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Will Going From 16" to 20" Make Sense For Observing Galaxy Groups?

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#1 Peter Natscher

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Posted 14 January 2018 - 05:08 PM

HI Reflectors Gang!

 

Well, here I am again immersed in the thought of increasing aperture to observe deep sky objects more easily.  I currently observe with a wonderful JP Astrocraft 16" Dob that is a joy to use.  I highly recommend this telescope for its portability, quality, observing various deep sky objects.  Using the 16 the past two years, I've completed a few brighter deep sky object lists (documenting, describing, and sketching) though the objects all have been no fainter than magnitude 15.0.  The fainter objects such as member galaxies of galaxy groups all required me to use averted vision to document and sketch.  I would like to see a few more galaxies in these groups. Having observed with larger Dobs I've owned in the past and having a larger transport vehicle to transport them to dark sites in, I'm back to desiring the larger aperture but now have a smaller vehicle and less will to manage 70lbs.-100lbs. telescope pieces.  I'm thinking a 20" Dob would still fit into the current vehicle, have lighter-weight pieces, and give me a easier time spotting more of the threshold galaxies listed in my next observing project - Galaxy Groups.  These galaxies have magnitudes ranging between 14-17. The 16" starts having a hard time with this magnitude range from my mag. 6.3 sites.  If I'm lucky, I get to spot one or two galaxies of a group of 4-5. I usually observe for magnitude 6.3 sites and twice a year travel to more remote star parties with magnitude 6.8. So, would going from 16" to 20" be a worth while move for me to observe and sketch galaxy groups in the magnitude 14-17 range?



#2 DLuders

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Posted 14 January 2018 - 05:20 PM

You could use the N.A.A. Telescope Calculator http://www.stargazin...a/scopemath.htm to determine the theoretical limiting magnitude of the 20" telescope vs. the 16" telescope.  



#3 Knasal

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Posted 14 January 2018 - 05:27 PM

Will the increase in aperture help you see fainter objects? All other things being equal, sure. Your sky conditions closely match mine, incidentally.

 

Here's a calculator that may help you:  http://www.cruxis.co...ngmagnitude.htm

 

But you hit on a key point - weight. And with that, your ability to take it out and set it up. If you're older, this becomes a key deciding factor in the investment of such a scope. 

 

There are lighter weight Dob options out there and perhaps that will suit you.

 

Do you plan to keep the 16"?

 

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#4 Jon Isaacs

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Posted 14 January 2018 - 05:35 PM

Peter:

 

I have a 16 inch and a 22 inch.  I don't think a 20 inch is going to get you where you want to go. Math says the 16 inch to 20 inch jump is about a half a magnitude.  I definitely go deeper with 22 inch than the 16 inch and seems somewhat more than the math predicts, it's no more than a magnitude and you're looking for 2.

 

Jon



#5 WilburTWildcat

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Posted 14 January 2018 - 05:35 PM

That's a fantastic resource!



#6 havasman

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Posted 14 January 2018 - 06:17 PM

My 16" is likely not quite as capable as yours but it is very good and the difference in observations that I am able to make and those I read of that others (IVM, Redbetter) with your level of experience make with 20" apertures is significant. Likely some of that difference is due to observing skill and maybe some is due to observing site differences but aperture is in play also. Whether the difference/cost makes sense is a call only you can make.



#7 Peter Natscher

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Posted 14 January 2018 - 07:11 PM

Hi Jon,

 

Ten years ago, I went from a 18 to a 24 and that was a very noticeable jump. A 16 to 20 jump I'm not so sure about either. The 24 had a 80lbs. removable heavy primary and cell which I hand carried into its mirror box (hint: a Starmaster 24) when I was 10 years younger. I don't want to do that now.  I would sell or trade my premium 16 for a premium 20 if it made enough of a difference. I could probably transport and observe wit a 20 for another 5-8 years. The weights for 20"ers parts seem ok with me, especially if rolled around.  I'm also in discussion with a person who is offering me his 22 Starmaster but the increase in box sizes and mirror/cell weights increase look unappetizing for at my 68 years age.  It was ok ten years ago for me but not now.



