APOD: The Colorful Clouds of Rho Ophiuchi (2013 Dec 03)

[Anthony Barreiro]

I much prefer an equatorial to an altazimuth mount. The equatorial is much more elegant and graceful, moving as things move across the sky, rather than needing to approximate an arc from line segments. Once you've done it a few times, a non-go-to equatorial mount takes less than a minute for a good-enough polar alignment. And the equatorial mount just loves to look straight up at the zenith! The only down side is that you'll have a harder time observing Sigma Octantis.

I do still have my first altazimuth mount, I use it mostly for quick looks at the Moon or planets when they're not too far from the horizon.

The things I like about my alt-az goto scope are:

1) It is programmable (I have a long term plan to develop various bits of software for any device using the published NexStar Communication Protocol, including better tracking accuracy ).

2) You can "shoot blind" with a camera at dim objects, because you have pretty accurate coordinates in RA,DE. This makes mosaics easier too.

3) I don't have to star-hop, and I can saves positions and re-find things quickly on subsequent nights.

4) It is easier to align with the sky accurately, no matter where I set the tripod (I need to fully unpack/pack the scope each session).

5) It typically allows for more comfortable viewing positions.

...

I would still like an equatorial mount as well, though, primarily for photography, but I'd want it to be a goto. I'm hooked on gotos.

I have a couple of go-to mounts, one altazimuth and one equatorial. The equatorial Celestron CG5 is my best mount. I really like go-to when I'm doing a public program --push a button and an object is centered in the eyepiece! But when I'm observing by myself, i'll often go to the bright star nearest the object I want to look at and star-hop to the object. I don't want to lose that skill, and there's a certain satisfaction in hunting something down.

As an astrophotographer you really should get an equatorial mount. We could start a charitable campaign!

[Chris Peterson]

The things I like about my alt-az goto scope are:

1) It is programmable (I have a long term plan to develop various bits of software for any device using the published NexStar Communication Protocol, including better tracking accuracy ).

Well, yes, but nearly all modern equatorial mounts have computerized goto, as well.

2) You can "shoot blind" with a camera at dim objects, because you have pretty accurate coordinates in RA,DE. This makes mosaics easier too.

See above.

3) I don't have to star-hop, and I can saves positions and re-find things quickly on subsequent nights.

See above.

4) It is easier to align with the sky accurately, no matter where I set the tripod (I need to fully unpack/pack the scope each session).

See above.

5) It typically allows for more comfortable viewing positions.

This is often the case. But it depends where you are viewing in the sky.

The primary limitation of an altaz mount is that it's nearly useless for imaging, since you are pushing things even with 30-second exposures, and that's way to short to get good results.

Another consideration is that while equatorial mounts do a poor job operating near the pole, altaz mounts fail at the zenith. And most people spend more time with their scopes near the zenith than near the pole.

Of course, all you need to use an altaz SCT in polar mode is an inexpensive wedge.

[stephen63]

The primary limitation of an altaz mount is that it's nearly useless for imaging, since you are pushing things even with 30-second exposures, and that's way to short to get good results.

Wait a minute! Aren't you the one who stated that short exposures collect the same amount of data as one long exposure of equal duration? In reality, the limiting factor for an Alt/Az mount is field rotation.

[Chris Peterson]

The primary limitation of an altaz mount is that it's nearly useless for imaging, since you are pushing things even with 30-second exposures, and that's way to short to get good results.

Wait a minute! Aren't you the one who stated that short exposures collect the same amount of data as one long exposure of equal duration? :lol2: In reality, the limiting factor for an Alt/Az mount is field rotation.

No, you misunderstood me. What I said is that the object S/N is the same for two exposures of the same length made at the same aperture, regardless of focal ratio.

Short exposures are a problem because they are dominated by readout noise. Typical sky conditions require minimum subexposure times of several minutes or longer. Field rotation is what limits the subexposure time. By itself, it's no limitation at all, since you can rotate frames when you stack them. But if your maximum exposure time is only a few seconds, you're never going to be able to get decent images of any but the very brightest objects.

