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LeeWilky

Are GoTo Dobsonians any good for AP?

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As the title says im interested in knowing if the GoTo Dobs are any good for AP?

I cant seem to find a definitive answer on google but have found a few people on Flikr who seem to get some pretty decent images with a GoTo Dob and considering that you can get the 12 inch SW on a GoTo Dob at £400 cheaper than the same scope on a Goto EQ6 plus the fact that Dobs are easier to set up and transport which is a big selling point for me id like to delve into this a little further.

Cheers :)

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only good for planetary imaging If you want to do long exposure dso work you need an eq mount

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Yeh i thought that would be the case as all the pics i found where Solar, Lunar or Planetary.  Shame really as a Dob would suit my needs so much easier than packing up my EQ with all the neccesary bits and carting them down to where i take my scope as our back garden is lined on all sides by high trees and LP isnt great. 

Oh well cheers for the anser mate.

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Ehmm.... have you checked out the 12inch goto dob in a store?

From 12inch and up.... the dobs become very Heavy and much less portable than you think. No less than a EQ6 setup.

The base of the 12inch dob is 23kg alone and the ota is 20kg.

The EQ6 tripod is 19kg and the Mount is 21kg.

So yes, With a EQ6 setup you have to walk Three times, instead of two times with the dob. But each individual piece still weights in the 20kg range.

Just some Food for thought. :)

Edited by GuillermoBarrancos
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Hmm - I don't know how well these GOTO Dobs track, but if you can manage 30secs then you should get excellent DSO images out of a big Dob, given all that aperture.

NigelM

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Hmm - I don't know how well these GOTO Dobs track, but if you can manage 30secs then you should get excellent DSO images out of a big Dob, given all that aperture.

NigelM

that was my thinking too and £400 is a lot of money to save when your budget doesn't stretch that far.

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Hmm - I don't know how well these GOTO Dobs track, but if you can manage 30secs then you should get excellent DSO images out of a big Dob, given all that aperture.

NigelM

Aperture is not the most relevant aspect in AP, a 12" F/5 newtonian will be the same brightness as an 80mm f/5 refractor.

Aperture is great in the visual world, but not so relevant in the AP one. It is a funny world, and not always as expected.

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What DSO imaging requires is long exposures, aperture is not important unless you wish to achieve a long focal length and retain a fast f/ratio. In terms of brightness and exposure times required f/5 is f/5 - no matter if it is on a 3" APO or 12" Newt.

A 12" dob would I'm sure give you some excellent planetary and lunar images with a webcam but would be useless for DSO images as it would begin to show field rotation after a few seconds of exposure, and you would be needing to set your minimum exposure time for DSO images in the minutes, not seconds.

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Aperture is not the most relevant aspect in AP, a 12" F/5 newtonian will be the same brightness as an 80mm f/5 refractor.

 

Aperture is great in the visual world, but not so relevant in the AP one. It is a funny world, and not always as expected.

Can't agree. The amount light a pixel gathers depends only on the aperture and the pixel scale (aparture area/pixel solid angle). Now this can be rewritten in different ways in terms of f-number, pixel size and focal length (e.g. pixel size squared/f-ratio squared) but just simplifying it to F-ratio can result in huge errors. The 12" Newt will gather about 14 times more light per pixel then the 80mm ED (depending on mirror reflectivity, lens transmission and central obstruction) if they both use the same image scale.

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Not perhaps (clearly so?) for classical, long exposure (guided etc.), DSO AP -

But an interesting prospect for (adjunct via) Video Astronomy techniques? ;)

In real time, modern video cameras have a sensitivity around (greater than) that of the naked eye. Add the automatic integration facility, over a few seconds, and you can significantly increase the (on screen) limiting magnitude. Stack the (avi) images obtained and you can go further. Essentially if the scope can remain steady over the integration time (few seconds) it will work well. Better still if a mount can *track* over (many) minutes - Software can stack, correct for Alt-Azimuth field rotation etc., over significantly long periods. No substitute for classical AP - To avoid controversy, I suggest the idea to increase the potential of a present (Dobsonian etc.) setup. <G> But a light gathering power equivalent to 2 - 3x (probably) the native aperture of any scope can be interesting. :)

