Two years ago I purchased a used AT72ED. The scope I wanted was the AT65EDQ, but it was too pricey for my budget.
The AT72 was an able performer when paired with the Orion Field Flattener for Short Refractors (see my Andromeda Galaxy image in this blog post. I didn't care for the mounting foot, but it was easy to replace with rings. The focuser was no Moonlite but it held my heavy CCD + Filter wheel without problems. If it had a fault it was a little blue haloing on brighter stars. (I can't be sure if that wasn't my fault for not being a diligent focuser.)
Eventually a local club member decided to part with his AT65 and I was able to pick it up for not much more than the AT72 cost me. This made the AT72 expendable and I put it up for sale. I let it sit idle for a while on the club's merchandise forum. Finally this fall I listed it on Cloudy Nights and it sold along with the FF. The new owner is interested in using it to get into wide-field DSLR astrophotography, for which it's wonderfully suited.
The AT65 has been great this fall for chasing down the larger Astronomical League Bright Nebula objects. I recommend it to anyone who wants to get into imaging without spending a lot on an expensive telescope.
One of my telescopes is becoming something of an orphan. My old ShortTube 80, originally purchased as a travel scope to take to the 1998 Solar Eclipse in Aruba, doesn't have a role any longer. I'm holding onto it for now because it's still a good travel scope, and it's great as a guide scope. The Orion mini-guider has replaced the ST on my refractors, but I'm uncertain if the little scope will work well with my C925. That determination will have to wait for the summer when the weather is better.
Speaking of the weather, it's not been helpful. I got the usual fall head cold a couple of weeks before Thanksgiving. Then after the holiday the weather turned cold (frequently going sub-zero). I can handle temperatures down to around 10 degrees F, but zero is outside my optimal range of operation. Cold and Clouds have continued all December. I'm hoping for a better 2014!
Oh yes, one more thing. Astrobin did a major reworking of their site code and all my image links are now broken. I'll be fixing them as time permits.
Showing posts with label autoguiding. Show all posts
Showing posts with label autoguiding. Show all posts
Sunday, December 29, 2013
Wednesday, August 7, 2013
Two images from last weekend: M17, M20/21; Processing workflow.
I'm REALLY pleased with how this image of M17 came out:
The previous night's image was not quite as successful--M20 and 21. More about why in the next blog entry.
Both images are full-frame. For all the gory details of acquisition, look at my gallery on AstroBin. By way of a quick summary, both are LRGB images made with an SBIG ST8300M CCD, TeleVue 102 apo and autoguided CGEM mount under mag 6+ skies. Dark and bias frames only, no flats.
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| Messier 17 (Omega or Swan Nebula), click for full size |
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| M20 (Triffid) and M21 |
Imaging is SO much faster doing LRGB in the dark than it is to use narrowband filters under light-polluted skies.
I realize that the colors in the M17 image may look a little muted to you, but I kind of like them this way. What's particularly pleasing about the image from my standpoint is three-fold: The stars are nice and round right out to the edge of the field, meaning that my field flattener is doing its job perfectly; The merging of the LRGB channels seems to be spot-on. And finally, guiding with the Orion mini-guider works great at the 700mm focal length of the TV102 to correct mount errors. I'll eventually try out the mini on my C925.
My processing workflow continues to evolve. These two images were made by ping-ponging back and forth between Images Plus and Photoshop:
- Calibrate, align and stack using IP
- Stretch using PS
- Noise cleanup and star size reduction in IP
- LRGB combine in PS using Annie's Actions
- Color balance in IP
- Masked sharpening in PS
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
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
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