[stephen63]

The primary limitation of an altaz mount is that it's nearly useless for imaging, since you are pushing things even with 30-second exposures, and that's way to short to get good results.

Wait a minute! Aren't you the one who stated that short exposures collect the same amount of data as one long exposure of equal duration? In reality, the limiting factor for an Alt/Az mount is field rotation.

No, you misunderstood me. What I said is that the object S/N is the same for two exposures of the same length made at the same aperture, regardless of focal ratio.

Short exposures are a problem because they are dominated by readout noise. Typical sky conditions require minimum subexposure times of several minutes or longer. Field rotation is what limits the subexposure time. By itself, it's no limitation at all, since you can rotate frames when you stack them. But if your maximum exposure time is only a few seconds, you're never going to be able to get decent images of any but the very brightest objects.

http://asterisk.apod.com/viewtopic.php?f=8&t=32335

The only reason the summed exposures aren't exactly equivalent to a single exposure is readout noise. Without that one factor, there is absolutely no difference, either theoretically or practically. In practice, a series of stacked exposures always produces a final image with worse S/N than a single image with the same exposure time.

At what point is the data dominated by readout noise?

[Chris Peterson]

At what point is the data dominated by readout noise?

The calculation for minimum subexposure time is based on the sky background. Once the sky background becomes the dominant noise source, there's limited gain in longer subexposures. There are a number of online subexposure calculators (e.g. this one). Typical values range from two or three minutes in heavily light polluted locations to 30 minutes or more in dark sites, or when using narrowband filters.

[stephen63]

All is not lost, Nitpicker! Your Alt/Az doesn't have to be, well, an Alt/Az. With the addition of a simple wedge, you can join the ever growing ranks of deep sky astrophotographers. Then you can start worrying about sky limited exposures!
http://sctscopes.net/Photo_Basics/Acces ... edges.html

[Nitpicker]

The things I like about my alt-az goto scope are:

1) It is programmable (I have a long term plan to develop various bits of software for any device using the published NexStar Communication Protocol, including better tracking accuracy ).

Well, yes, but nearly all modern equatorial mounts have computerized goto, as well.

2) You can "shoot blind" with a camera at dim objects, because you have pretty accurate coordinates in RA,DE. This makes mosaics easier too.

See above.

3) I don't have to star-hop, and I can saves positions and re-find things quickly on subsequent nights.

See above.

4) It is easier to align with the sky accurately, no matter where I set the tripod (I need to fully unpack/pack the scope each session).

See above.

5) It typically allows for more comfortable viewing positions.

This is often the case. But it depends where you are viewing in the sky.

The primary limitation of an altaz mount is that it's nearly useless for imaging, since you are pushing things even with 30-second exposures, and that's way to short to get good results.

Another consideration is that while equatorial mounts do a poor job operating near the pole, altaz mounts fail at the zenith. And most people spend more time with their scopes near the zenith than near the pole.

Of course, all you need to use an altaz SCT in polar mode is an inexpensive wedge.

Only my points 4 and 5 were in regard to alt-az mounts. The other points were in regard to goto mounts. I love goto mounts.

...

All is not lost, Nitpicker! Your Alt/Az doesn't have to be, well, an Alt/Az. With the addition of a simple wedge, you can join the ever growing ranks of deep sky astrophotographers. Then you can start worrying about sky limited exposures!
http://sctscopes.net/Photo_Basics/Acces ... edges.html

Thanks. I've contemplated getting a wedge. Not sure though, they look a little tippy and awkward for those of us near the tropics, especially with a heavier mount/scope to attach. I feel like I would be throwing good money after bad. I think I would be better served getting a dedicated equatorial mount (which can accept the Vixen bracket on my OTA). I haven't heard of many non-golfers who love their wedge. Besides, according to my calculations and observations, the imprecision in the spur gears of my alt-az mount, typically makes it hardware limited, even with the modest 16 bit software resolution it ships with (reprogrammable to 24 bit, which might be useful for a better worm-gear system). I think I want something a little more precise before I go any deeper. There's only so much fun one can have compensating for backlash.

