8" and 14" Telescopes not much difference?

[MarsG76]

2 hours 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!

 

1 hour ago, John said:

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

 

Yeah, 61 degrees and crisp views at max mag in the 8", hence the frustration... but I'm sure there is a very good and most likley simple reason for the 300X seeming limit.

 

[MarsG76]

2 hours ago, niallk said:

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

You know what? I just trusted that the ring sticker is in the middle.... but I use a cheshire eye piece to collimate, when the chesire lines up than wouldnt that also have the mirror centered?

BUT this is a very good tip...

 

[MarsG76]

1 hour ago, ollypenrice said:

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

I disagree, on clear nights the higher magnifications does make the details clearer and easier to see... however, going over board is counter productive...

[John]

I find the subtle details on Jupiter easier to see if I reduce the magnification so a smaller image scale and better empahsised contrast variations is desireable there. With Saturn though I find higher magnifications more rewarding because the nature of the detail that is being sought tends to have strong contrast differences and is more clearly defined.

I think you need to find what works with each planet rather than to lump them all together when it comes to magnification choices.

 

 

[MarsG76]

8 minutes ago, John said:

I find the subtle details on Jupiter easier to see if I reduce the magnification so a smaller image scale and better empahsised contrast variations is desireable there. With Saturn though I find higher magnifications more rewarding because the nature of the detail that is being sought tends to have strong contrast differences and is more clearly defined.

I think you need to find what works with each planet rather than to lump them all together when it comes to magnification choices.

 

 

Practice and adapt to the conditions.

[John]

1 hour ago, MarsG76 said:

Practice and adapt to the conditions.

Indeed.

[kirkster501]

Assuming all other things being equal, a huge difference.  The darker the skies, the more difference still.  I have seen delicate spiral structure with a C14 scope from a dark site that was breathtaking.  A globular cluster like M13 though a C14 is a sight to behold.

[MarsG76]

2 hours ago, kirkster501 said:

Assuming all other things being equal, a huge difference.  The darker the skies, the more difference still.  I have seen delicate spiral structure with a C14 scope from a dark site that was breathtaking.  A globular cluster like M13 though a C14 is a sight to behold.

My last observing session I did start to pick out more detail through the 14" than I saw through the 8" on the Helix Nebula, mind you, there was a massive amount of sky glow that night, so I'm sure that the key is and like you said "big difference" between the scope will be during dark skies.. the darker the better.

 

Talking about globulars.... If you liked M13, than it's a pity you cant see the Omega Centauri cluster and 47Tuc from Nottignham.... those through any scope are a sight to take breath away.. even in light pollution they are great, but during a dark sky, stars upon stars within stars all shrouded in twinkling stars, almost edge to edge in a low power EP all compressed into a sphere...

 

[kirkster501]

On 28/07/2017 at 19:35, MarsG76 said:

My last observing session I did start to pick out more detail through the 14" than I saw through the 8" on the Helix Nebula, mind you, there was a massive amount of sky glow that night, so I'm sure that the key is and like you said "big difference" between the scope will be during dark skies.. the darker the better.

 

Talking about globulars.... If you liked M13, than it's a pity you cant see the Omega Centauri cluster and 47Tuc from Nottignham.... those through any scope are a sight to take breath away.. even in light pollution they are great, but during a dark sky, stars upon stars within stars all shrouded in twinkling stars, almost edge to edge in a low power EP all compressed into a sphere...

 

My friend, when based in the UK and its climate you are grateful to see anything at all....  we are fed scraps from the table when it comes to getting to use out scopes. 

[MarsG76]

On 8/1/2017 at 20:50, kirkster501 said:

My friend, when based in the UK and its climate you are grateful to see anything at all....  we are fed scraps from the table when it comes to getting to use out scopes. 

Surely you have a few crystal clear nights every so often, at the very least.

 

[spaceboy]

I can count 3 in 7 years but they were nights imprinted in memory. Certainly lets us know what we are missing

[MarsG76]

On 7/23/2017 at 18:07, 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

Thank you all for replies and so much awesome info.... you guys are great...

So basically I'm at the point where I'm very sure that I have collimated properly.. very sure.... I've spent ages an collimation with the eyepiece, than the DMK21, fine tweaks on Saturn, fainter and brighter stars  and the stars are defocusing with the central obstruction in the center, getting close to focus doesn't have any "comet" effects either, but I did notice that there is astigmatism, most definitely... only slightly oval defocused stars, where the oval rotates 90 degrees when before or after focus. 

The last time, a couple of nights ago, when I was observing I had a very clear sky and Saturn actually looked very sharp and detailed up to 450X. It was a very nice view. Even at 660X it was softer but the Cassini division and cloud bands, with the shadow were coming into view occasionally... the scale of it in the eye piece was MASSIVE, but I was still returning to the 450X view since it was way more pleasing.