Edited by Macavity
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Can't agree. The amount light a pixel gathers depends only on the aperture and the pixel scale (aparture area/pixel solid angle). Now this can be rewritten in different ways in terms of f-number, pixel size and focal length (e.g. pixel size squared/f-ratio squared) but just simplifying it to F-ratio can result in huge errors. The 12" Newt will gather about 14 times more light per pixel then the 80mm ED (depending on mirror reflectivity, lens transmission and central obstruction) if they both use the same image scale.

The point is in your example yes, the 12" Newt will gather 14 times more light, but at f/5 it will have a much longer focal length and the light will be spread over a much larger area. When compared like for like with the f/5 ED80 it will be no brighter and require the same exposure time as its much smaller counter part.

Edited by johnrt

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The point is in your example yes, the 12" Newt will gather 14 times more light, but at f/5 it will have a much longer focal length and the light will be spread over a much larger area. When compared like for like with the f/5 ED80 it will be no brighter and require the same exposure time as its much smaller counter part.

Yes, the amount of light per area unit will be the same. Yes, if you just swap a camera between two telescopes F-ratio is enough. It also works with DSLRs with equal ISO settings since the difference in pixel size is (in theory) baked into the ISO settings (along with QE, etc.).

Now if (as I stated in my example) both systems have the same pixel scale F-ratio is not enough since the telescopes will use cameras with different sized pixels. One way of looking at it is that each pixel will gather 14 times more light because it will be 14 times larger.

A 12" tracking dob with the same pixel scale as a short APO (say 2-2,5"/px) is a light gathering monster. If 30 sec subs is possible before field rotation sets in the dob will gather the same amount of light per pixel as the 80 mm would in 7 minutes. I'm not saying it is a sensible setup, it would need a good 3" focuser, a 3" Wynne corrector and a Atik 11000M or similar to achieve this with a useful FoV and for that kind of money there a better ways to build a AP rig.

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Another way of looking at is, yes the flux per pixel is the same (for the same camera) at the same f-ratio, irrespective of aperture, BUT for a larger aperture you get more pixels per object. So a 4in f5 might have (lets say) 10 pixels covering a DSO, the 12in f5 will have 90 pixels, all with the same s/n as those in the 4in. Bascially, for a given DSO the latter is much better place to be than the former, and the result is that the 12in exposure will be deeper.

NigelM

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It's only the focal ratio that matters for determining the speed.

Proven by both theory and experiment. Signal received per pixel depends only on the focal ratio. Aperture plays no part. The experiment is proved by DSOs requiring very long exposures at f10 and much shorter at f3. Time to reach a given SNR is shorter with the f3 scope.

Perhaps it's counter intuitive but you shouldn't apply any prejudice you may have acquired from visual astronomy. The experiment and theory are in agreement.

There is a reason people to mad for the latest ultra fast scope. If aperture was king, fast refractors and the Ricardi Honders would be overlooked in favour of an SCT or large newt.

Aperture sets resolution, focal length sets image scale and focal ratio sets time to a given SNR.

I don't suppose a large aperture and a fast f ratio will hurt things. Apart from your wallet.

Paul

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focal ratio sets time to a given SNR.

Only per pixel, which is not what determines the depth of an image. Aperture determines this. It is a myth to think that you can image fainter objects by simply using a shorter f-ratio (or get to the same depth quicker).

NigelM

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It's only the focal ratio that matters for determining the speed.

Proven by both theory and experiment. Signal received per pixel depends only on the focal ratio. Aperture plays no part. The experiment is proved by DSOs requiring very long exposures at f10 and much shorter at f3. Time to reach a given SNR is shorter with the f3 scope.

You are simply wrong when it comes to the theory here, check any optics textbook. On top of that your experiment is not designed correctly. No matter if you are using Etendue or F-ratio you would get the same result in this case.