I no longer know why, but when I first chose my scope, I was convinced I was only interested in the brighter solar system objects and artificial satellites (for which an alt-az is perfectly well suited). It was only a few months after the purchase that I attached a DSLR to the scope and opened the shutter for long enough to detect all this stuff that I couldn't even see with my eyes at the eyepiece. It was only then that I started to think about field rotation as a problem. Sadly, it will probably be a year or two before I can convince the "accounting department" to fund an equatorial mount. (If you want to buy stuff under warranty in Australia, it is typically double the price of the USA, and that has little to do with exchange rate or taxes.)

[Anthony Barreiro]

Thanks. I've contemplated getting a wedge. Not sure though, they look a little tippy and awkward for those of us near the tropics, especially with a heavier mount/scope to attach. I feel like I would be throwing good money after bad. I think I would be better served getting a dedicated equatorial mount (which can accept the Vixen bracket on my OTA). I haven't heard of many non-golfers who love their wedge. Besides, according to my calculations and observations, the imprecision in the spur gears of my alt-az mount, typically makes it hardware limited, even with the modest 16 bit software resolution it ships with (reprogrammable to 24 bit, which might be useful for a better worm-gear system). I think I want something a little more precise before I go any deeper. There's only so much fun one can have compensating for backlash.

I no longer know why, but when I first chose my scope, I was convinced I was only interested in the brighter solar system objects and artificial satellites (for which an alt-az is perfectly well suited). It was only a few months after the purchase that I attached a DSLR to the scope and opened the shutter for long enough to detect all this stuff that I couldn't even see with my eyes at the eyepiece. It was only then that I started to think about field rotation as a problem. Sadly, it will probably be a year or two before I can convince the "accounting department" to fund an equatorial mount. (If you want to buy stuff under warranty in Australia, it is typically double the price of the USA, and that has little to do with exchange rate or taxes.)

I agree that a German equatorial mount is much better than a wedge. I would look on the various online marketplaces for a used mount. Astronomy enthusiasts are usually honest and fair, in my experience, and people often sell equipment they've only used a few times that doesn't suit their needs.

[Nitpicker]

I agree that a German equatorial mount is much better than a wedge. I would look on the various online marketplaces for a used mount. Astronomy enthusiasts are usually honest and fair, in my experience, and people often sell equipment they've only used a few times that doesn't suit their needs.

Cheers Anthony, you could be right. I happen to know the "accounting department" just loves to find a bargain on ebay.

[Chris Peterson]

I agree that a German equatorial mount is much better than a wedge

I'd go for an equatorial fork mount. It's more kinematically stable than a GEM, and generally superior for imaging given its ability to track the entire sky with no reversal. It's also easier to balance.

That said, most common equatorial mount designs can be problematic when used in the tropics. A modified English mount is good, but you're not going to find one commercially available.

[Nitpicker]

Cheers Anthony, you could be right. I happen to know the "accounting department" just loves to find a bargain on ebay.

Or maybe I'm just suffering from a mounting case of scope envy. My much-loved combination of a handy little 6" alt-az scope and DSLR, allows me to detect stars down to vmag 15. It has taken the number of stars I can identify -- just from by back yard and not including the northernmost stars -- from ~2,000 to ~100,000,000, at least according to: http://www.stargazing.net/david/constel ... stars.html

[Chris Peterson]

Or maybe I'm just suffering from a mounting case of scope envy. My much-loved combination of a handy little 6" alt-az scope and DSLR, allows me to detect stars down to vmag 15.

What is the make and model of your scope? With a wedge (which will probably work pretty well given a light weight mount) I'll bet your setup is good enough to track well for at least 5 minutes. That's long enough that you should be able to match many of the images that appear on APOD. Probably half the amateur images we see here are made with scopes of 6" or less aperture, and 5-minute subs don't require a very high end mount.

