6" Refractor vs 10-12" Dobsonian

[neil phillips]

15 minutes ago, Mr Spock said:

It doesn't matter what anecdotal evidence you provide, a 250mm is considerably brighter than a 100mm, while black remains black.  A scope without a central obstruction can produce a better MTF curve which would favour picking out faint contrasts on such as Jupiter, but the 100 to 250 ratio is overwhelming; even with a central obstruction the 250mm has significantly greater contrast.

I have a feeling this conversation is a dead end. People can have some funny opinions based on misunderstandings of the complexities of telescopes. Damien P comes to mind with that oh so large central obstruction. There is a reason he uses that rather than a tak sharp refractor with amazing contrast and no central obstruction. Its because he likes to work out body building every chance he gets and lifting a C14 is right useful for muscle mass. Anyone believing that needs to get a hat quick. And dive under the nearest table.

Edited May 20, 2021 by neil phillips

[johninderby]

This discussion reminds me of why so many of us own both refractors and reflectors. Why limit yourself to only one type? 🤔

 

Edited May 20, 2021 by johninderby

[John]

It's worth re-visiting the original post that started this thread. The original poster asked 2 questions I think:

Question 1:

"But, I'm finding one thing I'd like to see is galaxies and other DSOs, which I've had quite limited success so far (for example I'm reasonably sure I could see M13 but it was barely a smudge). I'm uncertain what the limits of what I should expect to see through the ST102 are, but aperture seems to be king so I presume I'll need to go bigger. But how big? Will a 6" fast refractor improve things significantly, or do I need to put in another garden store and go for a 10" or 12" Dobson? I've shied away from reflectors so far for simplicity sake."

Question 2:

"The other question is what effect higher quality glass has (I am clueless) - is the main benefit for resolving more detail on planets for example and more aesthetically pleasing images, or is there an element of compensation for aperture with better light transmission?"

Both entirely reasonable and quite different questions with different answers but I think they may have got conflated at times by some of the "this verses that" discussion ?

 

 

 

Edited May 20, 2021 by John

[neil phillips]

14 minutes ago, Stu said:

As I’ve said many times before, when conditions are good, I see more detail at high power in my 8” f8 than I do it my Tak. I don’t have @mikeDnights eyes unfortunately! When conditions are poor, it can be the other way around.

The key point is actually that the scope I had with me was the 4” refractor with a light weight mount (and actually a Heritage 150p), because my car was full of other stuff and the dob wouldn’t fit, plus I expected it to cloud over so not worth taking anything bigger. It’s the scope you have and use most that’s the best. Oh, and the chap wasn’t just being nice, it was a genuine unprompted comment.

But I still recommended the 12” dob in answer to the original question! 

I quite like it under this table anyway, so I’ll stick here for a while. The hat is keeping my head warm.... 🤪🤪

From your first top line statement. And the reasons we already know that can affect outcomes of different size telescopes of different designs. I think we see eye to eye quite clearly here Stu. Like you, I am a man with one foot in both camps i understand why they are there. Not sure thats a universal understanding on here. Wallet expectation bias might have a lot to answer for lol.

[Stu]

17 minutes ago, John said:

It's worth re-visiting the original post that started this thread. The original poster asked 2 questions I think:

Question 1:

"But, I'm finding one thing I'd like to see is galaxies and other DSOs, which I've had quite limited success so far (for example I'm reasonably sure I could see M13 but it was barely a smudge). I'm uncertain what the limits of what I should expect to see through the ST102 are, but aperture seems to be king so I presume I'll need to go bigger. But how big? Will a 6" fast refractor improve things significantly, or do I need to put in another garden store and go for a 10" or 12" Dobson? I've shied away from reflectors so far for simplicity sake."

Question 2:

"The other question is what effect higher quality glass has (I am clueless) - is the main benefit for resolving more detail on planets for example and more aesthetically pleasing images, or is there an element of compensation for aperture with better light transmission?"

Both entirely reasonable and quite different questions with different answers but I think they may have got conflated at times by some of the "this verses that" discussion ?