Also note that I’m just saying that using F-ratio in many cases is an oversimplification although in some cases it’s a quite large one. As I said in an earlier post, if all other factors are the same F-ratio still works with digital sensors. However the main factor that usually isn’t equal when it comes to astronomy is the pixel size/area/scale.

A good experiment would be to keep the F-ratio and aperture constant and change the pixel scale by testing, for instance, a 4.5 micron and 9 micron cameras with similar QE in the same telescope, with the same exposure time, apply darks/flats, subtracting the pedestal and then converting the ADU values into photons. The formula for Etendue says that the 9 micron camera should gather four times the amount of photons per pixel compared to the 4.5 micron camera while by going by just the F-ratio s they should be the same.

I'll have a go at explaining Etendue in terms of basic optical properties and then transform it into an expression using focal ratio since using F-ratio just won't die. ;)

1. Aperture area determines the rate the telescope gathers photons (total optical power/gain/etc.) and is pi*D^2.

2. The intensity (the rate of photons hitting a certain area) in the focal plane of the telescope is pi*D^2/f^2 (D – diameter, f – focal length) for a specific target. If we are using film or if all pixels had the same physical size we could stop here and F-ratio (squared and multiplied with a constant) would always give comparable results.

3. The rate of photons hitting a pixel is however also determined by the area of the pixel. This is called Etendue and is equal to pixel_size^2* pi*D^2/f^2.

In the textbooks pixel_size^2* pi*D^2/f^2 is rewritten as A*Omega where A is the aperture area (A=pi*D^2) and Omega is the solid angle of the pixel FoV (pixel_size^2/f^2) since this delivers a nice clean formula that also has the bonus gives some understanding into the physics. The A*Omega variant captures the perspective of how many “extra” photons you capture depending on the aperture of the optics (gain) and the field of view of the pixels (the area of the sky the pixel can gather photons from).

An advantage is that the numbers depends entirely on factors any astrophotographer worth his meat should know by heart, the aperture of the optics and the resolution of the pixels he’s using.

As an aside one of reasons small APOs and camera lenses gets such good results is that they trade a loss in resolution (high Omega) with a loss in aperture area (low A). For instance an 80 mm scope with a 2.5“/pixel camera matches a 200mm scope with 1”/pixel in light gathering (802*2.52=2002*12). Now if you use the same pixel scale the 8” beats the 80 mm by a factor of 6.25.

It is of course possible to leave the formula as is or to rewrite it in any number of other ways. One such way is to substitute F-ratio for aperture diameter and focal length in which case we get pi*(pixel_area^2/F-ratio^2).

This version however hides that a high focal length/slow scope need a camera with large pixels if you want good light gathering performance.

If we want to get the actual recorded signal (number of photons detected) instead of the rate of photons hitting the pixel we need to consider a few more details. The total signal is equal to the product of Etendue, exposure time and the quantum efficiency of the entire system (the product of all lens transmission/reflection/filter losses, obstructions, sensor QE, etc.)

 

Perhaps it's counter intuitive but you shouldn't apply any prejudice you may have acquired from visual astronomy.

You got my prejudices pretty much the wrong way around. I consider aperture to be mostly useless for visual astronomy; it just determines the range of useful magnifications since all scopes are equally bright at the same exit pupil. The target determines the tool and personally I rate the entire spectrum from 7x50mm binos to our clubs 45 cm Newton equally. One of my dreams is to build a pair of 3x21mm binos since you can’t buy that anywhere.

 

There is a reason people to mad for the latest ultra fast scope. If aperture was king, fast refractors and the Ricardi Honders would be overlooked in favour of an SCT or large newt.

That’s not strange. A fast scope is the obvious way to get most of the way to an AP rig with good Etendue. That is true even if very few amateur astronomer appears to know the entire story behind why they are getting good results. I’m just a guy with an interest but limited formal training in optics but I got very uncertain as this basic optical formula seemed like a novel idea on most AP forums so I checked this both with my local friendly optics researcher and the relevant chapters in my university textbook (I never took any physics beyond some electromagnetism/electronics courses).