[Anthony Barreiro]

I agree that a German equatorial mount is much better than a wedge

I'd go for an equatorial fork mount. It's more kinematically stable than a GEM, and generally superior for imaging given its ability to track the entire sky with no reversal. It's also easier to balance.

That said, most common equatorial mount designs can be problematic when used in the tropics. A modified English mount is good, but you're not going to find one commercially available.

Is there a difference between an equatorial fork mount and a wedge?

Regarding the difficulty of using a German equatorial mount in the tropics, the new iOptron ZEQ25 has an innovative design with the counterweight and OTA on opposite sides of the center of gravity. This makes for a more stable mount with a lighter weight, and allows the mount to be used at any latitude, even the equator. Given that Nitpicker needs to set up and break down equipment for each observing/imaging session, weight is a major consideration. I haven't used this mount personally, but it got a glowing review in Sky and Telescope.



It's on my wish list, in case anybody wants to start a charitable campaign.

[Chris Peterson]

Is there a difference between an equatorial fork mount and a wedge?

An equatorial fork mount is a fork mount that is operating with its primary axis aligned with the pole. A wedge is an adapter that allows a fork mount to be operated at different angles in order to achieve polar alignment. It certainly isn't the only way. Nearly all professional telescopes using equatorial mounts are on forks, but they don't use wedges.

[Anthony Barreiro]

Is there a difference between an equatorial fork mount and a wedge?

An equatorial fork mount is a fork mount that is operating with its primary axis aligned with the pole. A wedge is an adapter that allows a fork mount to be operated at different angles in order to achieve polar alignment. It certainly isn't the only way. Nearly all professional telescopes using equatorial mounts are on forks, but they don't use wedges.

Thanks Chris. I've seen these in dedicated observatories where the telescope is permanently mounted to a pier. They're massive. I can't recall seeing anybody use something like this in a parking lot or their back yard.

[Nitpicker]

Or maybe I'm just suffering from a mounting case of scope envy. My much-loved combination of a handy little 6" alt-az scope and DSLR, allows me to detect stars down to vmag 15.

What is the make and model of your scope? With a wedge (which will probably work pretty well given a light weight mount) I'll bet your setup is good enough to track well for at least 5 minutes. That's long enough that you should be able to match many of the images that appear on APOD. Probably half the amateur images we see here are made with scopes of 6" or less aperture, and 5-minute subs don't require a very high end mount.

I've probably overstated the stability issue with equatorial mounts near the tropics. I am still 27 degrees away from the equator and I'm sure there are loads of people happily using equatorial mounts nearer still to the equator. Given that storage, weight and price considerations are all important to me, I think I'd be inclined towards a GEM over any other kind of equatorial mount.

I have a Celestron 6SE, and as much as I love it, I really doubt the hardware (specifically the gearing) is precise enough to bother with a wedge and deep-as-possible sky photography. It is not typically sold for deep sky photography, although that may have been an early intention, as it has an auto-guiding port (for which I have only a theoretical understanding). Regardless, Celestron has since discontinued the wedge specifically for the 6SE and 8SE (which share the same alt-az mount). Whilst there may be independent commercial reasons for that decision, I suspect it stems from a technical issue. There is a heavy-duty wedge available from Celestron, which I have read can also be used with my scope, but it is a little hard to find in this part of the world, and is thus rather expensive for what it is. And if I had a bigger, heavier scope & mount, I suspect I'd have trouble lifting it and attaching it on such an incline.

As far as I can determine, the two DC servo motors on my alt-az mount (which, in general, both need to run at a variable speed to track a celestial object over longer periods) are set by the controlling software with a speed in increments of 0.25 arcsec/sec. Over time, the resulting analogue-to-digital quantization errors can accumulate to cause tracking errors, the magnitude of which is dependent on the part of the sky being observed. Some of these errors are periodic, some are not, but they are normally quite small. (Perversely, the mathematics of this problem of control engineering is another reason why I have a fondness for alt-az mounts.) However, the tracking errors I have observed in the field, typically swamp the much smaller errors caused by quantization. This suggests to me that the periodic and other errors inherent in the gears (compensation for which the 6/8 SE mount does not support) makes the overall system significantly hardware limited.

