Dob size upgrade question

[Treewick]

Looking to fill in the gaps on my understanding of what you get with a larger Dob scope versus a smaller one. I have had a 6” dob for almost a year now and have really enjoyed it. I was thinking of upgrading to a significantly larger one next year to try and pull in more detail on the DSO’s I have been observing now that I know how to find them. It is my understanding that the image size of the objects viewed would not be increased by the larger scope itself but that I could use higher magnification EP’s to gain greater clarity on the images because of the increased light. Is this correct? As for the how / why part, does the increased light collected alone allow you to go to higher magnification EP’s and retain clarity or does the length of a larger dob tube play a part as well? For sky darkness reference, if at my present viewing location I have been able to observe DSO’s around a magnitude 9.8 with my 6” Dob using a 25mm 1.25” EP, to what level could reasonably expect to view with a larger Dob (10” or 12”) at the same site using the same magnification?

[valleyman]

To answear one part of your questions, have a look her at the Skywatcher website where it gives Steller magnitude to each of its Dobs

TELESCOPE SUPPLIERS - SKY-WATCHER TELESCOPE

[swamp thing]

Hi Todd

As you are probably aware on extended DSO the surface brightness is often a more accurate guide as to what can actually be seen than just magnitude alone.

Objects with low surface brightness but a high magnitude can be a lot harder to observe than objects with lower magnitude but higher surface brightness.

You are indeed correct about the increase in light allowing for more magnification. The increase in resolution that comes with the larger aperture is also of great benefit in pulling out more detail.

Only the aperture will affect visual performance, the focal length (longer Dob tube) is of no importance except in storing the damn thing.

A 10" or 12" will both show a marked improvement over a 6"

Regards Steve

[tom33pr]

Apart from optics, performance etc, I'd also take into account the size of the scope.

10" is just about portable. I think you'd need a fairly big car, and some muscles to transport the 12". It is pretty big and heavy, indeed. ...unless you don't think you will be taking it anywhere.

[John]

Apart from optics, performance etc, I'd also take into account the size of the scope.

10" is just about portable. I think you'd need a fairly big car, and some muscles to transport the 12". It is pretty big and heavy, indeed. ...unless you don't think you will be taking it anywhere.

This is a good point - I had a 12" dob for a while and found it a little too much to move about easily. I downsized to 10" and the scope gets a lot more use.

Don't also underestimate the impact that getting the scope under dark skies can have. A couple of us reeled off loads of faint DSO's on a great night at the SGL star party in April this year - we were using "only" 6" newtonians

[acey]

It is my understanding that the image size of the objects viewed would not be increased by the larger scope itself but that I could use higher magnification EP’s to gain greater clarity on the images because of the increased light. Is this correct?

A telescope at its lowest useable power will give an image with the same surface brightness as the object itself (assuming no light loss), but at a larger size. Lowest useable power is aperture/eye-pupil.

Example: eye-pupil 6mm, aperture 60mm, lowest useable magnification is x10. With a 600mm scope it would be x100. Arm both scopes with lowest-power eyepieces and point them at a galaxy. The image surface brightness will be the same in both scopes (assuming they transmit light equally) but the larger scope shows it at ten times higher power. Not so good if you want to look at a large faint object like Barnard's Galaxy (better use binos or small scope for that) but great for hunting down hundreds of small NGCs, or for small details within galaxies (e.g. spiral arms).

As for the how / why part, does the increased light collected alone allow you to go to higher magnification EP’s and retain clarity or does the length of a larger dob tube play a part as well?

Yes, aperture is what does it. Focal length plays a role with regard to field of view, off-axis aberration (coma) and portability/convenience.

For sky darkness reference, if at my present viewing location I have been able to observe DSO’s around a magnitude 9.8 with my 6” Dob using a 25mm 1.25” EP, to what level could reasonably expect to view with a larger Dob (10” or 12”) at the same site using the same magnification?