#8 Allan Wade

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Posted 14 January 2018 - 07:49 PM

Peter I would only consider the jump to a similar quality 20” as your current, beautiful 16” JPA. I think Jon is on the money by describing the step up as noticeable, but not significant.



#9 Bob S.

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Posted 14 January 2018 - 07:57 PM

Peter, What others have said is right on the money. Adding only 1/2 mag of increased light generally does not significantly improve detection of faint objects. As you pointed out, the difference in dimensions for smaller 20"s is not that big a deal because the SFX-Q you formerly owned was placed in an 18" structure at time of manufacture. However, when you start talking about a 22" Starmaster, you are really upping the ante weight wise and certainly dimension wise. You once owned a 20" SFX-Q Starmaster where the combined mirror and cell weight were 48 pounds. The published combined weight for the 22" SFX Starmaster is 67 pounds which is 19 pounds heavier.

 

Like yourself, I have made 68 orbits around the Sun and have owned large scopes like yourself and find that my 16.5" FX Starmaster with a combined mirror and cell weight is 38 pounds and really the most I feel like toting around anymore. In fact, I just purchased a 14.5" FX Starmaster with a combined weight of 30 pounds that is MUCH more pleasant than even the 16.5" and they both sport 1.25" thick mirrors. Of course the dimensions and lack of the Hybrid feature on the 16.5" makes the 16.5" a bit less hospitable than the 14.5".

 

I suspect that the reality of aging and hassle factor are becoming more clear. As I and some others have done, we have gone to using image intensifying ep's which essentially increase the apparent aperture of a scope by a factor of 2-3x. Those faint objects are no longer faint in a 16" telescope. You own what is arguably one of the finest 16" scopes on the planet in that uber custom JP Astrocraft. I don't see how adding 4" is going to make your quest much better? Much cheaper and easier to just get a high-end image intensifying ep IMO. Bob Schilling


Edited by Bob S., 14 January 2018 - 08:13 PM.


#10 skyward_eyes

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Posted 14 January 2018 - 08:39 PM

I just did this in the last year. I’ve been using a 16” f/4.5 for over six years and this year picked up a 20” f/5.

There was more of a jump than I originally expected. Things in the 20 pop more and just about everything on the popular maps was visible. So after doing this personally a 16 to a 20 was a nice jump.

#11 sopticals

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Posted 14 January 2018 - 08:56 PM

Same difference as 4" to 5" or 8" to 10". Yes difference would be noticeable but not WOW.

 

Stephen.(45deg.S.)



#12 Peter Natscher

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Posted 14 January 2018 - 09:02 PM

I just did this in the last year. I’ve been using a 16” f/4.5 for over six years and this year picked up a 20” f/5.

There was more of a jump than I originally expected. Things in the 20 pop more and just about everything on the popular maps was visible. So after doing this personally a 16 to a 20 was a nice jump.

Interesting waytogo.gif



#13 Peter Natscher

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Posted 14 January 2018 - 09:17 PM

Peter, What others have said is right on the money. Adding only 1/2 mag of increased light generally does not significantly improve detection of faint objects. As you pointed out, the difference in dimensions for smaller 20"s is not that big a deal because the SFX-Q you formerly owned was placed in an 18" structure at time of manufacture. However, when you start talking about a 22" Starmaster, you are really upping the ante weight wise and certainly dimension wise. You once owned a 20" SFX-Q Starmaster where the combined mirror and cell weight were 48 pounds. The published combined weight for the 22" SFX Starmaster is 67 pounds which is 19 pounds heavier.