 

 

 

Did we establish what skies are like where the OP lives John? I ask because even with a 12” scope, galaxies are fairly meh from my Bortle 7 skies. Globs are certainly better resolved in the larger aperture but if the OP has poor skies then travel considerations come into play. If ok to transport a 12” dob then that makes sense, otherwise a smaller, more portable scope would get more use.

I wouldn’t necessarily go for a faster 6” refractor for deep sky, although one of the 152mm f5.9 clones may be worth considering. Quite chunky though and would need a decent mount, so not much easier to handle than a dob.

On the second point, I think the question is whether a better quality refractor will perform better on deep sky than a cheaper achromat. I would argue yes, due to likely better coatings, less CA and SA etc. Good for widefield deep sky (as we both enjoy), but still the light grasp and resolution benefits of the 12” dob would certainly win the day in everything other than widefield.

Neil has it right, and I think Piero also said similar. It’s about the right scope for the right job. I’m largely refractor biased because of the convenience and flexibility factors, but also I enjoy the purity of the views, even if I see less than in other scopes; I’ve said before, they just make my little heart sing, and that’s what it’s all about.

If my move somewhere darker comes off, then I am much more likely to get regular use from a 12” dob for deep sky observing and will enjoy that too.

[jetstream]

On 12/05/2021 at 16:06, chrispj said:

The other question is what effect higher quality glass has (I am clueless) - is the main benefit for resolving more detail on planets for example and more aesthetically pleasing images, or is there an element of compensation for aperture with better light transmission?

@John Glad you brought the question back to the forefront.

To the Op, as long as your telescope "system" is operating at "1/4" PV you are going to see excellent detail on the moon and planets IMHO. Smooth glass means less scatter, again my opinion.

With regard to transmission, yes it matters for DSO. All the losses stack up in the telescope system and mass produced SCT are particularly vulnerable to light loss through lowered transmission.

A newt with 2 mirrors and high reflectivity will operate around 94% transmission, good refractors a bit higher, depending on the number of lenses and the quality of coatings.Mass produced SCTs about 85%-88% (Starbright).

Most mass produced dobs are at the 1/4 PV mark or better IMHO and good refractors at least this, most likely better. Seeing is usually the limiting factor for lunar/planetary and transparency/darkness limits DSO views.

Hope this adds a bit of clarity among the "robust" discussions.

[neil phillips]

2 hours ago, John said:

It's worth re-visiting the original post that started this thread. The original poster asked 2 questions I think:

Question 1:

"But, I'm finding one thing I'd like to see is galaxies and other DSOs, which I've had quite limited success so far (for example I'm reasonably sure I could see M13 but it was barely a smudge). I'm uncertain what the limits of what I should expect to see through the ST102 are, but aperture seems to be king so I presume I'll need to go bigger. But how big? Will a 6" fast refractor improve things significantly, or do I need to put in another garden store and go for a 10" or 12" Dobson? I've shied away from reflectors so far for simplicity sake."

Question 2:

"The other question is what effect higher quality glass has (I am clueless) - is the main benefit for resolving more detail on planets for example and more aesthetically pleasing images, or is there an element of compensation for aperture with better light transmission?"

Both entirely reasonable and quite different questions with different answers but I think they may have got conflated at times by some of the "this verses that" discussion ?

 

 

 

A 8" F6 Newtonian will cover most bases from both questions. DSO and planets. Its just the collimation question. Being the sticking point. If he can learn it great. It will do most of what he wants i will bet

[jetstream]

2 hours ago, neil phillips said:

Damien P comes to mind with that oh so large central obstruction. There is a reason he uses that rather than a tak sharp refractor with amazing contrast and no central obstruction.

I have to say that your lunar images are excellent, its amazing what your doing with the 114mm newt.

With respect to the large central obstruction in the SCT's used for highly successful imaging- the effects of the central obstruction and other things can be dealt with in processing, frequency restoration etc, as you know. The large aperture provides more detail than smaller ones , just the way it is. I'm not saying this for you Neil, as you already know it- I'm hoping the OP @chrispj can sort some of this out from info in the thread.

 

[neil phillips]

1 minute ago, jetstream said:

I have to say that your lunar images are excellent, its amazing what your doing with the 114mm newt.