 

I don't suppose a large aperture and a fast f ratio will hurt things.

Since F-ratio depends on both aperture and focal length it’s a bit weird to say “large aperture and a fast f ratio” but I think I know what you mean.

Apart from your wallet.

I don’t think that any experienced astrophotographer would be surprised to hear that an improvement in light gathering of a factor 14 has an exponential impact on the price tag especially if you add the cost an EQ mount sturdy enough. There is a reason Spain had to spend £112 million on GTC (diameter 10.4m, F/1.6, Alt/Az mount with derotation).

I think there are three useful and equivalent ways to boil down light gathering for AP systems into bullet points. They are based on the Etendue formula rewritten and simplyfied in different ways.

A camera gathers lots of photons if your AP rig has a combination of:

Large aperture and a big pixel scale (arcsec/pixel)

or

Large aperture, short focal length and big pixels

or

Fast scope and big pixels.

If you want more detail than that you really need to run the actual numbers…

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I tried to do DSO imaging with a 12 inch auto tracking dob and a DSLR, thinking if I could get say 10 seconds I'd stack hundreds of images! Unfortunately, when I tried it, I couldn't even get one second without trailing, so gave up!

I now have a 16 inch goto dob and get a fair bit of drift with webcam imaging of planets at long focal length, so I haven't bothered to try with the DSLR again on DSO's. Planets are okay as I have a relatively large webcam chip so the planet can wander around a bit without causing a problem.

It would be great if the dob worked for DSO imaging. I'd love to image for just 30 mins or so at the end of a visual session!

re: speed:

I mostly agree that it's speed that's important for DSO imaging. Take my ED80 (F6.25) versus my C11 (F6.3 using a reducer). They both need about the same total exposure time if using the same camera (in my case, a DSLR), despite the massive aperture difference. What is different is that the SCT images at a much longer focal length, you are more "zoomed in".

The C11 collects many more photons, but they are spread over many more pixels. If you double the aperture of a scope, you get four times as many photons. But if you double its focal length as well, the photons are spread over four times as many pixels if using the same camera for both (doubling the size of an image gives you four times as many pixels).

That's how it makes sense in my head, anyhow! :grin:

Edited by Luke

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Hmm - I don't know how well these GOTO Dobs track, but if you can manage 30secs then you should get excellent DSO images out of a big Dob, given all that aperture.

NigelM

I don't think this is correct at all. It's the F ratio that matters. (Or the image scale in arcseconds per pixel.) I have two years' experience imaging with a 14 inch Dall Kirkham and during those two years we were finding it more and more productive to increase sub exposure length. We ended up routinely using thirty minute subs for luminance. 30 second exposures produced next to nothing.

I suspect the OP is thinkng that a large aperture scope must be better for deep sky imaging. Not so. A small, fastish apo gives good results and needs only a modest  German Equatorial mount.

85mm; http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-SVkkwzW/0/X3/ROSETTE%20FIN3WEB-X3.jpg

106mm; http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-3D2Hw7s/0/X3/M31%20Outer%20Halo-X3.jpg

140mm;  http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-TZRm3Cx/0/X3/M42%20CROP%20web-X3.jpg

Tracking Dobs for imaging? As already stated, planetary/webcam, 'Yes', and deep sky, 'Absolutely not.'

If using a DSLR, which cannot be made to work binned 2X2, there is a lot to be said for keeping to a short focal length so as to be imaging at a reasonable scale in arcseconds per pixel. At long focal lengths each pixel receives too little light.

Olly

Edited to fix link.

Edited by ollypenrice
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It can be done, but not to the level of the images that you will see on forums such as this.

For example, here's an image of the M82 supernova taken with a 10" Dob

http://www.flickr.com/photos/spicey_spiney/12141395926/

(from Mary Spencer's Flickr page)

With enough dedication and brute force of will you can do pretty much anything. So, yes, you can image with a Dob. Similarly, you can commute to work on a pogo stick. Whilst you might eventually get to the office, you are going to be sore, wet, and probably, will get strange looks from people.