 

Like yourself, I have made 68 orbits around the Sun and have owned large scopes like yourself and find that my 16.5" FX Starmaster with a combined mirror and cell weight is 38 pounds and really the most I feel like toting around anymore. In fact, I just purchased a 14.5" FX Starmaster with a combined weight of 30 pounds that is MUCH more pleasant than even the 16.5" and they both sport 1.25" thick mirrors. Of course the dimensions and lack of the Hybrid feature on the 16.5" makes the 16.5" a bit less hospitable than the 14.5".

 

I suspect that the reality of aging and hassle factor are becoming more clear. As I and some others have done, we have gone to using image intensifying ep's which essentially increase the apparent aperture of a scope by a factor of 2-3x. Those faint objects are no longer faint in a 16" telescope. You own what is arguably one of the finest 16" scopes on the planet in that uber custom JP Astrocraft. I don't see how adding 4" is going to make your quest much better? Much cheaper and easier to just get a high-end image intensifying ep IMO. Bob Schilling

Yes, Bob.  Logistics is a prime consideration.  My 16" JP Astrocraft Dob has a 22"W x 22"L rocker box leaving my Subaru Forester's cargo area a good amount of room for necessary astro-camping gear. It's super compact!  A telescope with 30"x30" boxes and a 30"x14" UTA alongside would take up more than half the Forester's cargo area. Compare the JPA boxes to the box sizes of 27.5"-29"L and W in a 20" Starmaster or 30"-31.5" in a 20" Teeter Classic, and also the weight increases.



#14 Peter Natscher

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Posted 14 January 2018 - 09:40 PM

Peter I would only consider the jump to a similar quality 20” as your current, beautiful 16” JPA. I think Jon is on the money by describing the step up as noticeable, but not significant.

I got a 0.6 magnitude gain going from a 18" to a 24" and the increase in galaxy detail was significant.  Why wouldn't a 0.5 gain going from 16 to 20 be worth while?  confused1.gif


Edited by Peter Natscher, 14 January 2018 - 09:41 PM.


#15 IVM

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Posted 14 January 2018 - 10:42 PM

I observed Alvin Huey's selection of galaxy groups, and much besides in the realm of the galaxies, from an SQM 21.2 dark site (for me, semi-dark - rural but not remote) with 16 and 20". 20 is better, no two ways about it. I now have a modern 20 that goes to the really dark skies. The 16 that you see in the avatar is ensconced in the Massive Equipment Vault (© Uncle Rod).



#16 dave brock

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Posted 14 January 2018 - 11:16 PM

I went from a 16" to a 20". The difference was subtle but there. What seems to be

a bigger difference is when after some time with the 20", you go back and look through a 16" again.

 

Dave



#17 Bob S.

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Posted 15 January 2018 - 04:44 AM

 

Peter I would only consider the jump to a similar quality 20” as your current, beautiful 16” JPA. I think Jon is on the money by describing the step up as noticeable, but not significant.

I got a 0.6 magnitude gain going from a 18" to a 24" and the increase in galaxy detail was significant.  Why wouldn't a 0.5 gain going from 16 to 20 be worth while?  confused1.gif

 

Peter, When I would compare the views in my mind of my 18" f/4.3 Zambuto Starmaster with my 24" f/4.2 Zambuto Starmaster, there was not doubt that the increase in aperture made faint DSO's pop. However, we are talking about a 6" gain in aperture and the fact that my receptor systems seem to respond quite favorably to the much larger 24" of aperture. However, the hassle factor of toting around a 24" vs. and 18" was pretty enormous. I can't remember the equation off hand for measuring the difference in square inches of reflective surface but the difference in overall surface area does not seem to necessarily correlate with human perception systems and how they perform with increased reflective surface area. We also need to factor in that as we age, generally our eyes/brain/body experience changes both positively and negatively that may negate the value of increased aperture.

