edge on and the effects of tidal interactions with two galaxies which lie slightly out of frame are evident in the tweaked dust lanes at the edges. ST90, 32 minutes

The blue halos around the bright stars in all ST90 images are the result of various factors... Factor one..refractors whose main objective lens consists of only two elements cannot fully correct for the fact that different colors of light bend different amounts when passing through the glass. This means the focus points are in different places, so when most of the other colors are in focus, the far end of blue is not. Hence the blue halos are the out of focus blue. Factor two, doublet lenses can get around this to a degree by using very long focal ratios (F10 and up) so that the light isn't bent as sharply as a shorter scope (F8ish and below is where it really starts to get noticeable) and thus the blue isn't as far out of focus. This scope is an F5.6 which shows a fair amount of this color aberration. Factor three, I do not wish to spend the 700+ bucks (equivalent aperture) to get a triple lens refractor assembly, using the new types of glass, which can do a near perfect correction. Stopgap for cheap scopes? A minus V filter. This is a bandpass filter which lets through most visible light but aggressively cuts off blue. It will result in a slight color shift, but the halos should be greatly reduced. We'll find out if this is acceptable, I have one coming. It should work as well in any of my other refractors, I am also very interested in trying it in the 120mm which is an F5 and also exhibits this aberration.

[MtnGoat]

Got this in on Friday night while battling a high, thin moonlit haze. The yellow to red tone of the cluster is not a processing effect, it consists of mostly relatively ancient stars which are nearly all in that color range at this point in their evolution.

M3 in Canes Venatici. This globular cluster is 34,000 light years away and is estimated to contain 500,000 stars. ST90, F5.6, 80 frames @ 4 secs, Meade 4504 mount, Meade DSI - C

[MtnGoat]

It's snowing, a full moon, and I'm stuck on motorizing my new mount for lack of hardware, once more. So I mined some older images and reprocessed them using some tricks I've learned but never applied to images done prior to the trick learnage. Also there is a recent one of the Sombrero galaxy.. So here they are, reaching back to August...particularly pleased with how M13 turned out

The famous Sombrero galaxy (M104). I bumped the mount at some point during the run and the bright stars show this screwup. I'll be back for more, this thing is cool looking.

Good old standby Great Nebula in Orion with the ETX70.

The famous Dumbell Nebula, M27 in Vulpecula. This is the gas shell ejected by a dying star, carrying heavy elements which will enrich the galactic medium and create new planets and stellar systems.

M106 is 23 million light years distant, and possesses an extremely active and violent core.. X rays, strong radio emission, and water maser radiation pour from this galaxy. Massive dust lanes wrap the exterior of the entire system, the spiral pattern behind them is evident when closely examined. High energy physics observations reveal particles being ejected from the core at relativistic velocities. The traces of a faint outer halo are barely evident as a faint brightening in spots beyond the rim.

M13 is cool

The Great Cluster in Hercules with the Schmidt Newt 8". Short subframes help keep star images tight. This huge cluster is the 2nd brightest in the sky, the brightest in the northern hemisphere, and is visible to the naked eye on a good night

[MtnGoat]

In spite of the waxing moon high in the sky lighting up a thin layer of haze, I got some postable stuff last sat night.

Another interesting one worth a clear night to reshoot. 47 million lights

not bad for a first go. should be much better minus the moon and haze. M100, 52 million lights shot on a bright moon, hazy night

M100 is kind of cool, worth another try. more distant back ground galaxies are a couple places with this shot. they clutter many of my shots in this region. it's lousy with those things out there, i tells ya

NGC 4490 just hit it's lil buddy and both are all out of whack. a couple other faint small faint fuzzballs in image are likewise still more galaxies.

good old M13. probably the second best shot i've ever done of this.