With an alt-az mount, one is limited to sub-exposures of about 30 seconds, to avoid field rotation. Again, this is dependent on the part of the sky being observed (avoid zenith and meridian where possible), as well as the FOV. My experience with this scope is that on a typical night, with a moon-size FOV, I will discard about three-quarters of my subs due to unacceptable tracking errors, completely unrelated to field rotation. If I were to instead use an equatorial wedge, so that only one motor would be running (and at a constant speed), I do suspect my success ratio would be a bit better. But I suspect it would be very rare to get acceptable tracking for a 5 minute sub-exposure. I would be most interested to hear from anyone who has had success using a wedge with this mount. I've not heard of anyone.

Anyway, for the time being I've decided to continue with my 30 second subs in alt-az mode. I still have a lot to learn about image stacking and processing, so I should focus on those aspects before forking out more cash on an equatorial system. Thanks for all the input and discussion.

Edit: I may not be 100% correct about my determination of how the in-built software actually controls the motors when tracking. I have assumed it works how the Nexstar Communication Protocol suggests it does. I might not be 100% correct about the exact cause of the more serious tracking errors actually observed, either. But my scope definitely has a nice orange colour.

[Chris Peterson]

I have a Celestron 6SE, and as much as I love it, I really doubt the hardware (specifically the gearing) is precise enough to bother with a wedge and deep-as-possible sky photography.

I think you might be surprised with what is possible. Here's a little known secret of astroimaging: better equipment doesn't produce better images (assuming you don't have junk... which you don't). What better equipment does is make imaging a lot easier. You spend less time fighting instrument limitations. But in fact, your equipment is capable of producing every bit as good of images as a 6" Astrophysics refractor mounted on a Paramount.

If the Accountant authorizes an equipment upgrade, go for it. While fork mounts are really better for imaging, they aren't as portable, so if that's an issue, by all means go with a GEM. But if no upgrades are on the horizon, consider equatorially mounting your scope. You can make a wedge out of wood- for 5-minute exposures, nothing fancy is required at all. And your scope will perform much better equatorially, because the altitude/declination axis will be unchanging (except for tiny corrections if you guide), and the azimuth/polar axis will be rotating at a steady rate. There is every reason to believe that you should be able to get 5-minute exposures, which is about the limit you'd want with a DSLR, and is probably a reasonable limit for your sky conditions, as well. And believe me, you'd be amazed by the difference in image quality if you use 5-minute subs versus 30-second subs.

[Nitpicker]

I have a Celestron 6SE, and as much as I love it, I really doubt the hardware (specifically the gearing) is precise enough to bother with a wedge and deep-as-possible sky photography.

I think you might be surprised with what is possible. Here's a little known secret of astroimaging: better equipment doesn't produce better images (assuming you don't have junk... which you don't). What better equipment does is make imaging a lot easier. You spend less time fighting instrument limitations. But in fact, your equipment is capable of producing every bit as good of images as a 6" Astrophysics refractor mounted on a Paramount.

If the Accountant authorizes an equipment upgrade, go for it. While fork mounts are really better for imaging, they aren't as portable, so if that's an issue, by all means go with a GEM. But if no upgrades are on the horizon, consider equatorially mounting your scope. You can make a wedge out of wood- for 5-minute exposures, nothing fancy is required at all. And your scope will perform much better equatorially, because the altitude/declination axis will be unchanging (except for tiny corrections if you guide), and the azimuth/polar axis will be rotating at a steady rate. There is every reason to believe that you should be able to get 5-minute exposures, which is about the limit you'd want with a DSLR, and is probably a reasonable limit for your sky conditions, as well. And believe me, you'd be amazed by the difference in image quality if you use 5-minute subs versus 30-second subs.

Thanks Chris. I want to think you're right. My engineering degree taught me many things, including exactly why I should be wary of building things myself. But I might just be foolish enough to have a go at a home-made wedge, maybe even in steel. I can hear myself swearing already.