With respect to the large central obstruction ina the SCT's used for highly successful imaging- the effects of the central obstruction and other things can be dealt with in processing, frequency restoration etc, as you know. The large aperture provides more detail than smaller ones , just the way it is. I'm not saying this for you Neil, as you already know it- I'm hoping the OP @chrispj can sort some of this out from info in the thread.

 

Think we all went a bit overboard here. Its a classic stand off debate isnt it. For the poster with my experiances i think a 8" F6 Newtonian will give him much of what he wants. Not too fast (planets) Not too slow (Deepsky) But collimation can be frustrating is my only worry

[Rusted]

At the risk of consternation from those "under the table" types..

I've always felt that those who keep their telescopes indoors.. 😱

are like runners who keep their costly "daps" in the coal bunker.

They "can't possibly" go out in them until they have had an hour of "scrubbing."

I'll get my [bullet proof] coat.

[Mr Spock]

Ah, the old debate - you say tomato, I say tomato. Except when they word is pronounced correctly both say tomato...

[michael.h.f.wilkinson]

50 minutes ago, Rusted said:

At the risk of consternation from those "under the table" types..

I've always felt that those who keep their telescopes indoors.. 😱

are like runners who keep their costly "daps" in the coal bunker.

They "can't possibly" go out in them until they have had an hour of "scrubbing."

I'll get my [bullet proof] coat.

I don't think I want my scope outside in the kind of weather we have had here recently. I do store them in a comparatively cold (but dry) garage (now mainly bike shed), so cooling time is limited.

[Rusted]

20 minutes ago, michael.h.f.wilkinson said:

I don't think I want my scope outside in the kind of weather we have had here recently.
I do store them in a comparatively cold (but dry) garage (now mainly bike shed), so cooling time is limited.

They should have a printed label with that wise recommendation on all telescopes.
Right beside the: "Do not look at the sun!" label.

Store in a cool, dry place! 

[Deadlake]

On 14/05/2021 at 11:53, chrispj said:

It's around 20.8 according to lightpollutionmap.info. If I pop up to the common behind the town I could apparently get to around 21.2.

@Stu some pages back. 
 

The other question I would ask is what kind of mount does @chrispj want to use?

For DSO’s I like to have goto (some of us don’t live under 21 SQM skies) and this helps a lot. In that case I would be tempted by a C9.25 and Mewlon, however apart from the design characteristics that @jetstream noted they also have mirror flop, which I know some people could not live with and sold them. Have spoken to people who have had both the Mewlon and C9.25, it’s make the jump to the 250 as no mirror flop and the built in fans work well and the smoother Tak mirrors give more contrast then the Celestrons for DSO work.

The other option would be 10” GSO Cassegrain, which have no mirror flop and cool quickly. The main issue is the weight. ATS uses the mirrors in there scope, I’ve attached the reflectivity report:

DatenblattCZ303-01 - Normal.pdf

GSO makes a research grade mirror as well,  but that needs to be parked up the top of a dry mountain otherwise the surface degrades in a year:

DatenblattCZ322-01 - Enhanced.pdf
 

In both cases I’d need to change my observing habits and the scope out hours before viewing with their built in fans running.

Obviously all depends on temperature gradient and time of year to how quickly if a reflector based scope acclimates in ever during the night.

[Piero]

11 hours ago, jetstream said:

@John

...

Smooth glass means less scatter, again my opinion.

...

Agreed. 

And light scattering is pretty much noise in the optical train. Effect? Star light spreads on a larger area, and faint stars can even become invisible as squashed with the background. 

In addition to that "structural" noise, there is the actual noise due to dust, each particle acting like a small scratch..... 

Edited May 21, 2021 by Piero

[chrispj]

OP back again. Thanks to all, this has been fascinating and very informative.

 

I have now dipped my toe in the world of reflectors with a GSO 10" dob, courtesy of @fwm891. So since bringing it home we have of course had 30 knot winds, 100% cloud cover and more or less continuous rain...

