The Jump

[davidem27]

Hi guys!

Aperture fever stole my mind.
I bought a brand new, larger, car.
I want to upgrade my 16" dobson after ~1600 different objects seen.

What do you think, in relation of your own personal experience, about grew from 16 inches to 20 inches mirror?

Some friend says that you can appreciate a truly significant difference among different apertures in a step of 4 inches.

[Howard B]

Hi Davide,

I understand, aperture fever has stolen my mind a few times too!

I've found the best way to determine if a larger scope is worth it is to compare the surface area of the primary mirrors - if the larger mirror has close to twice the surface area then it will be worth it. For instance, your 16 inch mirror has 201 square inches of surface area, and a 20 inch has 314 square inches. That's about a 50% increase in mirror surface area, so although you'll see more in the 20 inch you may not find the increase to be worth the larger size and weight of the scope.

But that's only a guess because you may indeed find the views in a 20 inch to be worth it - the only way to know for sure is to look through a 20 inch.

[Ivan Maly]

I find the difference well worth the upgrade but am still looking for a good transportable 20" design. The 20" I am using stays at our rural observatory. On a good night it's 21.4 mag/sq arcsec there - nice and usable but still bright to my taste. It beats my 16" that travels to a 0.5-mag darker site on stars but is about equal to it on the deep sky (minus the dark-sky aesthetics).

[Norman]

Hi Davide,

i am with Howard. I often had the chance to compare the views in 12"/ 16" under very good conditions. So the 4"-step-rule does not work for me. For me this never was somethin i´d call a "jump" or big leap. But it´s a very individual thing. You see the difference - but for me not worth changing anything.
Maybe it is not a big jump from 16 to 20 - but maybe the (personal) optimum of size/ power of the instrument and the given views. For me something around 20" would be the best choice - for i do not want to use a big ladder, stars a still fine and seeing not such an issue as in bigger instruments.

CS
Norman

[davidem27]

Thanks for the replies guys!
I saw many times the deepsky in a 20 inches telescope.
And, at almost 100% of times, I thought exactly what Norman and Howard are saying.

You can notice the light gain between 16 and 20, but it's not enough from how much is the worth of:

- ladder
- weight of the scope
- mirror acclimatation
- difference in cost

A 24" would, obviously, be better.
But things about observing will be surely change in front of this amount of upgrade:

- need to be in observing place before dawn, just to cool down enough the mirror
- change my observing programs just to sweep only 2-3 part of the sky, just to optimize the time at eyepiece than at the ladder
- understand what 24" could stay inside my car, without smashing me out :D

I usually observe objects fainter than 12.5 mag.

My observing programs are Herschel 2500, Arp Galaxies, Palomar GC, Hickson CG and Abell Galaxy Clusters and my usual skies are SQM (not lens) 21.2 to 21.6.

[davidem27]

Uhm...doesn't anyone else would like anything to add?

[Don Pensack]

Sure.
I see a profound difference when gaining a full magnitude on a scope.
That's the jump from 8" to 12.5", or the jump from 12.5" to 20", or the jump from 16" to 25".
Anything less, and I'm not impressed with how different the view is compared to how much extra trouble it is to transport and use the scope.
In pristine skies, I estimate somewhere in the vicinity of 50,000 objects might be detectable in a 16" scope.
You're at 1600, so you have a LONG way to go.
I only have 12.5" but my log has well over 11,000 objects in it.
I don't think you need a larger scope, you just need to spend more time observing.

That doesn't mean a larger scope might not be nice, but you are pretty close to the maximum size for no-ladder observing.
A 20" f/3.6 would be the same height, but many times the cost.
And a 25" would require some serious consideration ($$$) for transportation.

[davidem27]

Ohhhh Don, what a wise words!

Yeah, you're right in many of your words.
I agree with you at almost 100%.

BUT. I think life is short and life is now.
I spent about 100 nights with 16 inches through 8 years.
And now I would gather as much photons as I can.

Unfortunately "pristine skies" does not fit with my actual situation.
In June 2015 I went to Cali, Nevada, Colorado, Utah and Arizona.
Went to GCSP (south rim) and saw a great sky.
Then I went to Bryce Canyon sky.
If you go observing in places like that, spending 4-5 hrs drive, you have my compliments.

Near my home in Italy, I actually have only one good place to go observing.

1400 meters height.
SQM 21.30 mean.
3 hrs drive.

