10" dobsonian and astrophotography

[Ptarmigan]

fantastic images posted by members

Yes they are superb with the right equipment and time, but he wasnt asking if he could compete with the big boys, just could he "take deep space photos with it? " !

[RikM]

Yes they are superb with the right equipment and time, but he wasnt asking if he could compete with the big boys, just could he "take deep space photos with it? " !

Fair point

[Ptarmigan]

A driven dobsonian mount is Alt/Az and rotation of the object occurs, a rotated DSO is a blur.

It seems that Honis at al. take lots of short exposures then de-rotate and stack in software = no blur !

The Honis pics look good but that's a 20" jobbie !

Anybody on this forum tried it I wonder and can advise on efficacy ?

[umadog]

You can take lots of short exposures but there's a cost to doing so, which is why not more people do it. There are two classes of noise from the imaging chip: noise at the detector due to stray photons (including heat) and read-out noise. It's the second one which gets you.

Basically, the process of reading the pixel value is noisy. A single ten minute exposure will contain less noise than the average of 10 one minute exposures. The act of pulling in the image adds noise. Incidentally, this is the same reason why too many pixels is bad for low-light imaging. Averaging a number of small adjacent pixels on a high mega-pixel chip doesn't gain you as much as having one large pixel on a lower mega-pixel chip. Each pixel you pull in is associated with its own read-out noise.

The technique of taking many short exposures works much better on planets, which are bright, than DSOs, which are faint and closer to the noise floor.

[Ptarmigan]

Thanks, very interesting

so it comes down to signal-to-noise ratio and thresholds ?

My problem is, and implicit in the OPs original post I think,,, is it possible to image DSOs with Dobs. I see lots of "no way" "impossible" "blur" "dont even think it" to paraphrase ! !

Well so far I get :

(1)OPs orig preference for basic Dob then no, no way,

(2)but with auto tracking

then I see Bill Keicher's pdf and other refs. previously given

and this one newly found RASC Calgary Centre - Field Rotation

which suggest that indeed some DSOs can be imaged with Dobs !

If those images are acceptable may be in the eye of the beholder, , they may be interestingly acceptable (?) as an offshoot if our primary interest is visual and economy ? if you see what I mean.

Or is it truly impossible and a dead goose chase ?

A single ten minute exposure will contain less noise than the average of 10 one minute exposures.

Does this not also apply, in a limited degree, to the big-boys doing their 10x 1hr exposures? would not by that argument it be better to do a single 10hr exposure ?

I think I'll stop here, I'm in danger of confusing myself LOL, sorry - not trying to pick an argument ( I'm too new to do that) just tweaking out the details before spending a lot of brass

[umadog]

The problem is nowhere near as bad with 10 one hour exposures. You're right that the read noise is the same for the 1 hour as the 1 minute. The difference between the scenarios is that the recorded images contain far less noise in the longer exposures. In short exposures, the signal to noise of the read noise becomes a significant compared to the signal to noise of the image on the chip. As exposures get longer, the significance of the read noise becomes less. Does that make sense?

It's not impossible to take pretty good DSO photos with a tracking Dob. The link I posted earlier shows what's possible. What you need to know is that it's much easier to get good results if you have the right gear. You may find deep sky photography with a Dob to be pretty frustrating. Why don't you buy a copy of "Every Photon Counts" and read up about imaging? I think video astronomy with a tracking Dob is a better starting point. That is known to work pretty well and should produce acceptable results more quickly.

[RikM]

That sums it up nicely.

The point I was was trying to get across (in my own bludgeoning way) was that while it is not impossible to get some kind of image of DSO's with a dob, there are easier ways that will give you better results, for less money than a 10" basic Skyliner and far cheaper than an Auto / GOTO Dob.

For instance, here are a couple of shots taken using a 150P on an EQ3-2 with an unmodded Cannon 1000D. The whole lot costs less than an 10" Flextube Auto including camera. And this is bottom end of what is possible with the 'typical' deep sky kit.

I suppose it depends on what sort of images you want to take.

[todd8137]

am sure there is a goto 200p but they will not track for picsa smaller scope on a good eq mount and lots of cash is needed but the 10"dob is a great visual scope

[Ptarmigan]

Does that make sense?

Yes it does, I see now ! Brill. thanks for clearing the mists in my befuddled brain cell

It's not impossible to take pretty good DSO photos with a tracking Dob.

That is good to know if one is mainly interested in a big Dob for visual but with the option of some, but not the best, imaging as Rik is also pointing out.

Thanks to you both, it's back to which is most important and scratching of head time again !!

[Ptarmigan]

For instance, here are a couple of shots taken using a 150P on an EQ3-2 with an unmodded Cannon 1000D.

Gosh they are nice, only a 150 you say, wow!

And that way one has some nice images to admire when it is cold and wet outside !!!

You are tempting me back to the imaging side, oh decisions decisions.

I shall go quiet now and let TheEli have the thread back, sorry bout that.

[ScubaMike]

Aperture is not important for astrophography. Focal ratio speeds up your image taking and focal length affects magnification.

I don't really understand the first part of this statement.

All else equal, would a larger aperture not capture more light, and hence improve/brighten the image (or speed up the imaging process)?

I.e. A 10" F5 would be better than a 6" F5.

Or am I misunderstanding something?

[RikM]

I don't really understand the first part of this statement.

All else equal, would a larger aperture not capture more light, and hence improve/brighten the image (or speed up the imaging process)?

I.e. A 10" F5 would be better than a 6" F5.

Or am I misunderstanding something?

