quick galaxy question

[GrassLakeRon]

Hi all,

Just a quick question, how many galaxies are there brighter then magnitude 17.5? Thanks for the info.

Ron

[wvreeven]

Ron,

If you mean "visual brightness" then nobody knows because nobody has measured the brightnesses of galaxies in the visual range down to that level. All brightnesses fainter than magnitude 14 or so are photographic.

To my best of knowledge, the most complete database or extragalactic sources is Hyperleda. It depends a bit on your definition of galaxy but using their online database query tool you should be able to get a good indication of the answer to your question.

HTH, Wouter

[GrassLakeRon]

Ron,

If you mean "visual brightness" then nobody knows because nobody has measured the brightnesses of galaxies in the visual range down to that level. All brightnesses fainter than magnitude 14 or so are photographic.

To my best of knowledge, the most complete database or extragalactic sources is Hyperleda. It depends a bit on your definition of galaxy but using their online database query tool you should be able to get a good indication of the answer to your question.

HTH, Wouter

Thank you.

[Don Pensack]

Put it this way:
If your skies are dark enough, your 16" may be able to detect (not necessarily see well), well over 50,000 galaxies.
You won't live long enough to detect them all. And in many, the sizes are very small and they are visible only with averted vision.
Is that how deep you want to go? Is your goal to see all the objects visible in the 16" aperture? That may be impossible, even if you observe all night 300 nights a year.
My observing log, since 1982, has just under 13,000 objects in it, and I have barely scratched the surface in many constellations.
And I observe with a 12.5". A 16" will go a half magnitude deeper and see many thousands of objects more.
As Wouter noted, the visual magnitudes of many faint galaxies are not really accurately determined, so my basic advice would be to look for the galaxy.
If you can't see it, then log it as Not Found and try on another night. Transparency of the sky varies a lot. One night I can barely see a galaxy and on another night it is obvious.
Only after several NF results can you say you cannot see it.
Here is a list for you to use. When you have seen every object on the list, then you will be ready for the next 10,000:
https://www.saguaroastro.org/sac-downloads/
Download the v.8.1 database.

Here is another list of many thousands of galaxies:
https://heasarc.gsfc.nasa.gov/W3Browse/all/rc3.html
Make sure you download the guide to the columns.

The current version of the Nexus DSC Pro has almost 5 million galaxies in its database. There is pretty much noting you can see that is not in its database.
It's really overkill, but it is useful to have such a database at your fingertips in the field.

[obrazell]

If your interest is in the RC3 then I suggest you look at Harold Corwin's updated version at http://haroldcorwin.net/rc3/index.html rather than the HEARSAC one.

[Don Pensack]

Thanks, I downloaded that version, but I did not have the link.

[GrassLakeRon]

I guess what it boils down to is what do I set my filters on when making an observation list. I have ran my limiting magnitude settings and it says 17.3-17.5 mag for where I'm going to view. I was curious about the numbers in general. Don, I did get a hold of a company who will sell me a 16" dob and will ship it to me. Seems it's not a question of supply chain as much as US companies, dealers, are not asking for them. I find that funny.

[Don Pensack]

US importers buy from the factories, they do not buy from foreign retailers. A foreign retailer could very well still have stock when the US importer is sold out.
I would not set any parameters when sorting a list, because the magnitude figures when you reach 14.5 or so and go deeper are often in error.

[wvreeven]

To get back to the original question, on the HyperLeda website I submitted the SQL query "select count(*) from meandata where bt<17" and got the answer 523246. However, HyperLeda includes lots of other types of objects as well. I have a filtered file at home which I created about a year ago. It contains 260241 galaxies with bt < 17 and 11284 with vt < 17. That just goes to show that we really don't know the visual brightness of galaxies at all. Don't pin me on those numbers though. It is not unlikely that I didn't properly filter the data and that these numbers still are not correct.

[GrassLakeRon]

Thank for the heads up with HyperLeda. First Don, I emailed the company and they sent one to their local distributor with my name on it. The company is not getting orders to send them to the US for whatever reason. So a 16" is on the way..... As far as the galaxy question, with a 12" or a 16" is was looking at the Schaefer limiting magnitude curves and the limiting magnitude calculator. If I'm reading the data right, in Bortle 1 skies, a 12" "acts like" a 16" but a 16" like a 18 or better" in that type of environment. I just needed to know my limitations with the new scope. I have spent time reading Cloudy Nights over the last 6 months. Im looking at Naglers for high power viewing rather then Ethos. Folks seem to be leaning that way online for truly deep objects, I just wanted to know how deep to expect. Thanks for all your help.

[wvreeven]

I use Ethos with my 20" and get to go down to galaxies of (photographic) magnitude 17.2 at high magnifications. I once even went deeper. Not sure what a Nagler would add to that.

[GrassLakeRon]

I use Ethos with my 20" and get to go down to galaxies of (photographic) magnitude 17.2 at high magnifications. I once even went deeper. Not sure what a Nagler would add to that.

Which Ethos?

[wvreeven]

Usually I observe with the 8 mm (320x) and occasionally go up to the 6 mm (427x) or 4.7 mm (544x). With the 6 mm or, preferably, the 4.7 mm I can pick out those super faint, higher-than-mag-17 glows provided they're large enough for my eye to pick up.

I tried the 3.7 mm (692x) as well but that usually is too much for faint galaxies, even under good seeing conditions. It works wonderfully well on brighter objects like bright galaxies, planetary nebulae and planets though. I haven't tried this eye piece with super compact sources yet, though. It will probably work very well. Funny things, our eyes. When surface brightness drops, angular size becomes (much more?) important. Not too small and also not too large is key.

I have considerable difficulty spotting faint stars at those magnifications. Only the tips of the airy disks of very faint stars are visible then and they're not large enough to be seen by me.