Newts - maximum useful magnification?

[Flame Nebula]

6 minutes ago, John said:

I used to use 400x plus with my 12 inch OO dob when searching for faint planetary moons eg: Triton at Neptune, Titania and Oberon at Uranus. I've not managed Phobos or Deimos at Mars as yet.

Based on vlaiv's calculation, and assuming linear, you'd need a max of 350/200*290, to see all the details resolved in your scope. That's about mag 507x, or 42x per inch, certainly possible I suspect. 

Have you ever pushed it out that far, out of curiosity? 

Mark

[John]

5 minutes ago, Flame Nebula said:

Based on vlaiv's calculation, and assuming linear, you'd need a max of 350/200*290, to see all the details resolved in your scope. That's about mag 507x, or 42x per inch, certainly possible I suspect. 

Have you ever pushed it out that far, out of curiosity? 

Mark

Sometimes, more for fun than anything, I used the 2mm setting on the Nagler 2-4mm zoom with the 12 inch dob which gave 795x magnification. I used to observe the moon seeing what it might have been like to be an Apollo astronaut approaching the lunar surface 😁

For more serious observing I used to stay in the 200x - 350x range though, barring the occasional foray a bit higher when searching for faint point source targets such as planetary moons.

I've never really been interested in using maths to work out optimum magnifications for this and that - I much prefer to try things out and see what works and what does not 🙂

[ollypenrice]

1 hour ago, Flame Nebula said:

Can you imagine the neighbours seeing you with your head stuck down the end of your Dob? 🤣

Although you might be left with a cross on your head from the spider. 

I hadn't thought of actually venturing down the tube to do this. An input fan on one side and an output on the other would be more comfortable...

lly

[Piero]

14 hours ago, ollypenrice said:

Have you tried blowing air across the surface of the mirror? There's an argument which says that this is the best way to break the boundary layer.

I've never tried it and no longer have a Newt.

Olly

No, I have not. The mirror box of my 16" is only 11" deep. My 12" has a slightly deeper MB, but this has holes to the front and back at the level of the mirror surface (not the mirror edge!) to provide passive ventilation.

Both mirror cells are based on Kriege's design, so the back is fully open.

In either cases, the views become noticeable better (easily an extra 100x and more) as the light shroud is pulled about 3" up.

---

Update:

Here is a photo for the passive ventilation with the Kriege MC in my 12" F6. The mirror. Has it's lid (see dark circle). The counterweight balancing system has changed since this photo.

My 16" is shown in my profile photo.

Edited April 7 by Piero

[Flame Nebula]

39 minutes ago, John said:

Sometimes, more for fun than anything, I used the 2mm setting on the Nagler 2-4mm zoom with the 12 inch dob which gave 795x magnification. I used to observe the moon seeing what it might have been like to be an Apollo astronaut approaching the lunar surface 😁

For more serious observing I used to stay in the 200x - 350x range though, barring the occasional foray a bit higher when searching for faint point source targets such as planetary moons.

I've never really been interested in using maths to work out optimum magnifications for this and that - I much prefer to try things out and see what works and what does not 🙂

Yes, I'm quite interested in the maths, but I would tend to agree with you. I'm curious about how theory and practice compare to some extent. 

As a matter of interest, what did the moon look like at x795!? Lunar orbit? 

[John]

7 minutes ago, Flame Nebula said:

.....As a matter of interest, what did the moon look like at x795!? Lunar orbit? 

"Magnificent Desolation" 🙂

 

[Flame Nebula]

24 minutes ago, ollypenrice said:

I hadn't thought of actually venturing down the tube to do this. An input fan on one side and an output on the other would be more comfortable...

lly

Alternatively, a vacuum hose, placed at bottom of tube, suck out the air and allow displacement from air outside. 🤔

Just need to avoid scratching the mirror. 

I think I'll avoid this though. Might wake the neighbours (not because it's a really stupid idea. 😜) 

[Flame Nebula]

Just now, John said:

"Magnificent Desolation" 🙂

 

Ah yes, Buzz was certainly right. 

Did you take that photo John? 

[scarp15]

2 hours ago, Flame Nebula said:

Hi, 

Reason I ask, is I have a 6mm altair wave LER planetary eyepiece, and plan to use a gso x2 Barlow to get 3mm or 4.5mm(if you screw the endpiece of Barlow into end of eyepiece) giving roughly 330x and 200x respectively,  in 8" F5 ( when I get it). The Barlow retains the eye relief. But, I'm wondering if I should get a high quality short focal length eyepiece, with good eye relief, instead? 

Give it a go and take your time decision making, though as others have pointed out, there are one or two nice used e.p's currently on the used market. Much of the (currently infrequent) time outside and determined by the subject and seeing or atmospheric conditions, I remain at  between 150x and 200x, sometimes edging it to 240x. A 10mm and 6mm Delos probably gaining the most use. 

