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Are GoTo Dobsonians any good for AP?


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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 scie

Thanks very much for the kind words Ptarmigan, my personal view is that there could still be more done to improve those dob images, i'm reasonably new to image processing and in the hands of someone v

and just for the sake of it, i'll throw in a few more images, if someone stumbles across the thread in the future they can see what to expect from range of targets. some were taken without a coma corr

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hi,

i've done a bit of astrophotography with a DSLR on a goto 12" dob.

its perfectly fine on the brighter DSOs

http://www.flickr.com/photos/80336656@N07

depending of course on you standards...

but i will say that it is so easy and quick to set up. i've since purchased an EQ8 but haven't got that up and running yet mainly due to the weather.

good luck with the decision.

russ

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hi,

i've done a bit of astrophotography with a DSLR on a goto 12" dob.

its perfectly fine on the brighter DSOs

http://www.flickr.com/photos/80336656@N07

depending of course on you standards...

but i will say that it is so easy and quick to set up. i've since purchased an EQ8 but haven't got that up and running yet mainly due to the weather.

good luck with the decision.

russ

I am trully impressed.  25 sec subs without trailing.

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These various technologies converge? Recently I tried to convert between this plethora of "units"

re. camera sensitivity. During an internet search, I was amused by one guy, who commented that:

"quantum efficiency" was - "About as useful a saying you car had eight cylinders rather than six"!

Not entirely qualified to say, but... :D

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I am trully impressed.  25 sec subs without trailing.

takk for det Guillermo.

yes, 25 seconds is fairly safe however most of my images were done @ 20 second subs which gave more consistent results. I am aware of others who go out to 30 seconds, however I believe field rotation starts effecting the outer stars, cropping generally takes care of that but I also believe  your latitude is a factor in determining exp length. I find the tracking pretty good - if a bit of care was taken when setting up.

Actually reading this thread got me thinking of perhaps moving the mirror up to f4 for some extra signal ... hmmm! actually as its a flex tube maybe even just not extending it to its full length may work.

cheers

russell

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The closer you are to the poles the more a Dob approximates to an EQ. By the time you actually get to the pole your Dob is an EQ and would only need to track on one axis. A chilly solution to the problem, though!

Olly

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yes, 25 seconds is fairly safe however most of my images were done @ 20 second subs which gave more consistent results. I am aware of others who go out to 30 seconds, however I believe field rotation starts effecting the outer stars, cropping generally takes care of that but I also believe  your latitude is a factor in determining exp length. I find the tracking pretty good - if a bit of care was taken when setting up.

Nice to hear from some one with RL experience and I really like those images. You're right that the latitude is important but the azimuth and altitude the telescope is pointing at are also big factors. I can hook you up with the formula from Covingtons DSLR Astrophoto book if you like.

 

Actually reading this thread got me thinking of perhaps moving the mirror up to f4 for some extra signal ... hmmm! actually as its a flex tube maybe even just not extending it to its full length may work.

Moving the mirror up or under-extending the flextube won't change the focal length or focal ratio, that depends only on the shape of the optics. The only thing that will happen is that you get more back focus. Viable options for more light is a focal reducer/corrector or a more sensitive camera with larger pixels.

/Patrik

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Nice to hear from some one with RL experience and I really like those images. You're right that the latitude is important but the azimuth and altitude the telescope is pointing at are also big factors. I can hook you up with the formula from Covingtons DSLR Astrophoto book if you like.

Moving the mirror up or under-extending the flextube won't change the focal length or focal ratio, that depends only on the shape of the optics. The only thing that will happen is that you get more back focus. Viable options for more light is a focal reducer/corrector or a more sensitive camera with larger pixels.

/Patrik

Tack Patrik, that would be useful for people giving it a go. Thanks for the info re: changing the length, would upsizing the secondary mirror make any difference or is dependant on the primary curve?

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The formula (not quite the whole story but pretty accurate) is:

Time (sec) = amount of rotation (degrees)*cos(Alt)/(0.004167*cos(Lat)*cos(Az))

using the normal definitions of Latitude, Azimuth and Altitude.

