8" and 14" Telescopes not much difference?

[MarsG76]

Hi All,

After observing a few times with my new 14" Skywatcher Dob, I have a few questions and observation dillemas that I'm hoping someone can answer.

Firstly, exactly how much difference is there supposed to be between a 8" SCT and a 14" Dob when observing DSOs and Planets?

I find that on planets sure they are brighter than in the 8" SCT but as far as detail is concerned, I don't see that much difference in details between the two, namely Jupiter and Saturn. In both cases the amount of detail in both is about the same except like I stated, they're brighter in the 14".

Most recently with Saturn, on it I saw cloud bands, Cassini division and 6 moons around it, in the 8" SCT similar situation, except dimmer slightly and one less moon. Also in actual fact I find that regularly I can magnify higher with the 8" over the 14" where the view is still crisp.

The kind of nights I'm comparing to is night with good seeing, where basically I can magnify Saturn and Jupiter 406X and they look stable and detailed, but in the 14" getting to that magnification starts to go softer??!?!? I found that the sweet spot for the Dob is about 330X on a clear night where the SCT seems to be good at 406X.

I though that the 14" would show more detail and allow me to magnify to 500-600X where I might be able to pickup even the Encke Gap in Saturn's rings... or am I mistaken?

 

With DSOs, similar story, the objects are slightly brighter, but not 3X brighter when comparing mirrors where the 14" has 3X the light gathering compared to the 8".

Orion nebula looks nearly the same, Sombraro Galaxy looks very similar, the Tarantula Nebula and Omega Centauri looked brighter, but Centaurus A looks about the same... I was expecting a lot brighter DSOs with a 14" telescope compared to 8", or am I looking at this the wrong way?

The skies at my location are darker than average, and I'm comparing both scopes at the same conditions.

 

Imaging is even more of a difference, I attempted to image Jupiter and Saturn using both scopes on the same night, the 8" SCT was crisp and detailed on both Jupiter and Saturn but the 14" Dob image was soft and without details... surely the 14" should run rings around the 8"?

Does anyone have any words of wisdom for the similarities?

 

Thanks in advance.

 

[Stu]

Can we assume the dob has been properly collimated and cooled/stabilised?

Providing the seeing is excellent, a large dob should give significantly higher resolution views of planets than a smaller SCT. I've seen this myself when comparing an 8" Mak with a 16" dob on Jupiter. Even in quite poor conditions the dob was much better.

The curious thing about aperture with nebulae is that it does not actually increase the surface brightness of the object you are looking at. It can increase the size whilst maintaining the same surface brightness as a smaller scope at smaller size, that's all.

Being able to do this does allow you to see fainter objects such as small galaxies as you can magnify them more which helps your eye detect the contrast, but the SB can never get higher than with the naked eye.

Check out the collimation and have another go!

[Chris]

I've never owned a scope bigger than 8" so I can't really give a valid opinion, but I'll be following with interest to see that the general consensus is. As Stu mentions - collimation and cooling do spring do mind, along with optical figure which would be more tricky to check.

I feel for you that the extra aperture hasn't paid off so far, it's probably worth given it more time and experimenting a bit more, good luck!     

[Chris]

One other thing that springs to mind is contrast due to the truss design of the 14" SW Dob. Does the Dob have a shroud? 

[spaceboy]

I agree with Stu. You need to check collimation and allow a lifetime for the larger mirrors to cool. You also need to consider the SCT is a closed tube design so will often allow for a steadier view. Also consider that the smaller 8" is not going to magnify the atmosphere in the same way a larger scope is. What may come across as good seeing in the SCT may not necessarily in the larger 14" is what I mean.

It is my understanding that the larger you go the more light the mirror gathers and the brighter the object appears to the eye. As Stu points out though the eye is always going to be the weak link regards SB. I know a lot of the dob mob prefer larger scopes for the larger image scale but personally this was the one thing that put me off owning a larger scope. Each to their own I guess.

