8" and 14" Telescopes not much difference?

[Stu]

Just now, ollypenrice said:

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

lly

Agreed!  

Will be good to see future results, I'm sure the 14 will prove its worth!

[MarsG76]

Thank you everyone for the answers... I'm definitely missing something... and all of you are very helpful.

One thing that I do notice on the 14" dob is that I can definitely magnify more on DSO with no (or very little) loss of brightness... on the tarantula nebula when magnifying on the SCT and the Dob using the 31mm Nagler it looks very similar on both, but going further in the dob does keep the brightness about the same, where as the 8" dims the view when using the 17mm and Barlow... same with Carina, I can get in closer with the dob to nearly see the structure where the "mountains of creation are" where 200X in the 8" does become quite a bit fainter...

The Trifid nebula brightness and detail was about the same, although the 14" showed a bit of the "blue" part nebula glow where the 8" didn't so the 14" is a little bit better on DSO.

The main thing that concerned me most is when I was imaging Jupiter and Saturn, just how much better the images came out using he 8" SCT...

What I need to do, tomorrow if weather holds up, is set up both scopes again and do a detailed night of back to back comparisons.

With practice, I'm sure that it'll eventually be clear in what I'm doing wrong....

[Scooot]

The 8se is f10 and the Dob much faster isn't it, is it f4.6. So as well as mirror quality differences and collimation to consider there might also be some coma affecting the view on the Dob. When I use my 26mm nagler without the paracorr on either my dobs, the stars deteriorate very quickly as I look away from the centre. 

[Chris]

1 hour ago, ollypenrice said:

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

From trial and error I've found a 2mm exit pupil to be optimal for DSO's and my skies (I'm guessing due to the high contrast, all light entering eye, and large enough exit pupil to avoid floaters), but I shall be reading the thread you linked regarding exit pupil to see if this adds up. 

[Stu]

24 minutes ago, Lockie said:

From trial and error I've found a 2mm exit pupil to be optimal for DSO's and my skies (I'm guessing due to the high contrast, all light entering eye, and large enough exit pupil to avoid floaters), but I shall be reading the thread you linked regarding exit pupil to see if this adds up. 

2mm is supposed to be the optimum for DSO observing Chris so your findings seem to be in line with that  

[niallk]

For my 10", I used 3 EPs + a PM.  For my 15", I'm up to 5 EPs now + PM. I use them all, and really like the range of exit pupils available from 6mm to 0.75mm: all depends on the target, and conditions.  Not aligned with a minimal set approach I know, but hey - each to their own - I will set my 15" up just to observe the moon! I'll look at anything and everything I can

[Chris]

23 minutes ago, Stu said:

2mm is supposed to be the optimum for DSO observing Chris so your findings seem to be in line with that  

The best ever DSO observing experience I've had was with an Edge HD 8" SCT plus ES100 20mm at 2mm exit pupil....fond memories   

[ollypenrice]

Remember, also, that the SCT is the planetary imager's weapon of choice.

Olly

[mikeDnight]

Skywatcher reflectors don't all have a high level of optical figure, with many showing a turned edge along with zones and varying degrees of surface roughness. They are made to a price which offers large apertures capable of gathering a significant amount of light for studying faint dso's. Their lack of precision figure will have a detrimental effect on the telescopes resolving power and so for planetary study they will be somewhat limited. The SCT by its design demands the use of precisely figured primary and secondary mirrors, along with a precisely matched corrector plate for the system to work, making the SCT a much better choice as a planetary scope. 

[John]

2 hours ago, mikeDnight said:

Skywatcher reflectors don't all have a high level of optical figure, with many showing a turned edge along with zones and varying degrees of surface roughness. They are made to a price which offers large apertures capable of gathering a significant amount of light for studying faint dso's. Their lack of precision figure will have a detrimental effect on the telescopes resolving power and so for planetary study they will be somewhat limited. The SCT by its design demands the use of precisely figured primary and secondary mirrors, along with a precisely matched corrector plate for the system to work, making the SCT a much better choice as a planetary scope. 