The best way to image with a Dob is to rip it off it's base and mount it on a sturdy EQ mount.

img_2098.jpg

(from Gerhard Dangl's EQ8 site)


It would be great if the dob worked for DSO imaging. I'd love to image for just 30 mins or so at the end of a visual session!

30 minutes would net you very little data, even if you were using the Hubble. DSO imaging takes hours per image.

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Tracking Dobs for imaging? As already stated, planetary/webcam, 'Yes', and deep sky, 'Absolutely not.'

Or... VIDEO Astronomy? :p But these days I think / talk of imaging as "Classical Imaging". :)

Aside: Fastest way to convert Imagers to VISUAL Astronomy? Mention Video Astronomy! lol. 

They exit the "club room" faster than [my expletive]!!! But then *some* do return, intrigued.  ;)

I have (general) "mixed feelings" about Alt-Azimuth drives for "imaging". But if they are capable

of maintaining a scope *steady* for a FEW seconds? A deep sky snapshot versus Mona Lisa?

That said, an Ioptron Mk.I Alt-Az mount (Boo!) converted me to an Equatorial (HEQ5) user!  :o

But the BIG thing - You learn the techniques of Classical Imaging?  Stacking, Darks, Flats...

If I (already) had a big (driven) "Dob", I'd want to TRY Video Astronomy. But that's Just Me.  :D

A (justified?) ADDENDUM to above - Just "messing about"... Run out of Emoticons anyway! 

Edited by Macavity
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I don't think this is correct at all. It's the F ratio that matters.

If all that mattered was F-ratio, 5 mins with the WHT (4.2m, f2.8) would be the same depth as 5mins with a Boren-Simon 8" f/2.8 PowerNewton. I can assure you it will not be. The WHT will pick up 24th magnitude objects in this exposure time.

NigelM

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If all that mattered was F-ratio, 5 mins with the WHT (4.2m, f2.8) would be the same depth as 5mins with a Boren-Simon 8" f/2.8 PowerNewton. I can assure you it will not be. The WHT will pick up 24th magnitude objects in this exposure time.

NigelM

Indeed, clearly it won't be, but what is the sampling rate of the professional telescope?

I can assure you with equal confidence that 14 inches and 30 seconds don't get much data at 0.6 arcseconds per pixel. I spent two years and hundreds of imaging hours with this setup. Remember that the OP is talking about imaging with a DSLR which will have small pixels, smaller than the 7.2 micron I was using in the 14 inch.

In the context of a discussion about amateur telescopes I think it is wrong to expect much out of a tracking Dob in 30 seconds. Besides, the tracking is very unlikely to be smooth or particularly accurate. The example of huge professional telescopes has been used before to argue the case against F ratio but it's misleading. I'm certain that an 8 inch Boren SImon F2.8 with a DSLR will go deeper in 30 seconds than a 12 inch F4 Dob. I'm equally certain that for the same luggable mass a small refractor on a small GEM will go a sight deeper than any of them with a 5 minute exposure.

Olly

Edited by ollypenrice
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As a proud owner of the Skywatcher 300p skyliner Dob I can say

that it is only good for planetary AP as it only tracks in AltAz.

But I don't see that as a downfall as the optical views a Dob can give you

compared to the price of a good Eq Mount.

I would rather have my 12" Dob than a 8-10" SCT for double the price

of my Dob.

Cheers Roy

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As a proud owner of the Skywatcher 300p skyliner Dob I can say

that it is only good for planetary AP as it only tracks in AltAz.

But I don't see that as a downfall as the optical views a Dob can give you

compared to the price of a good Eq Mount.

I would rather have my 12" Dob than a 8-10" SCT for double the price

of my Dob.

Cheers Roy

Yes indeed, though that's a different story. To my mind AP is an activity apart and an complex and expensive one. A nice big visual Dob needs no additional praise from me. It's all been said. They are great visual instruments.

Olly

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