 

 

[dan_adi]

14 hours ago, neil phillips said:

Oh God not again. How about this for contrast and sharpness. A mass produced 15 year old Celestron power seeker F8  4.5" with pin holes in the primary. Trust me on this, when well collimated its supremely tak sharp (no pun intended ) and contrasty. It cost how much ? ? ?  I can see you under that table quite clearly

Well, put that Celestron on a real astroimaging test ;), the moon is way too easy and doesn't require much. Let's not fool our selfs, with regard to amateur astroimaging of DSOs the refractors will outperform many reflectors. They are well designed, well build, and a lot and I mean a lot more easy to work with. I tried for 2 years to work with a 12 inch SCT on a Mesu mount and the SCT ended up sitting in a corner collecting dust., no matter how much I tried to "optimize it". Try modeling a mount with a Celestron or Meade? forget about it, they simply are not built well enough. For lucky imaging of moon and planets, sure a Celestron or Meade will do because you don't need guiding or mount models or any kind of precision. If you want a good reflector that won't give you much trouble you still have to pay the big bucks .... see those 12-17 CDKs from Planewave (they are not cheap).. and even so they still need collimation from time to time, "thermal management" and all the management a reflector demands. Using a refractor demands almost 0 effort.. you have to focus before imaging and that's it. They cost much? Well that depends on the budget, but just for the "hassle factor" involved with imaging, I will gladly choose a refractor any day, even with their small aperture, keeping in mind that for long exposures the resolution will be limited by the seeing

[jetstream]

2 hours ago, chrispj said:

I have now dipped my toe in the world of reflectors with a GSO 10" dob

Congrats!

You will love the capability of this scope!

[jetstream]

3 hours ago, Piero said:

In addition to that "structural" noise, there is the actual noise due to dust, each particle acting like a small scratch..... 

Yes.

Contrary to some members beliefs I believe that a clean mirror is a must. A dirty mirror not only reduces DSO "contrast" through the eyepiece it adds scatter on bright objects reducing the quality of the view IMHO.

Some schools of thought maintain a dirty mirror is fine and brag about how long ago it was cleaned... I do not subscribe to this thought.

[Stu]

4 minutes ago, jetstream said:

Yes.

Contrary to some members beliefs I believe that a clean mirror is a must. A dirty mirror not only reduces DSO "contrast" through the eyepiece it adds scatter on bright objects reducing the quality of the view IMHO.

Some schools of thought maintain a dirty mirror is fine and brag about how long ago it was cleaned... I do not subscribe to this thought.

Have to agree with that Gerry. Providing you are cleaning the mirror carefully, it makes sense to keep it clean. A dirty mirror has to affect performance in contrast and scatter as you say.

[jetstream]

30 minutes ago, Stu said:

Have to agree with that Gerry. Providing you are cleaning the mirror carefully, it makes sense to keep it clean. A dirty mirror has to affect performance in contrast and scatter as you say.

I hadn't used my scopes much this winter but decided to do some "serious" DSO viewing in -30c. After wheeling the 15" out I noticed a bunch of dust on the primary... Previous views had me realize contrast was not up to par.

What to do for an emergency cold weather clean with 21.8 super transparent skies available?

I did a not recommended alcohol/paper towel clean. I placed paper towel on the primary (in the scope) the poured ethanol "rubbing" alcohol (99%) on the paper towel, then lifted the towels straight up and off. A few more repeats, the last lightly dragging the soaked towel across the mirror and the primary was excellently clean.

This is a risky not recommended way to clean the mirror- to all- don't do it. It did save that session that night, the alcohol evaporated, no frost and was super fast.

Again, dont do this, to all.

But a clean mirror is a must for top views IMHO, and should not be something to be scared of IMHO, if done properly as you say Stu.

Edited May 21, 2021 by jetstream

[JeremyS]

47 minutes ago, jetstream said:

I hadn't used my scopes much this winter but decided to do some "serious" DSO viewing in -30c. After wheeling the 15" out I noticed a bunch of dust on the primary... Previous views had me realize contrast was not up to par.

What to do for an emergency cold weather clean with 21.8 super transparent skies available?

I did a not recommended alcohol/paper towel clean. I placed paper towel on the primary (in the scope) the poured ethanol "rubbing" alcohol (99%) on the paper towel, then lifted the towels straight up and off. A few more repeats, the last lightly dragging the soaked towel across the mirror and the primary was excellently clean.