Theoretically the magnitude gain for a 12" over 6" is 5log(12/6) = 1.5. This would suggest that you should see down to around 11.3, though in practice it could be more or less, since total magnitude is not a perfect guide to galaxy visibility. With 10" the theoretical gain would be 5log(10/6) = 1.1 which is still a very big difference. Even the 0.6 gain of an 8" should be noticeable to someone with viewing experience such as yourself. The weight difference is also considerable, of course. An 8" dob can easily be carried in one piece (tube plus base), a 12" can't.

At a dark site a 6" can do a lot: the 19th-century astronomer Holetschek catalogued more than 600 DSOs that he viewed with a 6" refractor, finding total visual magnitudes for each of them. His estimates went down to about magnitude 12, though there were fainter objects that he could see. With a 12" the whole NGC comes within reach: the early 20th-century astronomer Bigourdan viewed all the NGCs above his horizon (over 6000 of them) using a 12" refractor: objects going down to mag 15 or 16.

Darkness really does count for everything. My back garden has a limiting magnitude of about 4.5, my dark site is 6. With regard to stars, that difference is like a doubling of aperture, but on galaxies it counts for far more, since a galaxy can only be visible if it has sufficient contrast against the sky. i.e. its surface brightness is sufficiently high in relation to the surface brightness of the sky. My garden sky has surface brightness of around 18-19 mag per square arcsecond, my dark site is about 21-21.5. DSO surface brightnesses range from about 16 down to 24 or fainter, the brightest things being galaxy cores and some planetary nebulae. At the dark site I can see outer parts of galaxies that I could never be able to see from my garden.

[Treewick]

Thank you all for the detailed information. This site is amazing, you guys are truly a resource to be valued for anyone starting out in this hobby!

[Nexus 6]

Excellent questions and some expert advice in response

[umadog]

Some excellent answers to your question. I heartily agree that dark skies are a must. We have a 25" Dob at out club "dark" site, where skies are about mag 5.25 to 5.75. I know, however, that my 10" will blow the pants off the 25" for most objects if I take it somewhere darker (better than mag 6.5). You can't compare dark skies to more aperture. If a galaxy is overwhelmed by light pollution then no scope will show it to you, since the sky-glow is brighter than the object. Whatever scope you buy, make sure it's portable for you.

I've owned a 12" and now have a 10" in its stead. The differences between what you can see in the two aren't huge by any means. If you're worried about portability and are deciding between 10" and 12" then go for the 10" and don't look back. If you're happy about handling a 12", then by all means go for it. A 12" truss Dob is very manageable, actually. Note, however, that you will have slightly longer set up time compared to a 10" solid tube.

In practical terms, aperture buys you magnification. You are able to magnify objects more and retain the brightness. Here's why. Imagine we have a 6" f/5 and a 12" f/5. A 10 mm eyepiece gives 76x in the 6" and 152x in the 12". So a much bigger image in the 12". However, the image brightness depends on the eyepiece exit pupil. Exit pupil size is calculated as eyepiece focal length divided by objective focal ratio. So both scopes will have the same exit pupil (10/5=2mm). The image is equally bright in both scopes but it's larger in the 12". Because it's larger, you can pick out more details. This is particularly useful for smaller objects such as planetary nebulae. Globular clusters are another class of object which improve substantially in larger scopes. It also allows you to make better use of light pollution filters.

A lot of guys who've been around the block a few times settle on a 12" as their ideal scope in terms of size vs portability. There are other considerations too. Firstly, the focal ratio. Faster focal ratios are good because they keep the tube shorter in larger scopes. However, faster scopes exhibit more coma and require more precise collimation. Plenty of people don't go faster than f/5 (or even f/6) because they don't like how the stars look. A coma corrector helps, however. Secondly, the larger you go the longer your focal length. This means that your lowest useful magnification increases. So large, extended, objects look better with a smaller scope since this frames them better. I find I enjoy the veil nebula more in my 10" than my 18", even though the bigger scope shows more detail. A 12" f/5 is a pretty good compromise focal length from this regard.