Blimey Mike - thats just not how I've found things when I've owned SCT's and Skywatcher newts. Maybe I have newtonian favouring eyes !

Still there we go - it would be odd if we all saw things the same way

[Ricochet]

I don't know about a turned edge because that is a term usually applied to poor figuring, but there does tend to be a bevel around the mirror edge that I would suggest masking with a baffle mounted to the mirror clips. If the parabolic surface of the mirror face went all the way to the vertical sides then you would have quite a sharp and fragile edge that I imagine would be prone to damage. 

[mikeDnight]

28 minutes ago, John said:

Blimey Mike - thats just not how I've found things when I've owned SCT's and Skywatcher newts. Maybe I have newtonian favouring eyes !

Still there we go - it would be odd if we all saw things the same way

I agree with you John, at least here in the UK. I've almost always been disappointed with SCT's no matter what the aperture and would personally prefer a SW Newtonian  anywhere between 6" and 10". However the OP lives in Australia and I can only assume the atmospheric conditions are better than ours. I know Peter Drew loves his SCT in Tenerife and sees amazing things through it, but he's reluctant to bring it back to the UK because of the seeing and thermal issues. I've only once seen a SCT work really well, it was an old orange C8, and the view it gave on just one night was truly breath taking.

I'd like to say that I've seen some very good views through SW reflectors and that they are not poor telescopes by any stretch of the imagination. 

[Chris]

3 hours ago, mikeDnight said:

Skywatcher reflectors don't all have a high level of optical figure, with many showing a turned edge along with zones and varying degrees of surface roughness. They are made to a price which offers large apertures capable of gathering a significant amount of light for studying faint dso's. Their lack of precision figure will have a detrimental effect on the telescopes resolving power and so for planetary study they will be somewhat limited. The SCT by its design demands the use of precisely figured primary and secondary mirrors, along with a precisely matched corrector plate for the system to work, making the SCT a much better choice as a planetary scope. 

I see what you're saying Mike, 'most' of my 8" SCT's have done well on planets for me at least (lots of image scale with plenty of detail once cooled) I've only found 8" f/5 Newts to be ok on planets (faint spikes and glare (but not from the disc being overly bright?) making detail trickier to see, so a bit less rewarding. When I tried viewing Jupiter with an f/4 Newt I found it especially un-rewarding but to be fair it was designed for DSO imaging so had rather a large central obstruction (but then again so do SCT's?). I honestly don't remember viewing planets with the 8" f/6 Dob I had, but remember DSO's being nice with it.

 Then in steps my current 6" f/8 Dob ! ! what a difference, the first Synta Newt to really impress me on planets! (very sharp detail, good contrast, no weird fuzz/glare, but still the faint spikes off course which I admit I'm trying my best to get used to. 

Adding to the above a direct comparison between my f/5 and f/8 6" Newts, on the same night at the same time with both scopes collimated the best I could, the f/8 was noticeably better on Jupiter! 

Sooo, analysing the above  - I think either my collimation tools and my collimation skills are not doing it for me, or Synta's fast production rate is favouring the slower scopes they make due to it being much easier to accurately figure a slow lens or mirror than a fast one. Maybe both of the above.

SCT's have a fast primary but it's spherical so maybe it's the figuring of the parabola that's key with the Synta Newts? 

One things for sure, the long and relatively expensive process of making my own mirror has given me the up most respect for Synta   How they manage to produce so many scopes to the standard they do, and at the price they do is beyond me! I'm still at the rough/medium grinding stage with my mirror, but I've already put so many hours and splurges of cash into it! Worth it though!       