This is a risky not recommended way to clean the mirror- to all- don't do it. It did save that session that night, the alcohol evaporated, no frost and was super fast.

Again, dont do this, to all.

But a clean mirror is a must for top views IMHO, and should not be something to be scared of IMHO, if done properly as you say Stu.

I bet it smelled nice too 👍🏻

[Piero]

38 minutes ago, jetstream said:

Some schools of thought maintain a dirty mirror is fine and brag about how long ago it was cleaned... I do not subscribe to this thought.

 

It's kind of shocking how much that school of thought is widespread..  I won't mention the location, but some years ago I had a chance to observe through a - what I believe was - 20" F4 Newtonian on an equatorial mount in an observatory. On the primary mirror the was a large (easily 4" diameter) bird squirt - possibly dropped by a seagull. Aside from that, the whole mirror was covered by plenty of dust and dirt. By the look of it, who knows when was the last time it was washed, if ever. A few guests pointed that out, and guess what, the person responsible for the observatory simply replied that, that dirt did not affect the views. We observed Saturn that time and the view was just a bit better than my prehistoric Celestron 114mm F8 Newtonian telescope (the one mounted on that kind of hyper shaky German "EQ 0" mount). 

[jetstream]

7 minutes ago, Piero said:

We observed Saturn that time and the view was just a bit better than my prehistoric Celestron 114mm F8 Newtonian telescope (the one mounted on that kind of hyper shaky German "EQ 0" mount). 

Some mention views through reflectors are sub par- they are like anything else, they need to be cleaned and maintained. I mean how many refractor owners have a bunch of dust on the lenses?

Dust is bad- some of it is organic, mix this with dew and the mixture eats coatings...

[neil phillips]

4 hours ago, dan_adi said:

Well, put that Celestron on a real astroimaging test ;), the moon is way too easy and doesn't require much. Let's not fool our selfs, with regard to amateur astroimaging of DSOs the refractors will outperform many reflectors. They are well designed, well build, and a lot and I mean a lot more easy to work with. I tried for 2 years to work with a 12 inch SCT on a Mesu mount and the SCT ended up sitting in a corner collecting dust., no matter how much I tried to "optimize it". Try modeling a mount with a Celestron or Meade? forget about it, they simply are not built well enough. For lucky imaging of moon and planets, sure a Celestron or Meade will do because you don't need guiding or mount models or any kind of precision. If you want a good reflector that won't give you much trouble you still have to pay the big bucks .... see those 12-17 CDKs from Planewave (they are not cheap).. and even so they still need collimation from time to time, "thermal management" and all the management a reflector demands. Using a refractor demands almost 0 effort.. you have to focus before imaging and that's it. They cost much? Well that depends on the budget, but just for the "hassle factor" involved with imaging, I will gladly choose a refractor any day, even with their small aperture, keeping in mind that for long exposures the resolution will be limited by the seeing

Think i have enough experiance to determine if a telescope has sharp optics or not. God knows ive had some lemons go through my hands in my time. All determined visually live. No faint deepsky imaging was required. I recently sent back a 90mm SW refractor F10 Achromatic.. Because i could tell the optics were not great. Contrary to you suggesting the moon is too easy. It was infact the perfect target, to evaluate how sharp this telescope is coming to focus. In this instance i kept trying to find a position of excellent focus. Only to find myself never really achieving it. The images later were not inspiring to say the least. I could determine how much seeing was affecting the outcome. The very next night i tried the small reflector. Under again similar seeing conditions. In around twenty seconds i could see how much sharper the optics were. With tiny little craterlets the size of a pin head revealing themselves very quickly while focusing. Deepsky imaging may well be a good tool to evaluate optics. I assure you i have enough experiance to be able to see how sharp optics are visually live on the moon. Trust me on that or dont. Next. Requirements for deepsky as you say are very different to lunar and planetary. To suggest lunar and planetary imagers can not evaluate how sharp optics are, without doing deepsky imaging is a bit presumttuous. Think we should leave it there. I have no doubt you are finding the best scopes for your art. Hopefully i can be given the same courtesy. I was Blessed many years ago to get one of the best Newtonians to go through my hands a 1/10th wave Orion optics F6.3 

Edited May 21, 2021 by neil phillips