In next few year this place will be difficult to reach. 2 children in 1.5 years are too much for my wife :D

So I will probabily go to 30 minutes far, 500 meters high, SQM 20.90 place to go observe.

Maybe the ladder will be the minor problem.
Maybe I would really see (faint) objects with details inside.
Maybe I'm just deciding to choose among 20 or 24 inches.
I want to make the right decision for my retina and for my brain :)

Thanks soooo much for your words, very very appreciated.

[Don Pensack]

Davide,
No matter what the site, the bigger scope will always go deeper.
If you can handle the larger scope and have the way to transport it and set it up, then the larger scope will show you more.
You do always have to evaluate how large a scope you can transport.
And, with two small children, how much telescope you can afford. :-)

[davidem27]

Eheheh.
If children are at home...everything is possible :D

I'm a man with a one-track mind (said Brian May). My track is MORE, MORE DIAMETER! :D

My point of question is how much a 22-24" can show more than a 20" paired with my actual 16".

[Don Pensack]

If we assume 16" as the "base" size,
20" reaches about a half magnitude deeper.
22" reaches about 0.7 magnitudes deeper
24" reaches about 0.9 magnitudes deeper
Obviously, the 16" to 20" change is the most significant difference, compared to 20" to 22", for example.
But the 16" to 24" change is the most significant of all.
The question is what you like to observe.
If it's star clusters, globulars, planetaries, the likelihood is that the 16" size is sufficient.
If it's galaxy groups, Arp objects, Abell planetaries, galaxies from 500 million light years out, then the larger the better.
The couple tenths of a magnitude difference between a 20" and 22" is close to the difference in sky brightness over a few hours of the night.
I don't regard it as significant if you want the image to be different enough to justify the extra size, weight, and cost of the larger aperture.
A half a magnitude is significant, but, even there, the difference will be felt more in the extremely faint end of the magnitude range.
Do you have a local club with large apertures so you can compare what you see to your 16"?
I have always gotten to compare my 12.5" (32cm) to larger scopes all the way up to 70 or 80cm where I observe, so I have come to know what to expect
when looking through the larger scopes. Larger scopes have more problems with cooling and seeing and longer focal lengths yielding smaller fields of view.
But they also handle the very high magnifications more easily, which is great for smaller objects.
If I used a 16", 20" would not be contemplated--there is too small a difference.
If you want a significant change, the 16" to 24" would be a lot more impressive.
But you need to look through a couple to decide for yourself. Otherwise, you're just guessing.

[davidem27]

Thanks for another "full featured" reply, Don.

My favourite targets are galaxies.

Arp ones, Abell Galaxy Clusters, Hicksons.
The faint and the grouped objects.

Palomar Cluster are also in my observing lists.
I want to complete the Herschels 2500.

Clusters and planetaries are secondary targets.

I already observed in 20" scopes.

And my observing mate had 24". He's a real machine gun (more than 20.000 DSOs seen and documented) and I never tried his scope as I deep as I would like to understand.

But I essentially know the differences 16-20 and 16-24, from SQM 21.50 skies.

My thought is that the 16-20 jump could be not enough...

I never observed in a 22 inches dobson.

Damnit... :thinking:

[andre8x8]

I went from a 8" to a 14" and now I have a 24" Dobson.
As for me it is not about how deep I can see, I want to see detail in what I see. I revisit the Messiers and other showpieces over and over.

If you want something now, don't wait until tomorrow. Go for it.
There is a rule of thumb, you go up in sizes with a factor 1.5 x
16" x 1.5 = 24"

Here's how I made my 24" f3.7 Encased GoTo Dobson (http://www.astrosurf.com/aheijkoop/Equipment/24%20inch%20Dobson/24%20inch%20f3.7%20Encased%20GoTo%20Dobson.htm)

[Norman]

Hi Davide,

I think the common opinion now is that 24" would be best for you. So there are only a few advices left: use the chance to experience the views in your mates 24" if it fits your needs! And last but not least: consider the seeing at the places you plan to use. Is the power you would use possible in most of the time at this site? Is it possible to magnify > 600 in rare nights or quite often? At least for me fine stars are more important than bigger smudges...

CS
Norman

[estwing]

kinda in the same boat...my next jump hopefully will be a 24" from my 18". I observe with 20" and a 22" buddies.
ARPs and Abell are my main interest and the 18" is capable. My dark skies are mid- Wales, a 3hr drive where we rough camp for the weekend.
but I just have the fever and the big 50 is around the corner!...you're only here once!