That said, the Cassini division was almost jumping left-right/up-down, almost vibrating like. The view of the, just a smidgen larger than, first quarter moon was sharp, where there were very fine details visible in the eye piece, BUT... there is always a BUT, isn't it?.... But when looking at the more contrasty parts of the moon, like the larger craters near the terminator where there was white and black close together, its like I couldn't 100% hit the "white" arcs on the craters to "overlap" in focus... with the same left-right/up-down almost like vibration at the closest point to perfect focus. I'm thinking that, and please correct me if I'm wrong, this is caused by the astigmatism???...

Imaging the moon or Saturn was not possible with any decent result either, resulting in a ghosted stack on craters and the Cassini division.

My question is how do I fix the astigmatism, is it even possible to? there are no side clips holding the mirror, basically a plate holding the mirror by the middle with the collimating screws pulling on the 1cm thick mirror plate, tilting the mirror... When I'm home, I'll take a photo of the mirror assembly.

I think that if I fixed the astigmatism issue than I'd have an all round winner here... last observation did show me more faint fuzzies and detail in them than the 8" but still I think the 8" is a better moon/planet scope than the 14", so as a worst case scenario, the 14 is a DSO telescope and the 8" the astro imaging/solar system observation scope... of course I'd love to have the dob as a all rounder and the 8" SCT /80mm frac piggy back setup permanently as a imaging scope.

 

[John]

The star test is by far the best way to detect astigmatism. Viewing the moon and planets at high powers will not be a realiable way to do it because there are too many other factors that can play a part.

The star test, properly carried out, is a very sensitive tool for assessing the state of collimation and the quality of the optics, as long as the seeing conditions are steady.

Is there anyone near you, from an astro society say, that could have a look at your star test for you and diagnose the issue ?

One thing that will happen, unless the seeing conditions are 100% perfect (very rare even in Australia I'd say), is that the moments of best sharpness and definition will come and go all the time as you are observing. Sometimes you just get a second or two of the good stuff, then you loose it, then it pops back for a second or two. This is due to atmospheric cells passing over head. It's quite a normal effect in my experience - you get these little glimpses of the full potential of the optics and your eye / brain has to sort of piece them together. This is why the recommendations to observe a target for a prolonged period are made on here !

 

[MarsG76]

1 minute ago, John said:

The star test is by far the best way to detect astigmatism. Viewing the moon and planets at high powers will not be a realiable way to do it.

The star test, properly carried out, is a very sensitive tool for assessing the state of collimation and the quality of the optics, as long as the seeing conditions are steady.

Is there anyone near you, from an astro society say, that could have a look at your star test for you and diagnose the issue ?

 

I did determine the astigmatism using a faint star and Jupiter's moons... I didn't get involved in any astronomical society, so no.

 

[MarsG76]

17 minutes ago, spaceboy said:

I can count 3 in 7 years but they were nights imprinted in memory. Certainly lets us know what we are missing

Oh really, that bad... I though I had heaps of clouds but, man, I'm starting to feel for you guys now...

I try to type a description up my better observing sessions, also post the better pics I end up with... once my obsy is finished and up and running I'll be posting more images too. 

[John]

You want a 2nd magnitude star or something close to that. We use Polaris a lot up here because it does not move much !

I've had a handfull of 100% perfect nights in the 30+ years that I've been observing. Most of the time it's 70%-80% so we go with that and make the best of it. Usually that means not using 450x magnification even with a 20" optically perfect scope !

Edit: it's not the clouds so much as the steadiness of the seeing that matters. Transparency for deep sky objects and steadiness for high power viewing of the moon and planets - different things really and getting both on the same night is often not the case.

 

[ollypenrice]

Your description of the defocused oval rotating through 90 degrees is the classic sign of astigmatism, as you say.

This may be coming from either one of the mirrors and it would be nice if it were the secondary! I was lucky in that this proved to be the case with our 20 inch. Optical genius Ralf Ottow confirmed this by taking out the secondary and propping it up on our kitchen windowsill so it was pointing at distant woods on the horizon. (Bear with me, this is remarkable!) He then set up his own self-built SCT at the back of the kitchen, pointing at the secondry mirror and tweaked everything till, in the SCT, he could see a point source of light coming through the trees. He then racked the SCT in and out of focus and the defocused point source turned 90 degrees. The secondary was causing the astigmatism. He made us a new one, bless him!

A more conventional test of your secondary, but on similar lines, would be easy enough to perform...