[MtnGoat]

Got some decent shots last night, 1 in the hour before the moon came up, a few after. However, this is the one that is the coolest. Just a smudge. A speck. Really tough to target and make sure I had the right spot. This smudge is probably one of the most distant things I will ever image...because it is so far away it is getting darn close to the age of the entire universe. The photons I caught from this image.... are 12 billion years old. Yup, this object is 12 billion light years distant, getting dang close to the estimated age of the universe. APM 08279 is a distant quasar visible only because there is an unseen galaxy between here and there whose gravity bends the light and makes a 'lens' effect...thus magnifying and brightening the far more distant object behind it, shown in the image. the light from this object has been on it's way here for twice as long as our entire solar system has existed. The field here is packed with small, distant galaxies. Nearly every smudge that is not clearly a star, is actually a galaxy.

APM 08279 is a gravity lens quasar and is about 12 billion light years distant.

[RayD]

Good stuff, Goat! It always blows me away when I read the distances to some of these light sources....the light that takes millions of years to travel here is still visible! Wow! Some energy, eh? I try to wrap my mind around what kind of train whistle could be heard across a continent.

[MtnGoat]

Another one from Sat nite. In spite of the full moon and haze, it looks pretty good. This is one I'll definitely return to for at least an hour with moonless skies. The blue blobs in the spiral arms are nebulas containing star formation regions.

M101 lies about 27 million light years away. It was discovered in 1781 and is considered one of the prototypical "Grand Design" spiral galaxies.

[MtnGoat]

Last weeks moonless nights and clear skies were a bonanza for me, I was up way too late 5 nights in a row. Luckily, some of them were even on non work nights. Spring is galaxy season and I've been a busy boy. Here's a lastest redo of M51 and one of a field with several large spirals in it. I'll post up the rest of the catch later.

The Whirlpool Galaxy and companion in a 38 minute exposure. The diffuse gas cloud surrounding the companion galaxy is clearly visible.

NGC4216 and Friends. NGC4216 is about 40 million light years away. At lower right is NGC4206 at about 50 million, and the smuge at upper right is IC771 at 240 million.

[MtnGoat]

Last night the moon was finally rising late enough to not blow out all the faint fuzzies, and Sunday's all day windfest got rid of the hot day haze that had been building. As a result, last night was clearer and darker than it's been for at least three weeks. Here's the result...a galactic sampler, three galaxies in decent detail amidst a lot of other more distant fuzzies which can be seen if you look closely. Two spirals, one face on, one edge on, and an elliptical system between them. This small association is a wonderfully compact arrangement which isn't very well known. Makes for very cool pictures! When I set up, my intent was to shoot 5985 only, the one at left. I had no idea it had friends. After I set up and aligned, and then slewed to target, I had some error and all I saw was the edge on one. Since it was distinctive, I pulled up Google Sky to see where I was looking and was thrilled to find I was near the right spot, and framing properly would bag me three for the price of one. This is what I like about this game, there is always a surprising new treat even when you think you have it all down. These galaxies are about 100 million light years distant. The image consists of a composite of 70 frames at 60 sec each. Taken with the Meade 8" F4 schmidt newtonian.

[MtnGoat]

Been getting some decent stuff in the last month or so, here's a sampler. I also made a moving .GIF of Pluto's motion over 48 hours, but Flickr doesn't store this format correctly, so no Pluto for you! Also added a couple variable stars to my program every night, both are explosive variables with a large brightness range and I intend to attempt a movie of them changing brightness over time. I figured two was a reasonable number to work, now that I am used to finding them it only adds about 15 minutes to my work each night, not so bad. When I started, both were near peak brightness and it's been interesting to see what is going on with them night by night. One has begun fading and continued, the other faded a bit, and now seems to be getting brighter again. anyhow...

Very bright and huge globular cluster M22 in Sagittarius

The Omega Nebula, M17

M16, Eagle Nebula, showing the "Pillars of Creation" dust features at center

The Bubble Nebula in Cassiopeia

The Cocoon Nebula, Cygnus

M24 in Sagittarius contains many, many stars and a dark nebula, seen at top center

A hot bright new star illuminates the nebula that gave birth to it