 

I vaguely remember from 35 years ago, some researchers who were looking at lifespan/health state related issues and were talking about a metric they came up with that were called QALY's=Quality-Adjusted Life Year. What they were suggesting was that there are steps that will increase the quantity of something but not necessarily increase the quality of the situation from a health standpoint. They were also factoring in the Time-Trade Off for achieving a different level of health. In your particular case, you have already identified that you have ample room in your Forester for your current 16" Zambuto/JPA and your camping gear and have been pretty pleased with how it all fits together. It seems to me that the urge for something different might be overpowering your need for something different. Having known you via these forums for the past 15 plus years, I suspect that you are afflicted (to a much lesser extent than my own affliction) with "changeitis" which is the "grass is greener" on the other side of the fence phenomenon in telescopes. If you consider the QALY's vs. the Time/Hassle Trade Off for increased aperture, I suspect that you may conclude that changing scopes may not greatly increase your "QALYs" when it comes to viewing faint DSO's? Again, I am suggesting that a simple image intensifying ep will crank up your level of detectability of faint objects without having to sacrifice "QALY's" to a certain extent waytogo.gif  Bob


Edited by Bob S., 15 January 2018 - 05:09 AM.


#18 Jon Isaacs

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Posted 15 January 2018 - 04:52 AM

 

Peter I would only consider the jump to a similar quality 20” as your current, beautiful 16” JPA. I think Jon is on the money by describing the step up as noticeable, but not significant.

I got a 0.6 magnitude gain going from a 18" to a 24" and the increase in galaxy detail was significant.  Why wouldn't a 0.5 gain going from 16 to 20 be worth while?  confused1.gif

 

Peter:

 

It's worthwhile.  I calculate the 16 inch to 20 inch step up as 0.485 magnitudes, the 18 inch to 24 inch as 0.625 magnitudes.  These are definitely significant.  As I said, my 22 inch goes 0.7 magnitudes deeper than my 16 inch and I find that to be big difference. I think magnitudes are not the best measure for determining the visual impact, the 22 inch is 90% brighter at a given magnification.. That's a number I can think about.  

 

However, in your initial post, you state that with your 16 inch you are working up to magnitude 15.0 galaxies and ask:

 

"So, would going from 16" to 20" be a worth while move for me to observe and sketch galaxy groups in the magnitude 14-17 range?"

 

If that is the question you want answered, I think it's safe to say that if your 16 inch limits you to magnitude 15.0 galaxies, a 20 inch is not going to take you 2 magnitudes deeper.  Even doubling the aperture to 32 inches only takes you 1.5 magnitudes deeper.  

 

As far as what one can manage. With ages our eyes get weaker, are arms and backs are not as strong as they once were, these are things we have to gauge knowing ourselves.  I'm a couple of years older than you guys but still can manage to lug a 70 pound mirror box around without too much thought.  Last year, I even hoisted the 22 incher's mirror box (with mirror) up and into the rocker box that was sitting on the tail gate of my Nissan Frontier.  That's well over 100 lbs but that was too much.. On to the bed itself is one thing but into the rocker box was too much.  

 

Jon



#19 Redbetter

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Posted 15 January 2018 - 06:42 AM

Good comments in the thread. 

 

I don't have a 16" but I sure wouldn't mind having a good one (in addition to the 20") because it would simplify and speed up some of my load/unloads.  While I am willing to put in the extra time on both ends for the views I get, there is no getting around the volume/weight/bulk factors even though I bought my truck with this very scope in the plans way back when.  There would be nights/events where a hypothetical 16" would make the trip and the 20" would stay home, even though the 20" would mostly get the nod.  But, as you recognize, an extra ~0.5 mag is a substantial thing, so weighing what your comfort limits are vs. "it's getting to be a hassle" are the most important aspects; and these are personal choices. 

 

As a hypothetical:  If you think a larger scope might add 5-10 minutes to loading and 5-10 minutes to set up does the length of your observing sessions suffer considerably?  Do you already do long sessions?  Or are you more likely to observe for 2 or 3 hours?  if you lose 20 minutes out of a 2 or 3 hour session then it is much more substantial than losing 20 minutes out of 6 hours.  I tend to do long sessions, so I accept the hassle; but I freely admit that at 3 or 4am I am weighing whether or not to start loading, or to target another group of objects and observe another hour. 