[mikeDnight]

Seeing conditions are the real equalizer providing the optics are excellent. Forget maximum resolution! You can increase magnification until you run out of exit pupil if the seeing allows, but bear in mind that different targets have sweet spots of their own. For example, Jupiter is unlikely to allow you to use the same high powers you might use on Saturn or Mars. Stars are wonderful at stupid high powers in a good scope on a good night, and the Moon too will eat magnification at times. So don't cage yourself in mentally because a book or popular opinion says this or that. You're the pilot of that spaceship so push it well out of its comfort zone if conditions permit. You'll not break it!

[John]

18 minutes ago, Flame Nebula said:

Ah yes, Buzz was certainly right. 

Did you take that photo John? 

No, I don't image apart from a few basic "mobile phone held to the eyepiece" type stuff. I just about managed to snap the Hadley Rille once, but only just !

The view through the eyepiece was much sharper than my old mobile phone camera could manage.

Edited April 6 by John

[Flame Nebula]

3 minutes ago, scarp15 said:

Give it a go and take your time decision making, though as others have pointed out, there are one or two nice used e.p's currently on the used market. Much of the (currently infrequent) time outside and determined by the subject and seeing or atmospheric conditions, I remain at  between 150x and 200x, sometimes edging it to 240x. A 10mm and 6mm Delos probably gaining the most use. 

Yes, thinking about it, if a 8" can achieve about 0.7" in perfect conditions, and the human eye can resolve 1', then I would think approx 100x would be minimum magnification needed. But, in reality, to make it easier for the eye to detect, pushing to 2-3', would need 200-300x for a 200mm scope, which is where most people push an 8", if they do need to push. 

Mark 

 

 

[Second Time Around]

1 hour ago, Flame Nebula said:

Hi John, 

Yes, you're right. I actually have the Barlow. I should have said use the 2.25, but I'd still fall a bit short of 290 mag, especially if I have a 0.95xcoma corrector in place. Having said that, might be able to avoid the corrector for planets. 

I'm wondering if something like the explore scientific 52° 3mm might be better than barlowing a 6mm.

Mark 

You can add an inexpensive extension tube(s) between the eyepiece and Barlow.   For instance Baader do an 18mm one, and each of these will add approximately 0.25x extra amplification. 

Go to https://www.firstlightoptics.com/adapters/baader-dt-4-1-31-8mm-nosepiece-extension-18mm-long.html

Edited April 6 by Second Time Around
Internet playing up

[Flame Nebula]

3 minutes ago, John said:

No, I don't image apart from a few basic "mobile phone held to the eyepiece" type stuff. I just about managed to snap the Hadley Rille once, but only just !

The view through the eyepiece was much sharper than my old mobile phone camera could manage.

OK, I was going to congratulate you on a stunning shot. 🙂

[davidc135]

I'm blessed with fairly dark, Bortle 3/4 skies with good seeing from time to time. If conditions are helpful, my 8.5'' f7.5 Dob with 21% co gives sharp lunar views at x400 which I prefer to the next step down, x220, partly because the 4mm ep is better quality. If quality was the same maybe x300 would be best of all but I expect that there is a range, especially on the moon.

My 71 yr old eyes probably appreciate a bit of extra magnification if there's plenty of light.

David

[Flame Nebula]

10 minutes ago, davidc135 said:

I'm blessed with fairly dark, Bortle 3/4 skies with good seeing from time to time. If conditions are helpful, my 8.5'' f7.5 Dob with 21% co gives sharp lunar views at x400 which I prefer to the next step down, x220, partly because the 4mm ep is better quality. If quality was the same maybe x300 would be best of all but I expect that there is a range, especially on the moon.

My 71 yr old eyes probably appreciate a bit of extra magnification if there's plenty of light.

David

I bet that's a nice planetary scope! 

[John]

One thing I have noticed, over and over again, is that a good refractor is capable of delivering to it's full performance potential much more often than larger aperture newtonians that I've owned.

50x per inch of aperture is an often cited top end "limit" for scopes. My newts have only very occasionally been useful to use at 50x per inch, most often half of that is where they have topped out, most nights.

Contrast that with my refractors where 50x per inch is very often entirely comfortable and quite frequently 75x per inch can be usefully employed.

I put this largely down to the UK seeing conditions but I guess obstructions, diffraction, reflective vs refractive efficiency and system optical accuracy (rather than just the primary) come into play as well.

Doesn't help with your decision making much though !

 

 

[Stu]

If I only ever used my refractors at 1mm exit pupil then I would have missed out on a huge part of their performance capability. I don’t really care what theory says, or that you don’t see any more detail over that, to me the additional image scale helps me to see what is there. So, I with happily use x200 or more in my 100mm fracs if the conditions allow, particularly on the Moon. Mostly on planets I use less, Jupiter x150 to x200. The limiting factor is often the exit pupil meaning floaters become an issue below 1mm and intrusive below 0.5mm which is why I use binoviewers.

Trouble is, the questions you ask don’t really have any fixed answers, because so much depends on the seeing conditions. In perfect seeing, all bets are off ie you can use what seems like crazy high mags, then the next night you might struggle to use much over x100 if the seeing is poor.