There is no denying that looking through a big dob is an experience. M15 through a 16" scope was almost blinding the globular was so bright, and M3 through a 20" looked like an open cluster it filled the view of a 100° eyepiece so much.

I think we set ourselves up with expectations not just as beginners but as seasoned astronomers. We forget that most improvements are only going to be subtle and we have to take time and patience to absorb details.

[Stu]

I think what you say is right Nick. The bigger globs through a big dob are just amazing, resolved to the core, Big and bright because the stars are point sources and don't dim in the same way that an extended object does.

My best experience with a large dob was my 16" at Lucksall. Seeing things like the Markarian's chain and a host of other lovely galaxies showed me just what a big dob can do. With a bigger mirror under a properly dark sky I can see the appeal. One of my bigger regrets are that the skies at the recent Peak Star Party were not better. I had a look through Damon's scope at some favourites including M51. It was lovely, with clear spirals but it obviously had so much more to give under better conditions it was almost disappointing if that makes sense? Nothing to do with the scope, just the rubbish transparency.

You still end up 'working hard' to see objects with a dob because you end up looking for fainter and fainter stuff.

[ollypenrice]

An interesting post. I must say that I don't use our 20 inch Dob as instrument of choice on the planets. It doesn't have such well figured optics as our smaller scopes and these usually win out on planets. (Colour on Jupiter is excellent in the big Dob, though.) Although I've never done so myself, some Dob users stop down their large mirrors with aperture masks for planetary observing.  One theory says that the seeing is affected by 'cells' of stability and that the aperture should not exceed the size of these cells. I can't comment on how accurate this idea is.

What hasn't come up is the matter of exit pupil. This might be the key to the OP's disappointment with his large Dob on extended targets. (To calculate exit pupil divide the EP focal length by the F ratio of the scope.) It is very easy, with fast scopes, to end up with an overly large exit pupil. When we used a 35mm Panoptic in the F4.1 Dob we had an exit pupil of 8.5mm, far too large for most people's pupils to pass. Switching to a 26mm Nagler brought the exit pupil down to 6.3mm which is still on the large side but far better. There's a discussion on optimal exit pupils here: https://www.cloudynights.com/topic/529169-ideal-exit-pupil-size/

There is surely nothing wrong with open truss designs. All big professional telescopes use them. They aid cooldown and prevent tube currents. A shroud eliminates stray light but, to be quite honest, I struggle to see any difference between 'shroud up' and 'shroud down' on ours. More dedicated visual observers do say they see a difference.

So I suspect this is all about exit pupil.

Olly

[ronin]

I can understand the title to an extent, I think that people interpret the use of the word "more" differently.

I would say a 14" newtonian will show more detail on say M42, depends on the overall quality of the mirror. That detail lbeing dependant on the gathered light.

Now a 14" newtonioan pointed at the galaxy cluster at the rear end of Leo will show more galaxies but they will remain small disks, is seeing 100 small disks in an 8" worse or not or no different to 150 small disks in a 14". So more numerically but not more in detail.

On planets I would half expect the 8" SCT to be likely better. A big mirror needs a lot of work done to be good and I would expect that figuring errors play a part. Will say I have yet to have a good planetary view through an SCT, either 8" or 14", have viewed through both.

The "advantage" of a 14" over an 8" is I suggest fundimentally that it collects more light, after that what the scope does with it and handles it comes into play and the newtonian and SCT are different scopes.

 

[Stu]

19 minutes ago, ollypenrice said:

Although I've never done so myself, some Dob users stop down their large mirrors with aperture masks for planetary observing.

I've seen this in action on Shane's 16" Olly, and it works very well. It gives a 170mm clear aperture scope without diffraction spikes which is very nice. Not the most compact way of achieving this but very effective.

Providing the optics are good, I think the full aperture will always give higher resolution when the seeing steadies, but you have to wait much longer for those steady moments if the seeing is variable. Put another way, the stopped down dob provides images which are a lower resolution but much steadier under variable conditions.