Seems like the only suitable mass-produced option currently in production is this:
http://www.celestron.com/astronomy/cele ... wedge.html
It looks fantastically over-engineered, weighs ~18kg, and even has azimuth adjustment (which I imagine is very handy). But I don't think I could get one delivered for much less that A$700, which I cannot justify until I've confirmed that my Az/RA drive is smooth enough running solo at a constant speed. Maybe I need to borrow or trial one of these wedges.

[geckzilla]

I can't figure out if astronomy is an elitist hobby or if things really need to be that expensive...

[neufer]

I can't figure out if astronomy is an elitist hobby or if things really need to be that expensive...

<<Milton Lasell Humason (August 19, 1891 – June 18, 1972) was an American astronomer. Much of the work Humason performed was actually credited to Hubble, the two of whom worked together for many years. Due to merest chance, Humason missed discovering Pluto. Eleven years before Clyde Tombaugh, Humason took a set of four photographs in which the image of Pluto appeared. There is persistent speculation that he missed discovering the planet because it fell on a defect in the photographic plate. This is unlikely, however, given that it appeared in four separate photographs over three different nights.

Humason dropped out of school and had no formal education past the age of 14. Because he loved the mountains, and Mount Wilson in particular, he became a "mule skinner" taking materials and equipment up the mountain while Mount Wilson Observatory was being built. In 1917, after a short stint on a ranch in La Verne, he became a janitor at the observatory. Out of sheer interest, he volunteered to be a night assistant at the observatory. His technical skill and quiet manner made him a favorite on the mountain. Recognizing his talent, in 1919, George Ellery Hale made him a Mt. Wilson staff member. This was unprecedented, as Humason did not have a Ph.D., or even a high school diploma. He soon proved Hale's judgment correct, as he made several key observational discoveries. He became known as a meticulous observer, obtaining photographs and difficult spectrograms of faint galaxies. His observations played a major role in the development of physical cosmology, including assisting Edwin Hubble in formulating Hubble's law. In 1950 he earned a D.Sc. from Lund University. He retired in 1957.>>

[stephen63]

I can't figure out if astronomy is an elitist hobby or if things really need to be that expensive...

No more so than owning a Harley Davidson. Talk about price gouging parts, in both cases!

[Chris Peterson]

I can't figure out if astronomy is an elitist hobby or if things really need to be that expensive...

Astronomy is not elitist. To a small degree, amateur astronomical imaging is, perhaps. That said, top notch equipment is a joy to work with, even if it doesn't generally deliver significantly better results than much less expensive equipment. It isn't overpriced, and most people who can afford it consider it worth every penny.

BTW, while high end equipment doesn't intrinsically produce better images, there is certainly a correlation between the best images and the best equipment. That can be explained by the fact that the most practiced, committed imagers have generally invested in very good equipment.

Compared with many other hobbies, imaging isn't really all that expensive. For the cost of a car you can have a really nice setup. And you can do really good work with much less than that.

[Nitpicker]

I can't figure out if astronomy is an elitist hobby or if things really need to be that expensive...

Astronomy is not elitist. To a small degree, amateur astronomical imaging is, perhaps. That said, top notch equipment is a joy to work with, even if it doesn't generally deliver significantly better results than much less expensive equipment. It isn't overpriced, and most people who can afford it consider it worth every penny.

BTW, while high end equipment doesn't intrinsically produce better images, there is certainly a correlation between the best images and the best equipment. That can be explained by the fact that the most practiced, committed imagers have generally invested in very good equipment.

Compared with many other hobbies, imaging isn't really all that expensive. For the cost of a car you can have a really nice setup. And you can do really good work with much less than that.

I agree astronomy is not an elitist hobby. One just has to go outside and look up to get started.

I might quibble with the fact that the only mass-produced wedge available for my scope is super high-end and that it looks to be about US$400 if purchased in the US, which is a more reasonable price for such a fancy wedge. What really rankles me is that it would almost certainly be manufactured in China, which is a good deal closer to me than the US. Globalisation means that all countries are equal, but some are more equal than others.