 

For galaxy groups there is little doubt that the 20" will prove more satisfactory.  However, if I was using a 16" I wouldn't have to do as much "homework" making charts/lists that go deeper than Uranometria before I take the 20" hunting. 

 

A bigger concern might be which scope would you take more frequently to darker skies?  The darker skies will likely provide as much or perhaps more benefit as the extra aperture for the targets you are after.  Will hassle factor prevent you from taking the 20" to darker skies as frequently?  Or will the extra reach actually give you even more incentive to take it out more often?  This is a matter of knowing yourself and your time available and what motivates or demotivates you. 

 

Where I am in life now, the 20" is calling me to take it out to dark skies.  It is motivating me to make the most of what I have.  There was a stretch of years where that was not the case due to a variety of things unrelated to the scope and more related to "life."  I had a lull in DSO observing as a result although I still did some planetary, comets, and solar...and a few short DSO sessions with the 8" SCT on camping trips. 

 

As Jon points out: if your galaxy mag limit is 15.0 now with a 16", a 20" will likely only get you to 15.5.  That still will increase the target list greatly and improve the views of things you already see, but it isn't going to take you to 17.  If you can get to skies that are half an MPSAS darker, then you can tack that on...to either aperture.  And you might be able to improve your technique with your existing 16" in the same skies you have to take you deeper...skills that can be applied to larger apertures and/or darker skies.  I am still working on this personally, learning to match magnification level to seeing and the object and sit longer staring into the eyepiece waiting to adapt fully to see even fainter galaxies.

 

One thing I have learned to appreciate in the past year (now that I have been paying closer attention) is that most reported galaxy magnitudes past about 14 are B rather than V--and many brighter ones as well.   So what I once thought was a 16 mag galaxy is actually about 15 to 15.5.  Depressing...but it explains some things.  Keeping that in mind I am now getting down to ~16.0 V mag for galaxies in 21.5 MPSAS conditions with regularity.  I will miss this level on diffuse very low surface brightness galaxies a mag or so short, but I go a little higher on compact higher surface brightness galaxies that are near stellar.   

 

Something to keep in mind about going deeper, and something I will weigh before I go to a gargantuan scope such as a 32" which would give me another magnitude:  if you want to see a magnitude deeper, the average galaxy will still have about the same innate surface brightness, which means that the dimensions are smaller because it is farther away.  So in order to detect it reliably as non-stellar you need more magnification...which is more dependent on seeing.  Various things become incrementally more challenging to scale as you increase aperture.  Or to put it more simply:  smaller aperture is more forgiving of seeing and thermal effects.

 

P.S.  Is your avatar a zoom in of NGC 1999?  I had somehow overlooked that nebula over the years until the other night and was completely taken aback by it in the 20".  foreheadslap.gif   That three armed/vaguely phallic dark hole in the center is one of the most unusual things I have seen in a small bright emission nebula.  Contrast that with the bright star and it makes quite an impression.



#20 Illinois

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Posted 15 January 2018 - 07:00 AM

I don't know jump from 16 to 20 inch would make big different but I know that darker sky is better for faint DSO! I used 10 inch in Chicago long time ago and I able to see faint NGC 7331 and faint M110 by M31 than my 4 inch refractor! I rather have 10 inch in very dark sky than 20 inch in Chicago!



#21 Peter Natscher

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Posted 15 January 2018 - 11:52 AM

Good comments in the thread. 

 

I don't have a 16" but I sure wouldn't mind having a good one (in addition to the 20") because it would simplify and speed up some of my load/unloads.  While I am willing to put in the extra time on both ends for the views I get, there is no getting around the volume/weight/bulk factors even though I bought my truck with this very scope in the plans way back when.  There would be nights/events where a hypothetical 16" would make the trip and the 20" would stay home, even though the 20" would mostly get the nod.  But, as you recognize, an extra ~0.5 mag is a substantial thing, so weighing what your comfort limits are vs. "it's getting to be a hassle" are the most important aspects; and these are personal choices. 