[Stu]

1 minute ago, ronin said:

Now a 14" newtonioan pointed at the galaxy cluster at the rear end of Leo will show more galaxies but they will remain small disks, is seeing 100 small disks in an 8" worse or not or no different to 150 small disks in a 14". So more numerically but not more in detail.

Ronin, that is just not correct. At the same magnification, the the larger scope will have a larger exit pupil and will show more galaxies in the same field. The real benefit comes when you want to see more detail. The larger scope will show the same galaxies at a larger image scale whilst retaining their surface brightness, your eye is better able to see low contrast features when they are larger so you will see more detail.

[spaceboy]

21 minutes ago, ollypenrice said:

What hasn't come up is the matter of exit pupil. This might be the key to the OP's disappointment with his large Dob on extended targets.

So I suspect this is all about exit pupil.

Olly

Given the OP no doubt has a selection of longer FL ep's for use in an SCT a valid point Olly but I suspect given the magnifications quoted in the opening post of x406-x600 the exit pupil will be small enough for even the mature observer ???

[MarsG76]

1 hour ago, Stu said:

Can we assume the dob has been properly collimated and cooled/stabilised?

Providing the seeing is excellent, a large dob should give significantly higher resolution views of planets than a smaller SCT. I've seen this myself when comparing an 8" Mak with a 16" dob on Jupiter. Even in quite poor conditions the dob was much better.

The curious thing about aperture with nebulae is that it does not actually increase the surface brightness of the object you are looking at. It can increase the size whilst maintaining the same surface brightness as a smaller scope at smaller size, that's all.

Being able to do this does allow you to see fainter objects such as small galaxies as you can magnify them more which helps your eye detect the contrast, but the SB can never get higher than with the naked eye.

Check out the collimation and have another go!

Yeah, collimation is the first thing I thought off and to be honest, I'm not 100% certain that I nailed it, perhaps close but not spot on.. and as far as cooling is concerned, I had the scope outside for the last few day so I dare say that the mirror temperature is ambient.

 

[MarsG76]

1 hour ago, Lockie said:

One other thing that springs to mind is contrast due to the truss design of the 14" SW Dob. Does the Dob have a shroud? 

Yeah, the dobsonian is shrouded.

 

[ollypenrice]

4 minutes ago, spaceboy said:

Given the OP no doubt has a selection of longer FL ep's for use in an SCT a valid point Olly but I suspect given the magnifications quoted in the opening post of x406-x600 the exit pupil will be small enough for even the mature observer ???

True on the planets but surely the OP doesn't use these powers on the deep sky objects. I'm not overwhelmingly surprised by the 8 inch beating the 14 inch on the planets but I'm very surprised by 14 not beating the 8 on the faint fuzzies.

Olly

[Stu]

Just now, MarsG76 said:

Yeah, collimation is the first thing I thought off and to be honest, I'm not 100% certain that I nailed it, perhaps close but not spot on.. and as far as cooling is concerned, I had the scope outside for the last few day so I dare say that the mirror temperature is ambient.

 

Perhaps, in that case, just try on a few more occasions to get a mix of conditions. Nick maybe right in that seeing which looks ok in the SCT may still be too bad for the dob at high power.

[ollypenrice]

Excellent collimation will matter on the planets but, at the risk of excommunication, I'd say it would matter less on the deep sky. It's the failure of the 14 to impress on the deep sky which seems a puzzle.

Olly

[spaceboy]

5 minutes ago, ollypenrice said:

True on the planets but surely the OP doesn't use these powers on the deep sky objects. I'm not overwhelmingly surprised by the 8 inch beating the 14 inch on the planets but I'm very surprised by 14 not beating in on the faint fuzzies.

Olly

Yes sorry for doubting you Olly. I think I read your post all wrong

Just checked on http://astronomy.tools/calculators/field_of_view/ and even in a large dob those are some pretty short FL ep's (small exit pupils) to reach those kinds of magnifications. As you so rightly say too small an exit pupil if anything for deep sky.