In summary- For planets, give me an SCT over a Synta F/5 Newt, and maybe give me a Synta f/8 Newt over an SCT    

[MarsG76]

On 7/23/2017 at 18:12, ollypenrice said:

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

I had a bit of a comparison between the 8" and the 14" on a couple of objects, and the 14" was picking up more of the faint fuzzies, like I mentioned before that it doesnt seem like it's 3X as bright but nonetheless on the Helix nebula there was definitely more visible, namely the 8" showed it as a very very hard to see with averted vision haze patch like something there, but through the 14" the actual ring was faintly visible with the top and bottom part of the ring being slightly brighter, as well as picking up the central pulsar/neutron star with averted vision that was not even a hope for through the 8", perhaps that is the 3X level but I have the wrong impression/expectation. That night was also particularly bright with skyglow, no moon and I was hidden away from lights but the sky was glowing ridiculously. Still I was happy to see the Helix and a few of the stars around it that I recognized from the image I took a while back.

 

On 7/23/2017 at 18:08, Stu said:

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.

That night I spent some time collimating the mirrors to as accurately as I could, ie, the secondary was a central circle, using a white paper behind it to make it obvious when it is not in the middle slotted in behind the secondary from the eyepiece view direction, than I used a chesire to align the primary, and fine tuned on a star and on Saturn.

Saturn detail looked exactly the same as in the SCT, details yes, but the 14" had a brighter view, nonetheless, no more extra details were visible... mind you tho, the seeing was not the best during the night, and I did see a crisper Saturn at different observing sessions in the past through both scopes, again, I was expecting to be able to magnify Saturn fair amount more, with retaining the clarity and perhaps pickup more detail like the Encke Gap....

I'm thinking that the seeing conditions are more critical on the 14" mirror as mentioned previously. 

I'm very determined to solve this...

[MarsG76]

On 7/23/2017 at 18:34, Stu said:

Agreed!  

Will be good to see future results, I'm sure the 14 will prove its worth!

Yup definitely developing some kind of an obsession to solve this dilemma.

 

[MarsG76]

On 7/23/2017 at 20:26, Stu said:

2mm is supposed to be the optimum for DSO observing Chris so your findings seem to be in line with that  

 

On 7/23/2017 at 20:01, Lockie said:

From trial and error I've found a 2mm exit pupil to be optimal for DSO's and my skies (I'm guessing due to the high contrast, all light entering eye, and large enough exit pupil to avoid floaters), but I shall be reading the thread you linked regarding exit pupil to see if this adds up. 

 

On 7/23/2017 at 20:54, Lockie said:

The best ever DSO observing experience I've had was with an Edge HD 8" SCT plus ES100 20mm at 2mm exit pupil....fond memories   

The exit pupils I tend to use on the 14" Dob are 6.7mm with the 31mm hagler and 3.7mm with the 17mm Ethos, 3.35mm and 1.85mm respectively with the 2X Powermate... so apart from the 6.7mm exitpupil generated by the 31mm Nagler, I seem to be in the ball park.

 

[MarsG76]

On 7/23/2017 at 21:36, ollypenrice said:

Remember, also, that the SCT is the planetary imager's weapon of choice.

Olly

Definitely seems to be that way, since the best planetary images I have seen are made by big SCTs.

 

On 7/24/2017 at 01:19, Lockie said:

I see what you're saying Mike, 'most' of my 8" SCT's have done well on planets for me at least (lots of image scale with plenty of detail once cooled) I've only found 8" f/5 Newts to be ok on planets (faint spikes and glare (but not from the disc being overly bright?) making detail trickier to see, so a bit less rewarding. When I tried viewing Jupiter with an f/4 Newt I found it especially un-rewarding but to be fair it was designed for DSO imaging so had rather a large central obstruction (but then again so do SCT's?). I honestly don't remember viewing planets with the 8" f/6 Dob I had, but remember DSO's being nice with it.

 Then in steps my current 6" f/8 Dob ! ! what a difference, the first Synta Newt to really impress me on planets! (very sharp detail, good contrast, no weird fuzz/glare, but still the faint spikes off course which I admit I'm trying my best to get used to. 

Adding to the above a direct comparison between my f/5 and f/8 6" Newts, on the same night at the same time with both scopes collimated the best I could, the f/8 was noticeably better on Jupiter! 