Covington suggests 0.1 degree as a rule of thumb for acceptable field rotation.

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tack igen Patrick .

from my location an object at an altitude of 45 comes out with a time of 30.4 seconds which sounds about right. I could potentially increase my exposure times accordingly depending of course on what elevation i'm imaging. although ... hopefully i have the EQ8 running soon but others searching for the info will find it handy.

cheers,

russell

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Covington suggests 0.1 degree as a rule of thumb for acceptable field rotation.

This "degree of rotation" cant be the whole story can it ?

I have not got Covington, was he writing for 35mm film ?

I am thinking outloud here, I may be wrong, please jump on any flaws :) !

For an acceptable image it is the (linear) displacement of an object in the image field which counts, not the angular rotation about the optical axis ?

So for a small sensor (webcam etc ) close to the optical axis the displacement at the edge of the field, at 0.1degree of rotation, will be small.

For a full frame dslr the displacement of a star at the edge will be much larger,

I think there is an   A.tan( B)  needed in there somewhere where A is radius to the edge and B is the rotation in the time interval (0.1deg or whatever floats your boat !)

A test exposure ** for a particular sensor would be needed ?

** as Rusty is doing very well, the image of Centaurus A is spectacular, thanks for showing.

Edited by Ptarmigan
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For 60N and 0.1 deg allowable rotation.

Direction           Alt 15 deg   Alt 45 deg    Alt 75 deg      South and North:     46 sec       34 sec        12 secE or W +-10deg      267 sec      195 sec        72 secNE, SE, SW and NE    66 sec       48 sec        18 sec 
The reason for "E or W +-10deg" is that there is no field rotation due west and east but objects won't stay there so I calculated the numbers for 80 degrees (identical to 100, 260, 280 degrees) instead. The best places to to image if you want to avoid field rotation is low and to the west and east. Around Zenith is the worst.
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So for a small sensor (webcam etc ) close to the optical axis the displacement at the edge of the field, at 0.1degree of rotation, will be small.

For a full frame dslr the displacement of a star at the edge will be much larger,

True, but it depends how you intend to display the final image. If you always do 6x4 prints, or a certain fixed size on the web, then it is the angular rotation which matters most.

NigelM

Edited by dph1nm
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Covington suggests 0.1 degree as a rule of thumb for acceptable field rotation.

This Alt/Az and field rotation stuff is quite interesting reading when it is cloudy/raining !

There is a google books snapshot of the Covington ref. online :

(I dont know if this big link is going to work, lets try)

http://books.google.co.uk/books?id=Agho6QnsrzsC&pg=PA112&lpg=PA112&dq=field+rotation+astro&source=bl&ots=Gi-qf-ZeMF&sig=UBjyWs2pwA2KZ9NITpYpVvvnNE8&hl=en&sa=X&ei=m_X3UuftDcWrhQfP5oDYDA&ved=0CIICEOgBMBc#v=onepage&q=field%20rotation%20astro&f=false

(looks good so far !)

So, having just about thrashed this field rotation to death (!) the next question is :

Which of the Alt/Az mounts and Dobs are best at tracking the altitude and azimuth of a guide star for the required time ?

>> goes back up the thread to see which mount Russell used for his outstanding examples ! >>>

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Indeed, i've learnt a bit out of this thread actually, its good to know that theory allows for longer subs if (planned correctly).

every second counts when you're ~20 seconds, so it could help bring in a few extra DSO targets / or allow one to reduce the iso setting on the dSLR.

Mine is the Skywatcher 12" goto dob.

M83

9672626330_46ff2e7033_k.jpg

Carina mosaic - having a crack at stitching

10977906944_b1efc290fe_k.jpg

Clear skies

Rusty

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Mine is the Skywatcher 12" goto dob.

Thanks for the reminder of the 12"

and WOW! Words almost fail me !! :)

I'm not normally a fan of the vernacular "gobsmacked" but I think I shall use it here !!

The stich is beautiful

and M83 is amazing, can easily see that it is a barred spiral.

Is that the full frame from your dslr or is it a crop ?