[MarsG76]

1 hour ago, spaceboy said:

I agree with Stu. You need to check collimation and allow a lifetime for the larger mirrors to cool. You also need to consider the SCT is a closed tube design so will often allow for a steadier view. Also consider that the smaller 8" is not going to magnify the atmosphere in the same way a larger scope is. What may come across as good seeing in the SCT may not necessarily in the larger 14" is what I mean.

It is my understanding that the larger you go the more light the mirror gathers and the brighter the object appears to the eye. As Stu points out though the eye is always going to be the weak link regards SB. I know a lot of the dob mob prefer larger scopes for the larger image scale but personally this was the one thing that put me off owning a larger scope. Each to their own I guess.

There is no denying that looking through a big dob is an experience. M15 through a 16" scope was almost blinding the globular was so bright, and M3 through a 20" looked like an open cluster it filled the view of a 100° eyepiece so much.

I think we set ourselves up with expectations not just as beginners but as seasoned astronomers. We forget that most improvements are only going to be subtle and we have to take time and patience to absorb details.

 

52 minutes ago, Stu said:

I think what you say is right Nick. The bigger globs through a big dob are just amazing, resolved to the core, Big and bright because the stars are point sources and don't dim in the same way that an extended object does.

My best experience with a large dob was my 16" at Lucksall. Seeing things like the Markarian's chain and a host of other lovely galaxies showed me just what a big dob can do. With a bigger mirror under a properly dark sky I can see the appeal. One of my bigger regrets are that the skies at the recent Peak Star Party were not better. I had a look through Damon's scope at some favourites including M51. It was lovely, with clear spirals but it obviously had so much more to give under better conditions it was almost disappointing if that makes sense? Nothing to do with the scope, just the rubbish transparency.

You still end up 'working hard' to see objects with a dob because you end up looking for fainter and fainter stuff.

 

31 minutes ago, ollypenrice said:

An interesting post. I must say that I don't use our 20 inch Dob as instrument of choice on the planets. It doesn't have such well figured optics as our smaller scopes and these usually win out on planets. (Colour on Jupiter is excellent in the big Dob, though.) Although I've never done so myself, some Dob users stop down their large mirrors with aperture masks for planetary observing.  One theory says that the seeing is affected by 'cells' of stability and that the aperture should not exceed the size of these cells. I can't comment on how accurate this idea is.

What hasn't come up is the matter of exit pupil. This might be the key to the OP's disappointment with his large Dob on extended targets. (To calculate exit pupil divide the EP focal length by the F ratio of the scope.) It is very easy, with fast scopes, to end up with an overly large exit pupil. When we used a 35mm Panoptic in the F4.1 Dob we had an exit pupil of 8.5mm, far too large for most people's pupils to pass. Switching to a 26mm Nagler brought the exit pupil down to 6.3mm which is still on the large side but far better. There's a discussion on optimal exit pupils here: https://www.cloudynights.com/topic/529169-ideal-exit-pupil-size/

There is surely nothing wrong with open truss designs. All big professional telescopes use them. They aid cooldown and prevent tube currents. A shroud eliminates stray light but, to be quite honest, I struggle to see any difference between 'shroud up' and 'shroud down' on ours. More dedicated visual observers do say they see a difference.

So I suspect this is all about exit pupil.

Olly

 

12 minutes ago, ronin said:

I can understand the title to an extent, I think that people interpret the use of the word "more" differently.

I would say a 14" newtonian will show more detail on say M42, depends on the overall quality of the mirror. That detail lbeing dependant on the gathered light.

Now a 14" newtonioan pointed at the galaxy cluster at the rear end of Leo will show more galaxies but they will remain small disks, is seeing 100 small disks in an 8" worse or not or no different to 150 small disks in a 14". So more numerically but not more in detail.

On planets I would half expect the 8" SCT to be likely better. A big mirror needs a lot of work done to be good and I would expect that figuring errors play a part. Will say I have yet to have a good planetary view through an SCT, either 8" or 14", have viewed through both.

