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Qualitative Comparison Between Astronomical Telescopes

02/21/2014 12:39 AM

Hello

I want to purchase a new telescope for Astronomy photography and observations.

Can any one has experience and can guide me about the best type of telescope for photography, considering the various types of optical defects... Coma, Viganetting, Sharpness, astigmatism.....

The photography is to be done using full frame DSLR and CCDs

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#1

Re: Qualitative comparison between Astronomical Telescopes

02/21/2014 1:13 AM

Well it depends on several variables aptly laid out at this link.....

http://www.astropix.com/HTML/I_ASTROP/SCOPES.HTM

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#2

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 8:09 AM

Like anything, it depends on your budget, which you did not state.

The Schmidt-Cassegrains are the popular picks for photography. Meade and Celestron are the two major brands.

Pay particular attention to the mounts and drives. You will need both extra stability and precision in the drive system to do photographic work.

I highly recommend finding a local astronomy club and buddy-up with someone there that has experience with your goals.

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#3

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 8:10 AM

I've been an astronomer for years and have used a variety of scopes.

The best astro-photographs I've seen have been taken using apochromatic refractors. These are relatively expensive telescopes compared to other designs (reflectors or standard achromatic refractors) of the same aperture and f/ratio. But by using a 3-lens system including low-dispersion glass, these apochromats produce stunning images with DSLRs and astronomical CCD cameras.

You'll need to mount the telescope on a high quality 'goto' style equatorial mount that has a port for an auto-guider system. You'd need to spend at least $800 US to get one capable of tracking properly for long-exposure photography (like the Celestron advanced VX mount, or better). [Personally I'd avoid Meade; I've had a couple bad experiences with them.]

There are a variety of telescopes and telescope mounts sold by OPT: http://www.optcorp.com/ .

Orion sells a nice auto-guider package, which includes an 80 mm guide scope and the auxiliary guide camera. Autoguiding-Solutions

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#4
In reply to #3

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 8:18 AM

Also, a number of guys on the photography blog I visit give rave remarks on the All-View Sky-Watcher camera mount. The price is extraordinary for what it does.

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#5
In reply to #4

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 11:51 AM

It's from Celestron (or at least it's badged as Celestron), and everything I've gotten from Celestron has been very good. [Orion and iOptron make similar devices.]

It's an altazimuth mount, which will produce field rotation on the CCD surface. The user would have to take short exposures and then stack them and de-rotate the images in the stacking process. (As you probably know.) Or the user could buy a 'de-rotator' device that holds the camera and compensates for field rotation.

It just seems easier to me to buy a equatorial mount and not deal with those extra steps and know that I can take really long exposures without much fuss. I'd use it with an auto-guider, or pay a bit more for an astro-camera that includes a secondary CCD for auto-guiding.

My own set up uses a home-built 10 inch scope on a Celestron CGEM. I've been very pleased with its performance thus far.

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#7
In reply to #5

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 1:05 PM

I think image stacking is always a better choice as it reduces the thermal read noise, at least somewhat.

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#8
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Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 2:04 PM

Agreed, but you can stack long exposures, too. In my experience stack of 12 5-minute exposures gives better results with less effort than a stack of 60 1-minute exposures. And with a EQ mount, you get the whole CCD frame for every image. With an Alt-az mount, the field rotation is going to cause the loss of the image in the corners, since it gets clipped off in the stacking process.

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#11
In reply to #8

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 3:46 PM

I can see that. No pun intended. :-)

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#12
In reply to #5

Re: Qualitative Comparison Between Astronomical Telescopes

02/23/2014 10:18 AM

"It just seems easier to me to buy a equatorial mount and not deal with those extra steps and know that I can take really long exposures without much fuss. I'd use it with an auto-guider, or pay a bit more for an astro-camera that includes a secondary CCD for auto-guiding."

Equatorial mounts are definitely better for imaging purposes as you don't have the problem of image rotation in the field of view as the telescope tracks. However, if you aren't using a permanent mount that's aligned to a degree of accuracy that impossible with a portable mount then forget about even trying to use an equatorial mount. Getting them properly aligned is extremely difficult and if you're using a portable tripod you'll spend all night trying to get the thing aligned properly and unless you get it spot on you will still have movement of the stars in the field of view.

To start with you're better off using an alt/azimuth mount and limiting your exposure times to 30 seconds or so which if you are using a DSLR rather than a specially designed and manufactured astronomical imaging device is the maximum exposure time that will be possible before the electronic noise starts to become a problem.

De-rotating images isn't a problem anymore especially if you are using any of the numerous image stacking applications that are available on the net that use NASAs Drizzle software that was developed for the Hubble Space Telescope as the core of the application.

If you are using a DSLR as your imaging device then make sure you set it to save the images in the camera's raw format, DO NOT use JPEG format images as thy will wipe out too many small objects. Most image stacking programs both free and purchased use a piece of open source code called DCRAW which can decode the raw images from just about any camera manufacturer and as new cameras come on the market they are added to the code.

As for the argument about refractors versus reflectors let me just say one thing, all the telescopes the professional astronomers use including the HST and it replacement are reflectors with possibly a couple of exceptions where you are observing the Sun. Which incidentally is definitely not something for a first time astronomer to even contemplate doing. Mess up while observing the Sun and you will not only destroy your equipment, but potentially any future astronomy aspirations because you've wrecked your eyesight.