To more directly answer your question, I have an Orion Optics 8” f8 with 1/10th wave optics. It is a lovely scope and, in the right conditions can give amazing views. The best views I’ve had were probably of Mars when at opposition in 2020. I used x360 that night as the seeing was very good and Mars was at decent altitude. Not a common occurrence but nice when it happens. So, in larger scopes, the x1 mag per mm rule makes more sense, particularly for scopes over 300mm say but still, seeing conditions tend to add a layer of variability over this.

I guess what I’m saying is that theory is theory, but we observe in the real world. Even very modest scopes can give fantastic views when seeing is excellent, whilst top end scopes can be humbled to the point you think something is wrong with them when seeing is poor.

[niallk]

One night of exceptional seeing, I went to 560x on Jupiter in my 15" f4.43 dob: PMx2 & 6E.  However I backed off to the 8E ~480x for greater drift time across the fov, with a Paracorr 2 added.

On a night of very good seeing for planetary, I use a P2 + 6E for 320x.  On just a good night, I use P2 + 8E for 240x, ... and if it's not a good night I generally don't bother on planets!

You also asked about mirror quality: the cert I got claims 0.97 strehl for the primary (1/38 waves RMS wavefront), and 1/10 PV wavefront for the secondary.

So I go nowhere near 50x/inch under southern Irish skies!

Edited April 6 by niallk

[Flame Nebula]

1 hour ago, Stu said:

If I only ever used my refractors at 1mm exit pupil then I would have missed out on a huge part of their performance capability. I don’t really care what theory says, or that you don’t see any more detail over that, to me the additional image scale helps me to see what is there. So, I with happily use x200 or more in my 100mm fracs if the conditions allow, particularly on the Moon. Mostly on planets I use less, Jupiter x150 to x200. The limiting factor is often the exit pupil meaning floaters become an issue below 1mm and intrusive below 0.5mm which is why I use binoviewers.

Trouble is, the questions you ask don’t really have any fixed answers, because so much depends on the seeing conditions. In perfect seeing, all bets are off ie you can use what seems like crazy high mags, then the next night you might struggle to use much over x100 if the seeing is poor.

To more directly answer your question, I have an Orion Optics 8” f8 with 1/10th wave optics. It is a lovely scope and, in the right conditions can give amazing views. The best views I’ve had were probably of Mars when at opposition in 2020. I used x360 that night as the seeing was very good and Mars was at decent altitude. Not a common occurrence but nice when it happens. So, in larger scopes, the x1 mag per mm rule makes more sense, particularly for scopes over 300mm say but still, seeing conditions tend to add a layer of variability over this.

I guess what I’m saying is that theory is theory, but we observe in the real world. Even very modest scopes can give fantastic views when seeing is excellent, whilst top end scopes can be humbled to the point you think something is wrong with them when seeing is poor.

Thanks Stu. 

[lunator]

I have used upto x300 on my 8" f6 OO newt. I could probably go higher but the shortest FL eyepiece I have is a 4mm ortho and I don't like to Barlow them. Also as it is on an undriven Dob base It can get tricky tracking. 

As I have said before Paul Couteau said that "resolving magnification" on double stars is 2x aperture. I would use this magnification if I was on a driven mount. 

As said above seeing is key and if I can add patience is the best way to get the best seeing.

cheers

Ian

[Flame Nebula]

1 hour ago, lunator said:

I have used upto x300 on my 8" f6 OO newt. I could probably go higher but the shortest FL eyepiece I have is a 4mm ortho and I don't like to Barlow them. Also as it is on an undriven Dob base It can get tricky tracking. 

As I have said before Paul Couteau said that "resolving magnification" on double stars is 2x aperture. I would use this magnification if I was on a driven mount. 

As said above seeing is key and if I can add patience is the best way to get the best seeing.

cheers

Ian

Thanks Ian, 

I'm interested in double stars, where I'm hoping the 8" will be able to resolve sub 1" pairs, with up to 400x seeing permitting. 

 

[lunator]

Hi Mark

That is certainly doable in the 8". 

The closest pair I have resolved in the 8" was around 0.5"  I think. I will have a check back through my records. 

A good example zeta Cancri AB it is about 1"  and it can be clearly resolved at x150 and split at x200. 

Cheers

Ian

[Flame Nebula]

5 hours ago, lunator said:

Hi Mark

That is certainly doable in the 8". 

The closest pair I have resolved in the 8" was around 0.5"  I think. I will have a check back through my records. 

A good example zeta Cancri AB it is about 1"  and it can be clearly resolved at x150 and split at x200. 

Cheers

Ian

Thanks Ian. 

I think the 8" resolves to 0.6"? 

Marj

[Paz]

19 hours ago, John said:

Sometimes, more for fun than anything, I used the 2mm setting on the Nagler 2-4mm zoom with the 12 inch dob which gave 795x magnification. I used to observe the moon seeing what it might have been like to be an Apollo astronaut approaching the lunar surface 

I've done similar, trying a 2.5mm SLV in my VX14 with a paracorr, so going over 700x, locating the leading edge and then letting a dim, grainy and wobbly view of the moon fly by as if I'm looking at footage on an old black and white tv.

Ignoring the varoius limitations of observing with a scope, at that kind of magnification the view is as it would be if you were only hundreds of kilometeres above the moon.