The "advantage" of a 14" over an 8" is I suggest fundimentally that it collects more light, after that what the scope does with it and handles it comes into play and the newtonian and SCT are different scopes.

 

Some very good posts there. I might be having the wrong idea about what to expect with a bigger mirror...

the exit pupil I'm using is 6.7mm at most, the 31mm Nagler...

 

At this stage I guess more practice and collimation...

 

 

[niallk]

I can only comment a little on a 15" vs 10" - that's a factor of 2.25x light gathering compared to a 14" vs 8" which is an bigger 3x increase.

Globs, star clusters are dramatically improved: real 'wow' factor territory.  Jupiter at 200x or so is similar: brighter in the 15" with a little higher resolution. I had my 10" for 8 years, and the 15" for 2yrs: I routinely use higher mag with my 15" observing at 330x, and sometimes higher on rare really stable nights.  On Saturn, I see more shading on the planet, and haven't failed to see the Cassini division with the 15", and also see the A and B rings as distinctly different colours, and have seen the C ring.  Saturn is brighter at higher mag in the 15".  Now Saturn has been lower altitude but with the rings far more open since I got the 15", and I haven't done a side by side comparison on Saturn like I have done on Jupiter (I have to travel to get a low enough horizon!).  I've also had my best views of Mars with the 15".

I like to observe planetary nebs, and here I also find that the 15" is a dramatic enhancement over the 10": I find that I observe at higher mag while preserving sufficient brightness - ie larger image scale. At the same mag, I detect hints of greeny blue colour more in nebs.  Things are brighter when using filters at higher poweraiding teasing out more detail.

Galaxies are a favourite type of target for me, and my first light with the 15" wowed me: I looked at M81/82 (ok not a faint targets to push limits ) but M81 was just huge with the extended arms, and the detail discernable in M82 was great and I could up the mag on its 'core'.  I've finally seen spiral arms in  A M51 from my back garden. Ifind success observing galaxies to be highly variable and sensitive to sky conditions - transparency and how the crud in the atmosphere is catching the light pollution.

There's no doubt for me: I love the increased aperture.  The 15" takes a little more effort to set up/ tear down, and I give my mirror >2hrs with a fan to cool compared to 45mins for the 10" if observing at high powers.  90% of the time, I choose the 15"!!

 

[Stu]

Great post Niall  

[ollypenrice]

Rather than thinking in terms of magnification when doing deep sky in the 14, it might be worth experimenting with different exit pupils. I'd prepare a few EPs in advance, giving you a range to try.  If you can try some around 3mm and either side of that it might be instructive.

We've just acquired a 14 inch F10 SCT and I've been a little concerned by the tiny exit pupils it gives, 2.6mm being the largest we can do with present EPs. However, these seem to give excellent views. A 1.3mm exit pupil on M51 was thrilling.

Olly

[MarsG76]

17 minutes ago, ollypenrice said:

True on the planets but surely the OP doesn't use these powers on the deep sky objects. I'm not overwhelmingly surprised by the 8 inch beating the 14 inch on the planets but I'm very surprised by 14 not beating the 8 on the faint fuzzies.

Olly

Same, I'm surprised also...

For observations of deep sky objects, 99% of the time I use the 31mm Nagler and a 17mm Ethos, with and without a 2X power mate... so magnification is quite modest on DSOs.

[ollypenrice]

3 minutes ago, MarsG76 said:

Same, I'm surprised also...

For observations of deep sky objects, 99% of the time I use the 31mm Nagler and a 17mm Ethos, with and without a 2X power mate... so magnification is quite modest on DSOs.

Very similar to my default DS eyepieces, a 13 Ethos and 26 Nagler. They do 99% of the work here, as well.

Olly

[Stu]

What a great thread this is

[ollypenrice]

Just now, Stu said:

What a great thread this is

Indeed. I think we're all rooting for the OP to gt that 14 inch to sing!

lly