All across the interweb and in the fora, it is said that you cant do AP with alt/az.

Well I'm going to send them all here !

Edited by Ptarmigan
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Thanks for the reminder of the 12"

and WOW! Words almost fail me !! :)

I'm not normally a fan of the vernacular "gobsmacked" but I think I shall use it here !!

The stich is beautiful

and M83 is amazing, can easily see that it is a barred spiral.

Is that the full frame from your dslr or is it a crop ?

All across the interweb and in the fora, it is said that you cant do AP with alt/az.

Well I'm going to send them all here !

Send them here by all means and hats off to the Dob images on here. No doubt about that. But if you are going to choose an imaging setup specifically for DS, should it be a Dob? Come on, it shouldn't. That would be silly. If you have a Dob and want to stick with it for some AP then, yes, this thread should be an inspiration.

Olly

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Thanks very much for the kind words Ptarmigan, my personal view is that there could still be more done to improve those dob images, i'm reasonably new to image processing and in the hands of someone very skilled would do much better i'm sure of it these were simple processing techniques in photoshop only - and i haven't captured any DS data for a long time. add a cooled dSLR also the results would be even nicer. all that coupled with the mathematical models of how long to shoot for, in relation to where it would be - you'd be doing quite nicely.

Olly raises a fair point, of course, if you are interested in DS photography you will want a GEM. I myself picked up a new EQ8 as it was a natural progression for me. now i have a deep space astrograph setup, and the goto dob for planetary or some visual work, perfect as far as i'm concerned. so the dob hasn't become obsolete despite buying an upgraded mount.

But then who would a goto dob satisfy?

- Perhaps someone who wants an 'all rounder' telescope system  to do both planetary with a bit of deep space on the side and visual... the trifecta - although acknowledging the DSO limitations. 

- you want some aperture and not spend a fortune on astro gear.

- Someone who doesn't have a lot of time and wants quick results. Two star align and away you go. I always considered it to be the 'lazy man's astrophotography' because it really is so quick and easy to set up and go.

- Infact some of the comments in this thread were from goto dob owners who were convinced that DSOs weren't doable at all, now they can attach a dSLR and get at it and produce a nice image of a DSO. Will it be APOD material or a David Malin winner? of course not, but they've now got more value out of their telescope setup and opened up another avenue to enjoying the night sky.

Cheers,

Rusty

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and just for the sake of it, i'll throw in a few more images, if someone stumbles across the thread in the future they can see what to expect from range of targets.

some were taken without a coma corrector, some with, some when i had no idea about processing or capturing and some in the heat of an Australian summer (= DSLR noise). but its an honest look at what kind of quality you can expect. you could do better no doubt but if you want a lot better then a GEM is the only way to go.

triffid

9672602542_9677626236_k.jpg

Swan pre coma corrector

9672610486_8e038b4810_k.jpg

Tarantula

12163421945_678ac7f295_k.jpg

Horsehead ? faint and noisey but why not?

8352726418_97830320cf_h.jpg

Running man

8261447680_7db6d2174b_k.jpg

Over toasted M42

8195533832_265b6ccdb7_k.jpg

Centaurus A

9672645018_ad90a3648d_k.jpg

Omega Centaurus

7739804884_95b41017b4_h.jpg

Apparently Alt Az dobs are capable of Planetary stuff as well

7365276796_6dd5aebcbd_h.jpg

7365276788_c76730ec4f_h.jpg

9500145825_dcd6049d43_b.jpg

And even a bit of Solar while you're at it

7365371530_ccc3e04004_b.jpg

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Rusty > "But then who would a goto dob satisfy?

- Perhaps someone   " etc.

Well said !

"Nothing ventured, nothing gained" :)

I love the pics, and you had fun doing them, I can tell :)

like "Horsehead ? faint and noisey but why not?"

yes indeed why not, you got it and it's all yours !! Great pic.

In fact great pics all round, thanks,

very interesting,

more more ,, :) :)

meanwhile, in another thread, I am still wondering if the Nexstar 8SE will be ok or should I try to squeeze in a 12" like yours heheee , , ,

Edited by Ptarmigan
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