 

As a hypothetical:  If you think a larger scope might add 5-10 minutes to loading and 5-10 minutes to set up does the length of your observing sessions suffer considerably?  Do you already do long sessions?  Or are you more likely to observe for 2 or 3 hours?  if you lose 20 minutes out of a 2 or 3 hour session then it is much more substantial than losing 20 minutes out of 6 hours.  I tend to do long sessions, so I accept the hassle; but I freely admit that at 3 or 4am I am weighing whether or not to start loading, or to target another group of objects and observe another hour. 

 

For galaxy groups there is little doubt that the 20" will prove more satisfactory.  However, if I was using a 16" I wouldn't have to do as much "homework" making charts/lists that go deeper than Uranometria before I take the 20" hunting. 

 

A bigger concern might be which scope would you take more frequently to darker skies?  The darker skies will likely provide as much or perhaps more benefit as the extra aperture for the targets you are after.  Will hassle factor prevent you from taking the 20" to darker skies as frequently?  Or will the extra reach actually give you even more incentive to take it out more often?  This is a matter of knowing yourself and your time available and what motivates or demotivates you. 

 

Where I am in life now, the 20" is calling me to take it out to dark skies.  It is motivating me to make the most of what I have.  There was a stretch of years where that was not the case due to a variety of things unrelated to the scope and more related to "life."  I had a lull in DSO observing as a result although I still did some planetary, comets, and solar...and a few short DSO sessions with the 8" SCT on camping trips. 

 

As Jon points out: if your galaxy mag limit is 15.0 now with a 16", a 20" will likely only get you to 15.5.  That still will increase the target list greatly and improve the views of things you already see, but it isn't going to take you to 17.  If you can get to skies that are half an MPSAS darker, then you can tack that on...to either aperture.  And you might be able to improve your technique with your existing 16" in the same skies you have to take you deeper...skills that can be applied to larger apertures and/or darker skies.  I am still working on this personally, learning to match magnification level to seeing and the object and sit longer staring into the eyepiece waiting to adapt fully to see even fainter galaxies.

 

One thing I have learned to appreciate in the past year (now that I have been paying closer attention) is that most reported galaxy magnitudes past about 14 are B rather than V--and many brighter ones as well.   So what I once thought was a 16 mag galaxy is actually about 15 to 15.5.  Depressing...but it explains some things.  Keeping that in mind I am now getting down to ~16.0 V mag for galaxies in 21.5 MPSAS conditions with regularity.  I will miss this level on diffuse very low surface brightness galaxies a mag or so short, but I go a little higher on compact higher surface brightness galaxies that are near stellar.   

 

Something to keep in mind about going deeper, and something I will weigh before I go to a gargantuan scope such as a 32" which would give me another magnitude:  if you want to see a magnitude deeper, the average galaxy will still have about the same innate surface brightness, which means that the dimensions are smaller because it is farther away.  So in order to detect it reliably as non-stellar you need more magnification...which is more dependent on seeing.  Various things become incrementally more challenging to scale as you increase aperture.  Or to put it more simply:  smaller aperture is more forgiving of seeing and thermal effects.

 

P.S.  Is your avatar a zoom in of NGC 1999?  I had somehow overlooked that nebula over the years until the other night and was completely taken aback by it in the 20".  foreheadslap.gif   That three armed/vaguely phallic dark hole in the center is one of the most unusual things I have seen in a small bright emission nebula.  Contrast that with the bright star and it makes quite an impression.

Yes, that's NGC 1999, one of my favorite wintertime objects.  In larger apertures and 400X, it's stunning.