Olly

[MarsG76]

4 minutes ago, John said:

You want a 2nd magnitude star or something close to that. We use Polaris a lot up here because it does not move much !

I've had a handfull of 100% perfect nights in the 30+ years that I've been observing. Most of the time it's 70%-80% so we go with that and make the best of it. Usually that means not using 450x magnification even with a 20" optically perfect scope !

 

20" ??? nice..... did you see actual color, reddish/pinkish color in the Orion Nebula with that monster?

 

[MarsG76]

4 minutes ago, ollypenrice said:

Your description of the defocused oval rotating through 90 degrees is the classic sign of astigmatism, as you say.

This may be coming from either one of the mirrors and it would be nice if it were the secondary! I was lucky in that this proved to be the case with our 20 inch. Optical genius Ralf Ottow confirmed this by taking out the secondary and propping it up on our kitchen windowsill so it was pointing at distant woods on the horizon. (Bear with me, this is remarkable!) He then set up his own self-built SCT at the back of the kitchen, pointing at the secondry mirror and tweaked everything till, in the SCT, he could see a point source of light coming through the trees. He then racked the SCT in and out of focus and the defocused point source turned 90 degrees. The secondary was causing the astigmatism. He made us a new one, bless him!

A more conventional test of your secondary, but on similar lines, would be easy enough to perform...

Olly

Another 20"... I'm the little guy in the club....

Interesting test, I guess the secondary would be a easier and/or cheaper fix...

[ollypenrice]

2 minutes ago, MarsG76 said:

 

Interesting test, I guess the secondary would be a easier and/or cheaper fix...

Much easier!

Olly

[John]

The 20" is not a scope that I own - I was using it as an example that, perhaps, your expectations of the seeing conditions, plus mass produced F/4.5 optics, might be a touch on the high side if you are seeking 400x magnification to be readilly useable often with crystal clear, sharp results.

If you do have atmospheric issues then the smaller aperture scope will be less affected by them and therefore will appear to "punch above it's weight" wheras the 14" dob might not. Seeing conditions can be a great equaliser of aperture I've found

If you have astigmatism in your optical system (which it sounds like you have) then it needs to be sorted otherwise it will hold the scope back, no doubt about that. Sometimes it's the way the mirrors are mounted (eg: too tight cells or clips) which is relatively easy to sort, sometimes the primary cell can deform slightly if you are using the collimation locking bolts / screws (I never use mine on my 12" dob for this reason) or in the worst scenario the figure of the secondary or primary mirror is not as good as it should be. In a new scope thats a warranty issue.

 

[MarsG76]

11 minutes ago, John said:

The 20" is not a scope that I own - I was using it as an example that, perhaps, your expectations of the seeing conditions, plus mass produced F/4.5 optics, might be a touch on the high side if you are seeking 400x magnification to be readilly useable often with crystal clear, sharp results.

If you do have atmospheric issues then the smaller aperture scope will be less affected by them and therefore will appear to "punch above it's weight" wheras the 14" dob might not. Seeing conditions can be a great equaliser of aperture I've found

If you have astigmatism in your optical system (which it sounds like you have) then it needs to be sorted otherwise it will hold the scope back, no doubt about that. Sometimes it's the way the mirrors are mounted (eg: too tight cells or clips) which is relatively easy to sort, sometimes the primary cell can deform slightly if you are using the collimation locking bolts / screws (I never use mine on my 12" dob for this reason) or in the worst scenario the figure of the secondary or primary mirror is not as good as it should be. In a new scope thats a warranty issue.

 

I don't use the locking screw either... I'm going to try the above mentioned secondary test, and, yeah, if there is resolvable astigmatism, than warranty repair might be the necessary step.

 

[Peter Drew]

Although a secondary is cheaper to replace I would first of all do a quick check of the primary mirror. Make a note of the axis of the astigmatism then rotate the mirror 45 degrees and see if the astigmatic image has also rotated. If it doesn't then the fault lies with the secondary.     

[MarsG76]

2 minutes ago, Peter Drew said:

Although a secondary is cheaper to replace I would first of all do a quick check of the primary mirror. Make a note of the axis of the astigmatism then rotate the mirror 45 degrees and see if the astigmatic image has also rotated. If it doesn't then the fault lies with the secondary.     

Good idea... Thanks for the tip...

[cloudsweeper]

On 23/07/2017 at 07:36, Stu said:

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.

Good point Stu.  This theme is one that seems to polarise opinion.

A bigger aperture gives you more mag for the same SB.

Some then say this makes the extended object seem brighter just because it is larger, and contrast plays a part in this.

Others further say that since the image occupies a bigger area, then the total or integrated brightness increases, which is after all to be expected since greater light flux has been captured.

Hmmmm........

Doug.