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LeeWilky

Are GoTo Dobsonians any good for AP?

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

The pros often go nuts when it comes to the number of cameras, instruments and different optical configurations possible with a single telescope so there is no single value. The WHT Prime Focus Imaging Platform (PFIP) has an image scale of 0.27"/px and their median seeing is 0.7".

 

I can assure you with equal confidence that 14 inches and 30 seconds don't get much data at 0.6 arcseconds per pixel.

Yeah, going from 1"/px to 0.6"/px will reduce the amount of light per pixel by a factor of almost three. In fact a 80 mm scope with 2.5" pixels or a 8 inch scope with 1"/px will match that setup. Improving resolution always costs a lot of light grasp (inverse square law).

 

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.

Sure with the same camera that is true. With different cameras (both with the same QE) that gives the same pixel scale the 12" will go 2.25 times deeper.

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Another thought. Focal reducers reduce exposure time without increasing aperture. Why do they do this? Is it because they bring more arcseconds to bear on each pixel? If not, then why do they work?

Olly

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If not, then why do they work?

They don't. You may think your images looks nicer/brighter etc,  but scientifically speaking it is no deeper than one taken at a higher f-ratio (caveat - if your pixels at the higher f-ratio are dominated by read-noise. then you will get some increase in depth from lowering the f-ratio for the same aperture. For small aperture scopes and or short exposures this may often be the case of course).

Actually, if your pixels are much larger than the seeing at the low f-ratio this image will be less deep!

Anyway, I would be more than happy imaging with 30 sec shots at 0.6"/pixel and 14" aperture, but it does require the tracking to be reasonably good, so if the Dob mounts aren't up to it then so be it.

NigelM

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Another thought. Focal reducers reduce exposure time without increasing aperture. Why do they do this? Is it because they bring more arcseconds to bear on each pixel? If not, then why do they work?

Olly

They do what it says on the tin, a focal reducer reduces the focal length. There are several ways to look at why you get more light per pixel with a focal reducer, the increase in the pixel scale (as you mentioned) is one, lowering the F-ratio is another and and shortening the focal length is the third (basically the three variants of the Etendue formula I presented earlier).

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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.

Have you tried taking images of say 15, 30, 60 seconds with it? It would be interesting to see how good the tracking really is by looking at the center of the image.

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only good for planetary AP as it only tracks in AltAz

I dont know the 300p but the AltAz thing may not be a total barrier for you and LeeWilkie, OP,,, depending ! :)

A read through this pdf by Bill Keicher http://www.autostarsuite.net/forums/storage/19/4981/Field%20Rotation%20V3.pdf may give you some ideas along with glappkaeft's suggestion

( + lots of stacking of course !! )

Edited by Ptarmigan

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Oh, and PS :

When someone has tried the Keicher method they might post results?

Cos I am also thinking of a big Dob, primarily for visual but it would be nice if I could take 'holiday snaps' thro' it as well !!

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They don't. You may think your images looks nicer/brighter etc,  but scientifically speaking it is no deeper than one taken at a higher f-ratio (caveat - if your pixels at the higher f-ratio are dominated by read-noise. then you will get some increase in depth from lowering the f-ratio for the same aperture. For small aperture scopes and or short exposures this may often be the case of course).

Actually, if your pixels are much larger than the seeing at the low f-ratio this image will be less deep!

Anyway, I would be more than happy imaging with 30 sec shots at 0.6"/pixel and 14" aperture, but it does require the tracking to be reasonably good, so if the Dob mounts aren't up to it then so be it.

NigelM

Focal reducers don't add to the object photons collected so they don't take you deeper. This I understand. (The F ratio myth.) But they do work in that they change the picture captured (to a wider field one) and fill up the new, wider, picure with sufficient light for an attractive result.

But with regard to the OP I think it's wrong to assert that just because he has 12 inches of aperture he should be able to get something aesthetically decent in 30 seconds. He'd get something aesthetically comparable in 30 seconds with an F4 3 inch refractor. It wouldn't be the same picture. It would be wider.