 

I've already completed the observation, descriptions, and sketching of 15 galaxy groups in the Astronmical League's "Observe Galaxy Groups and Clusters" book.  I would like to complete the required minimum of 120 groups.  A lot of the galaxy members in this book's lists are in the mag. 15 area that I'd like to see more directly.  Seeing two or three members of a Hickson group are a real challenge from this list for the 16. For instance, from my two local and darker sites with NELM 6.2 skies, mag 15.4 galaxies appear to me with averted vision, only after staring at the exact object location for a minute or two and at 300X. Tracking is mandatory.  I'm sure 20" telescope would make these objects appear as direct vision galaxies. 



#22 Jon Isaacs

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Posted 15 January 2018 - 01:16 PM

For instance, from my two local and darker sites with NELM 6.2 skies, mag 15.4 galaxies appear to me with averted vision, only after staring at the exact object location for a minute or two and at 300X. Tracking is mandatory.  I'm sure 20" telescope would make these objects appear as direct vision galaxies.

 

 

I don't think so..  Averted vision is worth more than 0.4 magnitudes. 

 

Do 15.0 magnitude galaxies appear in direct vision in your 16 inch?? 

 

Jon



#23 Starman1

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Posted 15 January 2018 - 01:44 PM

HI Reflectors Gang!

 

Well, here I am again immersed in the thought of increasing aperture to observe deep sky objects more easily.  I currently observe with a wonderful JP Astrocraft 16" Dob that is a joy to use.  I highly recommend this telescope for its portability, quality, observing various deep sky objects.  Using the 16 the past two years, I've completed a few brighter deep sky object lists (documenting, describing, and sketching) though the objects all have been no fainter than magnitude 15.0.  The fainter objects such as member galaxies of galaxy groups all required me to use averted vision to document and sketch.  I would like to see a few more galaxies in these groups. Having observed with larger Dobs I've owned in the past and having a larger transport vehicle to transport them to dark sites in, I'm back to desiring the larger aperture but now have a smaller vehicle and less will to manage 70lbs.-100lbs. telescope pieces.  I'm thinking a 20" Dob would still fit into the current vehicle, have lighter-weight pieces, and give me a easier time spotting more of the threshold galaxies listed in my next observing project - Galaxy Groups.  These galaxies have magnitudes ranging between 14-17. The 16" starts having a hard time with this magnitude range from my mag. 6.3 sites.  If I'm lucky, I get to spot one or two galaxies of a group of 4-5. I usually observe for magnitude 6.3 sites and twice a year travel to more remote star parties with magnitude 6.8. So, would going from 16" to 20" be a worth while move for me to observe and sketch galaxy groups in the magnitude 14-17 range?

You'll gain about 0.5 magnitude.  Will this expand the range at the limit?  Yes.  Will it make visible a galaxy with direct vision that was formerly visible only with averted vision?  Probably not.

Will it make that galaxy appear a bit more detailed?  Maybe.

If you have the two scopes sitting side by side, you'd probably see that nearly everything visible in one was visible in the other, but that in the 20" all those faint galaxies are just a tad easier to see.

And that, at the very limit, a few more galaxies may be visible.

 

For example: In Burbidge's Chain near NGC247, MCG -4-3-12 is an "absolute limit" observation for my 12.5".  It's visible a small % of the time with averted vision only (at a dark site with exceptional transparency)and I could not tell you the shape or angle.  In a friend's 20" (a gain of a full magnitude), it is still an averted vision object, but it is visible a little more of the time, and the position angle is occasionally seen.  It's still not easy, and it isn't visible all the time in the 20".

 

But, a more direct answer to your question was a test 15 years ago with an 8", 12.5" and 28" scope on the galaxy group AGC426 in Perseus.

In a 1° field, the 8" saw 8 galaxies.  The 12.5" saw 18 galaxies in a 50' field.  And the 28" saw 51 in a 45' field.

It was a casual test, and it wasn't a long-term viewing with charts to push the observer for a limit observation.

The gain from the 8" to the 12.5" was a full magnitude, and the 28" was over a magnitude and a half over the 12.5", and since these galaxies were right at

the magnitude where the gain was most effective, the number of galaxies rose a lot.