Sooo, analysing the above  - I think either my collimation tools and my collimation skills are not doing it for me, or Synta's fast production rate is favouring the slower scopes they make due to it being much easier to accurately figure a slow lens or mirror than a fast one. Maybe both of the above.

SCT's have a fast primary but it's spherical so maybe it's the figuring of the parabola that's key with the Synta Newts? 

One things for sure, the long and relatively expensive process of making my own mirror has given me the up most respect for Synta   How they manage to produce so many scopes to the standard they do, and at the price they do is beyond me! I'm still at the rough/medium grinding stage with my mirror, but I've already put so many hours and splurges of cash into it! Worth it though!       

In summary- For planets, give me an SCT over a Synta F/5 Newt, and maybe give me a Synta f/8 Newt over an SCT    

Perhaps the difference if that slower mirrors have a much bigger sweet spot, ie error tolerance, when it comes to collimating, so the f8 or faster scopes seem to hold up better on high power applications like observing planetary detail.

 

[MarsG76]

On 7/23/2017 at 21:51, mikeDnight said:

Skywatcher reflectors don't all have a high level of optical figure, with many showing a turned edge along with zones and varying degrees of surface roughness. They are made to a price which offers large apertures capable of gathering a significant amount of light for studying faint dso's. Their lack of precision figure will have a detrimental effect on the telescopes resolving power and so for planetary study they will be somewhat limited. The SCT by its design demands the use of precisely figured primary and secondary mirrors, along with a precisely matched corrector plate for the system to work, making the SCT a much better choice as a planetary scope. 

 

On 7/24/2017 at 00:30, John said:

Blimey Mike - thats just not how I've found things when I've owned SCT's and Skywatcher newts. Maybe I have newtonian favouring eyes !

Still there we go - it would be odd if we all saw things the same way

 

On 7/24/2017 at 01:07, Ricochet said:

I don't know about a turned edge because that is a term usually applied to poor figuring, but there does tend to be a bevel around the mirror edge that I would suggest masking with a baffle mounted to the mirror clips. If the parabolic surface of the mirror face went all the way to the vertical sides then you would have quite a sharp and fragile edge that I imagine would be prone to damage. 

 

On 7/24/2017 at 01:13, mikeDnight said:

I agree with you John, at least here in the UK. I've almost always been disappointed with SCT's no matter what the aperture and would personally prefer a SW Newtonian  anywhere between 6" and 10". However the OP lives in Australia and I can only assume the atmospheric conditions are better than ours. I know Peter Drew loves his SCT in Tenerife and sees amazing things through it, but he's reluctant to bring it back to the UK because of the seeing and thermal issues. I've only once seen a SCT work really well, it was an old orange C8, and the view it gave on just one night was truly breath taking.

I'd like to say that I've seen some very good views through SW reflectors and that they are not poor telescopes by any stretch of the imagination. 

I'm still going to hold on to the fact that perhaps my collimation is to blame or that seeing is more crucial to a larger mirror compared to the 8" and that eventually the problem will be solved... and it's just that I just happened to be observing during the less than perfect seeing conditions. The times I had perfect skies in February and May, when everything was still and sky was black, I probably didn't have the mirrors lined up to what I believe they are now since the last observing outing and so at above 330X Saturn was starting to soften up, was OK until 470X but softer than 330X.

There was a time where Jupiters details were crisp but just as with Saturn anything above 330X was starting to go softer, that was a night of good seeing too, 24th February is when I imaged my best Jupiter to date using the 8" SCT.

Don't mis-understand me, the views were great, and there were a couple of moons more visible around Saturn through the 14 than the 8, the fact that the 14 gathers more light is not up for question (especially with the extra detail picked up on the Helix Nebula the other night), the fact that I'm not seeing any more details on the planets than through the 8" is the puzzling phenomenon to me and the fact that I can magnify with the 8" upto 406X and I had crisp views of Saturn and Jupiter, as opposed to seeming like I'm maxing out at 330X on the 14.