I'll see I have any short exposures in linear form from both the 14 inch and the 3 inch and make them available. I think that they key difference between amateurs and professionals regarding F ratio is that the professionals want data on object X. The only way to get more object photons is to have more aperture. Amateurs, however, will often accept that object X is too small on the sky for their telescopes and will settle for a wider field image at a faster F ratio with sufficient signal to noise to be able to present what is in that field with suffiicient S/N ratio to make it attractive.

Olly

Edited by ollypenrice

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Oh, and PS :

When someone has tried the Keicher method they might post results?

Cos I am also thinking of a big Dob, primarily for visual but it would be nice if I could take 'holiday snaps' thro' it as well !!

Unfortunately I didn't record the exact settings. But this is the sort of thing I was getting with a MAK150 on an Ioptron Alt-Azimuth mount. But of course, with an integrating video camera (Watec 120N+). A "Registax" stack of 10 second (256x 1/125s internally stacked) exposures I suspect. But often a 10-20 min AVI file? At the time the Watec boasted perhaps the most sensitive mono chip around? As trade-off - It is a 1/2" chip, has (big) 8.5 micros pixels etc. etc. ;)

But the MAK150 was a under-powered (re. light gathering) for galaxies, diffuse nebulae etc., so I moved to an 8" / F4 Newt. There aren't many "budget" Alt-Azimuth mounts capable of carrying an 8" OTA, so I also moved to an HEQ5. But if it's a "video diary" (OK, holiday snaps? lol) you want, genuinely think about integrating video cameras? The Messiers would ALL and easily be within range of a 12" / F5 Dob. The real time (on screen) display would go down to about magnitude +17 once you get over 8" in aperture. Many galaxies have surface brightness > + 14. etc. etc.   :)

Hard to predict what another (camera) setup might see. Although I am familiar with video camera sensitivity in "lux @ f 1.4" etc., heaven knows what to make of (classical imager) "quantum efficiency"... Heck CCD data sheet "Volts / Candela" (whatever?), can present a bit of a challenge too. :p

post-539-0-10175400-1391020044_thumb.jpg

Edited by Macavity
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Hi Glappkaeft

Yes, f ratio only isn't enough. The problem is that everything is intertwined. Changing one whilst varying the other will affect the third. The etendue argument seems good and I remember from my days as an undergrad. I suppose 2 set ups are identical if the deliver the same etendue per pixel. I will check the derivation myself as every scientist should do! :)

Cheers

Paul

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Thanks Chris/Macavity, very interesting, I had not yet thought about video, ho hum yet another factor to be thrown into the cauldron** !
My use of 'holiday snaps' was my abbrieviated way of distinguishing humble efforts/requirements by me from the superb efforts of the dedicated imagers of this parish :)
One problem that I think I see with the Keicher method is that typical sensor size is a lot bigger now than in 2005 and a large or even full frame DSLR would be in trouble I suppose !?

** I am still watching and puzzling and getting confused over the aperture vs. Fratio debate, I keep wondering that if an ED80 is such a wizzard thing over and above a light bucket why isnt an ED80 in orbit instead of Hubble

Ok, I wont hijack the thread from LeeWilky any longer but I'll be reading with interest, thanks to all.
 

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** I am still watching and puzzling and getting confused over the aperture vs. Fratio debate, I keep wondering that if an ED80 is such a wizzard thing over and above a light bucket why isnt an ED80 in orbit instead of Hubble

Because the Hubble wants more object photons to get more detail on an object?

Users of the ED80 want to take images of extended objects, not obtain vast amounts of data on a tiny object within that field or on an object within it so faint as not to register at all with small aperture.

Olly

Edited by ollypenrice
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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 :)

A 12" reflecting telescope will do AP just fine, you might find your barrier with a dob for AP is the mount. Limiting your exposure times before field rotation plays a part, which is why EQ mounts are preferred among other reasons.

Sent from my GT-I9505 using Tapatalk

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Because the Hubble wants more object photons to get more detail on an object?

Users of the ED80 want to take images of extended objects, not obtain vast amounts of data on a tiny object within that field or on an object within it so faint as not to register at all with small aperture.