Yes, I am afraid you are misunderstanding. I can't explain it very well but it is something due to the focal length being different spreading the light captured over a larger area so the relative brightness....I don't know. Anyway, the exposure time is dependent only on the focal ratio not apperture.

[umadog]

Mike,

This stuff's confusing (to me also) but here goes!

Telescopes don't brighten the image, actually. That's a common misconception. Even the moon and the sun are no brighter through a scope than naked eye. The moon through a 6" scope is the same brightness as the moon through a 25" scope. This is dictated by the brightness theorem, which is fundamental to optics.

What a telescope does is magnify. The reason objects appear brighter through a telescope is because, basically, they fill your field of view. Apparent brightness depends on the exit pupil of the eyepiece. An eyepiece with a 7 mm exit pupil produces a "light pencil" that fills your eye's pupil. That's a lot more photons then you'd get by looking at the moon naked eye, as the moon subtends only half a degree in the sky. A simple eyepiece has an apparent field of 50 degree, on the other hand. Eyepieces that magnify more have smaller exit pupils (exit pupil = eyepiece focal length / telescope f ratio) and apparent brightness degreases as exit pupil diameter gets smaller. That's why higher powers look dimmer. This is important for the photography situation.

In photography we have the telescope objective and and a CCD chip. We form an image on the chip using the scope. The longer the focal length of the scope, the more the view will be magnified on the chip. What, then, controls the apparent brightness? What is the equivalent of the eyepiece exit pupil? The answer is that it is dictated by the angle subtended by the objective as viewed from the chip. Imagine you're a pixel on the chip and looking at the lens of the scope (mirror is the same if it's a reflector): the lens occupies a certain portion of your field of view. If you swap the lens with a faster f/ratio optic of the same size then the lens comes closer. It now occupies more of your field of view, which means that you're getting photons over a larger range of angles. The pixel is collecting more photons from the object and increasing apparent brightness. However, the image will be of lower power since the focal length has gone down. Just like our eyepiece, see?

So why doesn't bigger aperture always mean more photons? Remember that, in imaging, focal length is magnification and apparent size of objective is apparent brightness. Focal length is objective diameter multiplied by focal ratio. Imagine a 6" f/5 and a 12" f5. The larger objective will be further from the chip at focus. This means it magnifies more but the larger distance ensures it isn't delivering more photons (it's not subtending a larger angle as viewed from the chip). However, a 6" f/5 and a 12" f/2.5 is a different ballgame. Now the objectives are the same distance from the chip but the 12" optic appears much larger. They now magnify by the same amount but the 12" is pumping more photons onto the chip (bigger range of angles, again). Your exposure times can therefore be shorter by a factor of 12^2/6^2 (I guess).

If you're confused about the brightness theorem, the thing to remember is "photon flux." You can't increase photon flux with an optical device. If the object you're looking at emits 10 photons per square cm then you can't build an optical device which packs more than 10 photons per sq cm onto the image plane. In other words, you can't cram the photons together more tightly. It boils down to conservation of energy.

Aperture does help resolution, however. It's limited by turbulence but, in theory, larger objectives will produce smaller point spread functions and so smaller stars and higher resolution images. However, the ability to capture this is also limited by the resolution of the chip.

[Ptarmigan]

Hi, it's me again ! What an interesting thread

Now I may be wrong (it's been a long time since I studied these things ) but Rick's focal ratio thing and U's excellent description of it are true for extended objects such as the moon, nebulae and ordinary daylight things. But, theoretically, for a point source (star) then the exposure/brightnes is only dependant upon the aperture not the Fratio.

All the extra photons captured by the increased aperture all end up on the same spot of the eye/film/chip, not spread out.

Er well not quite because things are not perfect in real life ! but it is a sufficiently point-like situation that the larger aperture sees fainter stars, but not quite the theoretical increase.

I think :)

stands back scratching head again

Malcolm.

[Dave]

If i were to buy the 10", is it possible to later on remove the tube from the Dobsobian mount and place it on an equatorial one?

I have a 10" Dob and I have fitted a dovetail onto the tube, so It can be mounted on my EQ6 and I can image through it. And very sucessfully, I might add. Have a look at my web site for some examples.

I also take images of the ISS with a webcam using it as a Dob. Much easier to manually attempt tracking a moving object with a mount like this.

[umadog]

Malcolm,

Yes, I keep hearing that point sources are different but I haven't yet ingested why this is the case. I should look into it again, actually.

Cheers!

[Ptarmigan]

but I haven't yet ingested why this is the case.

Yes me too, while I was typing that I felt a wee paradox creeping into my brain cell -

If the aperture is increased but the fratio stays the same then the nebula would stay the same but the stars would increase, in the limit the stars would out-shine the nebula and overwhelm the sensor till the nebulosity became insignificant, argh ! Nurse, nurse, help

I think some googling is needed,,,

[RikM]

Olly Penrice explained this really well in a thread a while back but I can find it. Sorry.

Are point sources really point sources or does the airy disc act like a (very small) extended object? If it were really a point (as far as our imaging kit were concerned), all the light from a star would end up just on one pixel, but clearly it doesn't.

[ScubaMike]

Thanks for the detailed explaination Umadog.

I'll re-read it once I have had more coffee!

It sounds counter-intuative though.

So what are the requirements in terms of Astrophotography...

I appreciate that a low Focal ratio is first consideration (F4 is better than F6 which is better than F8 etc), but have not worked out whether a short focal length is better than a long focal length if the Focal ratio stays the same.

more coffee & thought required.

[RikM]

Chose the focal length based on the field of view you want.