Where you are located is important as if you live inland away from the coast having a sealed optical path is not as critical. For a beginner I would recommend a Catadioptric design or one of the many variations thereof. They have two big advantages, they are sealed and less prone to contamination of the optical elements (extremely important if you live near the coast as sea air will destroy the mirroring very quickly) and they are compact having much longer optical focal lengths than their actual size. For example I have a 125 mm diameter by 1,900 mm focal length Maksutov-Cassegrain that is physically only around 450 mm long.

It all depends on how serious you are, where you are located and how much money you want to spend. Ideally a large reflecting telescope, with a fixed equatorial mount that is cemented into the ground, something like a Finger Lake Instruments ProLine imaging unit that keeps the CCD sensor 70°C below the ambient temperature to minimize noise, with a proper permanent structure to protect everything from the elements would be perfect for any amateur astronomer. But you wouldn't get any change from 50 to 60 thousand US dollars for such a setup. At the other end if you already have a descent DSLR camera then the largest Maksutov-Cassegrain or similar type of telescope you can afford with an appropriate adaptor for your camera will get you going for between 1 and 2 thousand dollars.

Astronomy can be phenomenally rewarding and just last December in the southern hemisphere we were treated to a star going nova that became bright enough to see with the naked eye and Jupiter with its many moons is looking absolutely spectacular at the moment as we have just passed the point where we are closest to it. However, it can also be extremely frustrating if you have to spend hours setting everything up, aligning the telescope, getting the tracking system working, making sure you have focused the imaging unit properly only for the clouds to come over before you have had a chance to do any observing or imaging.

As someone else suggested contacting your local astronomy club/organisation and if possible attending one of their observing nights where they will have a range of telescopes is definitely a good starting point.

However, from your initial post it sounds like you are starting out as a novice so I would definitely suggest something along the lines of a Cassegrain telescope or one of the many variations on the theme as your initial telescope. They are robust, compact and relatively cheap now they are being mass produced. There are plenty of telescopes of this style on the marked that usually come with a GoTo mount that has an automatic alignment system, but I would suggest getting one with an inbuilt GPS receiver as it makes aligning the telescope that much simpler particularly if you intend to move it about to different locations. Start out using and Altitude/Azimuth mount as its easier to set up and align and then as you build up experience you can always move to an equatorial mount later on. Proper astronomical imaging units are fairly to phenomenally expensive but if you already have a DSLR camera then for about US$50.00 you will be able to purchase an adaptor that in conjunction with some free downloaded software like Deep Sky Stacker that will not only stack your images but de‑rotate them and you'll be up and running.

Happy stargazing

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#6
In reply to #3

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 1:04 PM

Though I am replying Usbport, in fact I am replying to all.

Let me add some more information... I am Ex President of India's oldest Amateur Astronomer's association "Jyotirvidya Parisanstha", which was established in 1944 (yes in 1944)

Though I said, that I want to purchase, in fact we want to purchase a telescope for our club.

We have already 10" Celestron Newtonian telescope with Skywatcher EQ 6 mount. We just discarded the original CG mount of Celestron, because it was giving no satisfactory tracking and pointing accuracy. EQ 6 is giving good tracking and pointing accuracy with Celestron GOTO system

Thus, mount is not a problem for us.

We are already in serious observations, and our variable star observations are already accepted by AAVSO. We have successfully done and doing observations of Asteroids. Even we have photographically recorded few Asteroid occultations.

But, we are facing serious problems of Coma and not so good flats with present system.

We understand that these problems will be less in Schmit Cassegrain system. We tried to search comparison on internet between Newtonian and Schmit Cassegrain, but failed to get any information. We could not get any experience person who has worked with both these type of telescopes.

Refractor is not the solution, as we feel, as there is size limitation on refractors. While going for new telescope, we wish reach to lower magnitude. Presently, we can record the stars up to 14th magnitude. With bigger size, we wish to reach at least 16 or 17 magnitude. But, we want to avoid the problems we are facing with present system.

Is there anyone who has photographying experience on both the types of telescope ?

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#9
In reply to #6

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 2:12 PM

Well, all of that information would have been useful to know at the beginning.

OK - You don't say what f/ratio your 10" scope is. If it is between f/3.5 and f/6 then buy a Baader Coma Corrector, available via Orion. It does an excellent job of eliminating coma for moderately fast Newtonian reflectors. It's a good solution and much less expensive than a new scope. Try it, and then decide if you need a new scope of, say, the Ritchey-Chretien design that has very low coma.

Here's the link: http://www.telescope.coBaader-MPCC-Mark-III-Multi-Purpose-Coma-Corrector/

Here is a link to OPT's page on Ritchey-Chretien telescopes:astrograph-telescopes

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#10
In reply to #9

Re: Qualitative Comparison Between Astronomical Telescopes

02/21/2014 2:35 PM

Another option to the Ritchey-Chretien design (if you really think you need a new scope) would be a Celestron Edge HD scope. Like the Ritchey-Chretien it has almost no coma, but it also has a flat focal plane, unlike the RC which has a curved focal plane.

http://www.celestron.com/portal/articles/cat/celestron-videos/post/edgehd-tour/

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#13
In reply to #9

Re: Qualitative Comparison Between Astronomical Telescopes

02/27/2014 11:06 AM

Here's a photo of M42, unprocessed except to do a bit of image compression in order to post it here. No other processing has been done. It has not been cropped.

This photo shows the effect of the Baader Coma Corrector.

The photo was taken using a 10 Inch f/4.5 newtonian, at the newtonian focus, with an APS-C format DSLR. This is the entire image. Note that the stars are round right to the corners of the photo. Without the Baader Coma Corrector the stars over the outer 2/3rds of the image would show increasing amounts of coma.

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