Olly

Exactly that.  This is my ED80 / Focal Reducer / DSLR version of M101:

M101_V1_3x.png

I'm very happy with it, but you couldn't exactly argue that it has the same amount of detail as you'd get with Hubble   :) :

hs-2006-10-a-2560x1024_wallpaper.jpg

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No please all of you hijack away I think I have the answer I never thought of field rotation but there's an interesting discussion going on now so keep it going :)

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No please all of you hijack away I think I have the answer I never thought of field rotation but there's an interesting discussion going on now so keep it going :)

This is unfair. In my view you were given misleading advice suggesting that, because a big Dob has lots of aperture it will, therefore, be effective at giving you a good picture in short exposures.* From your point of view, wanting to get a pleasing image, the aperture is not what matters and the 12 inch scope of a set F ratio has no advantage over a small one of the same F ratio. Indeed it has a considerable disadvantage because its long focal length will require far more accurate tracking, which it doesn't have on a Dob mount. In my view the discussion has been about precisely that, with underlying reasons.

Olly

* I run lots of astrophotography courses and this is an assumption I frequently meet and try to set into context. That's why I harp on about it here.

Edited by ollypenrice

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** I am still watching and puzzling and getting confused over the aperture vs. Fratio debate, I keep wondering that if an ED80 is such a wizzard thing over and above a light bucket why isnt an ED80 in orbit instead of Hubble

I'm not a APO owner (at least not yet) and the ED80 is not very common in Sweden so I just realized one thing. The ED80 is pretty slow telescope even with the corrector/reducer. So the quality people get from ED80 telecopes is really an argument against the F-ratio argument.

Surfing around the internet I have come across many heated discussions on the F-ratio myth, the aperture myth, the big pixel myth and probably some other "myths" I can't recall. I suggest that if there is anything worthy of calling a imaging myth the idea that there is just a single important factor should be it (lets call it the single number myth).

I can't think of a single way of usefully characterising a photographic telescope without at least three numbers (quantum efficiency, Etendue and pixel scale) and you need some additional ones (focal length, aperture and pixel size) just to calculate those three numbers. That will give you a good idea of the light gathering ability of the scope, the amount of photons/signal you'll get and the resolution of the telescope.

/Patrik

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The ED80 is pretty slow telescope even with the corrector/reducer. So the quality people get from ED80 telecopes is really an argument against the F-ratio argument.

You are right that ED80-type scopes are not particularly fast.  You do have to shoot a fair few hours of subs on most targets to get a half-decent result, so personally I wouldn't consider it a valid counter-argument to whether f-ratio matters or not.  Even reduced it is absolutely not going to beat a (say) FSQ85 on any measure, including speed.  On the other hand one with a reducer costs about £500 and the other is more like £3,500 so you 'gets what you pays for' as they say.

I think f-ratio is a useful short-cut to the relative speed of different scopes, but you do have to consider all the other parameters to really know whether you're comparing apples with apples or not. Personally I find the first number you have to take in to account is the budget that your other half will agree to, and that will constrain your choice of camera, scope and mount more rapidly than anything else.  Or is it just me in that situation? :)

Edited by IanL

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I must say this has been a really interesting thread - to be honest if you want to do standard astrophotography with long exposures, before you even think about aperture, focal length and all the science behind it etc. it's all about the mount. You need a mount that will comfortably hold your scope, camera - DSLR/dedicated CCD, filter wheel (if mono), guide camera/scope or finder guider and only track/guide in one axis so you don't have to worry about any field rotation.

Now that aside big dobs - for example 12" Felxtube's are great (as already said) for planetary and lunar work as aperture (resolution) really is king along with a long focal length, fast exposures and a fast frame rate CCD/CMOS camera to cut through the 'seeing'.

Here's an example - I'm using my new 5x Powermate so imaging at 7.5m with my 300P Flextube and for the first time I've picked up detail not only on surface of Jupiter but also on the surface of one of the moons Callisto and that's with a colour DFK so it's not as sensitive as it's mono counterpart due to the bayer matrix. 