If I solve the problem or get the needed seeing where I can magnify Jupiter, Saturn, Mars or the Moon at 500X-600X and have crisp views, I know that I'll get goose bumps and a smile where the corners of my lips will tickle my earlobes, but if the 14 will only be better on DSOs, than I'll use the Dob for deep space observation and for higher planetary observations I'll still have my trusty 8SE.

I am in the process of setting up a permanent observatory to have the 8SE with the 80mm frac piggybacked as a permanent astroimaging setup with as accurate polar alignment as I can get, with the occasional planetary observation (depending on the outcome of the limitation of the dob), and the dob will be the goto scope for observing... Although I would still love to image and view the planets through the maximum capability that a 14" mirror can offer.

 

 

[MarsG76]

Thank you everyone for the comments and the tips. I'm grateful and with your help I know I'll get to the bottom of this.

I guess new toys always come with a learning curve, it's all part of the adventure.

 

[John]

My 12" Orion Optics dob has excellent quality mirrors in good collimation but I rarely find more than 250x useful on Jupiter and 300x is about the most that I use on Saturn. Mars is often a tiny target so high magnifications are needed to get a disk large enough to pick out the detail. 300x - 350x is my max usually with that one.

I doubt you are going to get crisp planetary views at 500x - 600x with any scope apart from very, very rare occasions.

[MarsG76]

30 minutes ago, John said:

My 12" Orion Optics dob has excellent quality mirrors in good collimation but I rarely find more than 250x useful on Jupiter and 300x is about the most that I use on Saturn. Mars is often a tiny target so high magnifications are needed to get a disk large enough to pick out the detail. 300x - 350x is my max usually with that one.

I doubt you are going to get crisp planetary views at 500x - 600x with any scope apart from very, very rare occasions.

So far definitely looks that way... still those rare occasions will be what will keep me hoping.

 

[Stu]

37 minutes ago, John said:

My 12" Orion Optics dob has excellent quality mirrors in good collimation but I rarely find more than 250x useful on Jupiter and 300x is about the most that I use on Saturn. Mars is often a tiny target so high magnifications are needed to get a disk large enough to pick out the detail. 300x - 350x is my max usually with that one.

I doubt you are going to get crisp planetary views at 500x - 600x with any scope apart from very, very rare occasions.

Jupiter is at over 61 degrees in Sydney and I expect your seeing will be more stable over there, so I'm sure you will hold higher mags than in the U.K. Quite how high remains to be seen!

[niallk]

Hi,

Apologies if it has already been mentioned in this thread, but is your primary mirror centre spot accurately positioned?  My f4.43 mirror has the centre accurately etched, in the glass, but the sticker ring was frustratingly off by several mm!!  I realized this after less than stellar images of Jupiter vs my 10" dob. I re-spotted, and have enjoyed great views of Jupiter this last apparition, usually employing 330x mag, 430x on very good nights, and up to 560x once - holding image crispness.  So much is down to atmosphere, cooling and collimation For planetary viewing, I'm pretty selective on the nights that I go out: the jetstream forecast really has to be promising!

While I do like upping the mag, it's not the be-all; my scope is manual so undisturbed drift time through the tfov is an important tradeoff to me, even though 560x was ok for tracking with smooth fraction-of-the-fov movement with quick damping.

Best of luck,

-Niall

[John]

47 minutes ago, Stu said:

Jupiter is at over 61 degrees in Sydney and I expect your seeing will be more stable over there, so I'm sure you will hold higher mags than in the U.K. Quite how high remains to be seen!

Good point Stu - I forgot that MarsG76 is down under !

 

[ollypenrice]

I think these very high magnifications are over rated. They don't define what you can actually see. It's interesting to take a piece of text at close to the limit of your scope's resolution, a sign, for instance, and try to read it backwards. It has to be backwards or your brain will operate a bit like predictive text and invent what it doesn't see.  I'm lucky to have a local government water filter sign on a hillside visible in my scopes. Making it bigger does not make it clearer.

Olly