Had the galaxies been fainter or brighter, the increase may not have been as profound.

 

You'll see more with a 20", but the scope will be a lot larger and harder to move.  If you're going to that trouble, why not gain a full magnitude and go with a 25"?

You can get a short f/3 25" and not need a tall ladder.



#24 Vic Menard

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Posted 15 January 2018 - 03:10 PM

I'm coming up on 20 years of observing with a 22-inch aperture f/4 Newtonian. When I purchased my first 22 (Go-To StarMaster), a sizable part of the decision process was the structure. Back then, a 20 StarMaster had the same truss tubes as a 16 or 18, but the 22 had the larger truss tubes you would find on the 24 (and later on the 30). The upshot was I wanted the structure to be robust so the collimation and DSC alignment would be less susceptible to gravitational flexure and other torque forces. In the long haul, I think it paid off. I later upgraded my StarMaster to a StarStructure 22 (also Go-To) which was similarly robust, but also built to be one-man "portable".

 

Recently, I began entertaining the idea that I could get by with a lighter, more compact 18-inch f/4 (or maybe even f/3.6 or 3.7), until I spent an evening observing with the scope I was "considering" (an 18 Zambuto/StarMaster, also Go-To).

 

While the 18 can easily hold its own on planets and "bright" stuff, my 22 simply shows more on everything else. Hicksons or Abells, extra galactic star clouds and HII regions, (easily) discernible color in planetary and diffuse nebulae, distant quasars...the list is long. And while I know that a 24 makes threshold objects like the Twin Lensing Quasar even easier, I had to draw the line somewhere (and that line is the primary mirror size that I still feel relatively comfortable handling/cleaning--I've cleaned a few 24s--my 22 is much easier).

 

Perhaps an image intensifier or something like a Mallincam would fit your observing need and allow you to keep your 16. If it's something you're considering, a test drive may help you decide. 



#25 Starman1

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Posted 15 January 2018 - 07:02 PM

I always like to ask how much time the observer gets to use his scope.

At 1000 new objects per year, it would take 30+ years to exhaust the list a 12.5" can see.

I'd guess that having 16" of aperture adds at least 10-20K objects to that list, making it take 40-50 years at 1000 new objects per year

to exhaust what a 16" can show you.

I recommend spending time learning to see objects and details at the limit until it becomes second nature.

By then, you'll no longer want to increase the aperture and you'll wonder why you ever wanted to.

I've observed for years where there are tons of scopes larger than mine, and, after having looked at the images I see in those scopes,

I have no temptation whatsoever to go to a larger size scope.

 

Do they see fainter objects?  Sure.

Do they see details in objects I can't see?  Sure.

Do the views tempt me to get a bigger scope so I can see more?  Not at all--I won't live long enough to even begin to

see all the 12.5" can see.  And when most or nearly all of the objects I'd be able to see better are 1' or smaller galaxies, or 10" or smaller planetaries, I think I'll pass.

Sure, I'm ruling out viewing so many thousands of objects, but I always wonder if what the owners of big scopes are seeking is the ability to see fainter targets with direct vision

when I have no issues whatsoever with using averted vision to see objects or details therein.

 

I do have the luxury of viewing in skies of 21.4 and darker, which many people in the US don't have without a lot of travel.

 

We all draw the line at particular sizes of scope.  Me, I'd rather my choice of car determine the scope size than the scope size determine my car.

And, I value my back lifting things in and out of the car, and even the 12.5" is getting close to the maximum I care to lift.

 

If I owned the OP's 16", I'd spend at least another decade with it before deciding I need something larger.

Improve the ability to see before getting the larger scope.

 

The exception?  A permanent observatory under dark skies.  There, the only limit is how high on the ladder you'd like to climb.

I'd build it large enough for 5 different scopes instead of one giant one, but I can understand the allure of a large aperture.




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