12078532816_4f79a879df_o.png

Jupiter and Callisto 12/01/14 Winjupos combo by Steven Gray (stev74), on Flickr

For deep sky and a 12" dob the way to do it is as Chris (Macavity) has said - Video Astronomy which shouldn't be confused with astrophotography in the sense of a final 'polished' image from (sometimes) hours of data, it's much closer to what you will see visually. Here's an example of M82 the first one is the 'live' view where the camera integrates an image over 2.5secs and the second is 118 frames plus darks stacked in DeepSkyStacker which controls the noise (a little bit anyway) - 

12126939106_7f0b487d7d_z.jpg

Observing (monitor) and imaging (laptop AVI) Supernova SN 2014J in M82 by Steven Gray (stev74), on Flickr

12126276115_2f7ba5fca9_z.jpg

M82 Supernova SN 2014J by Steven Gray (stev74), on Flickr

So to answer the OP question yes big dobs can be used for AP but don't expect the same results as a guided EQ mounted scope. Still to me it's an amazing combination for visual (spiral arms in galaxies, surface details on planets), planetary imaging and video astronomy with an ease of set up that means you are up and running in under 10mins every time.  :smiley:

Edited by stev74
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Steve, that is an excellent Jupiter image - well done :icon_salut:.

I definitely agree about the mount - it has to be able to cope with whatever you're putting on it :).

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A picture is worth a thousand words? Thanks, Steve - A great summary of the options. :)

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This is unfair. In my view you were given misleading advice suggesting that, because a big Dob has lots of aperture it will, therefore, be effective at giving you a good picture in short exposures.

While large apertures  of any sensible f-ratio are never particularly good for wide field or large  objects, for exposures of most single DSOs they will outperform small aperture scopes. It is easy to see why. If I have a 4" f4 and a 16" f4 taking a 30sec exposure (with the same camera in the same conditions), I can bin up the 16" pixels 4x4 and get the same size pixels in arcseconds as the 4", but with 16x as many photons in them! i.e. it is like having an 8min exposure with the 4". Yes I get a smaller field of view, but for a single object this doesn't matter (providing it fits on the chip on the 16" of course). It is best to do this binning on-chip if you can, as this mitigates tracking errors, and makes data reduction easier. Otherwise (and particularly with a DSLR) you have to do it in software, probably before you start stacking the data. This is a bit tricky, of course, and probably not for the beginner, but it can be done.

NigelM

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While large apertures  of any sensible f-ratio are never particularly good for wide field or large  objects, for exposures of most single DSOs they will outperform small aperture scopes. It is easy to see why. If I have a 4" f4 and a 16" f4 taking a 30sec exposure (with the same camera in the same conditions), I can bin up the 16" pixels 4x4 and get the same size pixels in arcseconds as the 4", but with 16x as many photons in them! i.e. it is like having an 8min exposure with the 4". Yes I get a smaller field of view, but for a single object this doesn't matter (providing it fits on the chip on the 16" of course). It is best to do this binning on-chip if you can, as this mitigates tracking errors, and makes data reduction easier. Otherwise (and particularly with a DSLR) you have to do it in software, probably before you start stacking the data. This is a bit tricky, of course, and probably not for the beginner, but it can be done.

NigelM

I've already conceded this point. 'Most single DSOs' is the bone of (amicable) contention! Amateur astronomers are not generally trying to image 'a single DSO,' they are trying to fill their frame with an attractive and informative image. If this is the objective then F ratio is very important.

To stick strictly to the issue posed by the OP, would he be better on a tracking alt azimuth mount to use a small F4 refractor or a large F4 Newtonian? If his aim is to image a small target like M82 then he should use the large Newt. But it will, across the chip, be a lousy picture. If he chooses the smaller telescope of shorter focal length he will not get a better image of M82 but he will get a better image which will contain a small M82 and a lot of other interesting things.

This is the difference between the amateur and professional worlds. I can't speak for other amateur imagers but if I don't have the focal length/aperture for an object I don't image it.

Olly

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The larger the telescope, the higher the resolution resolving Power. That's where aperture comes into play.

Hubble is able to make those awesome Pictures, because with the cameras it has on Board, together With it's focal length of 57,6 meters it can reach a resolving power of up to 0,04 arcsec pixels.

Edited by GuillermoBarrancos

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