Newts - maximum useful magnification?

[Flame Nebula]

Hi, 

So, I'd like to hear the experiences from newt users, past and present, regarding how far you've been able to push the magnification on your scope. 

I am particularly interested in any experience with 8" Newts. 

So, details can hopefully include 

1. Scope aperture, F ratio, and mirror quality (if known) 

2. Target(s) 

3. Seeing, if you remember. 

4. Bortle sky number, if remembered 

I think that should do. 👍🙂

Thanks 

Mark 

 

[scarp15]

I have 'pushed the envelope', in terms of high power, with my Orion Optics VX8L (200mm) dobsonian to 343x / 0.58mm exit pupil, using a 3.5mm Delos. An evening focused upon Lunar observing under very steady atmospheric conditions. The Newtonian is their standard F6 / 1200mm, though supplied with 1/10 PV wavefront grade mirrors, that is perhaps in terms of contrast and clarity an asset, in stable circumstances at such high mags.  

Edited April 6 by scarp15

[vlaiv]

You are likely to hear many different accounts on what works for people - and there is a reason for it.

I'm going to tell you what is maximum useful magnification based on objective criteria (and it varies from person to person), but there is no way to account for subjective criteria (at least not that I know of).

There is something called visual acuity of observer and it varies from a person to person:

https://en.wikipedia.org/wiki/Visual_acuity

There is a table on above wiki page - that lists MAR value. That is minimum angle of resolution. Person with 20/20 vision will have MAR of 1 arc minute. Person will be able to see the contrast change that is this small in the image.

On the other hand - there are several criteria of what the resolving power of telescope is compared to the aperture size. I'm going to use the "strictest" one - sampling rate. We are going to equate MAR of person with the size of a single pixel when critically sampling for given aperture at 400nm of light (lower bound of visible light) - that is absolutely the "worst case" scenario or maximum magnification that we can justify to be able to see all the detail that telescope offers.

We can do calculation for 200mm of aperture and it goes like this:

Let's say we have F/10 system with 2um pixel size (for critical sampling at 400nm we need F/ratio to be x5 of pixel size). That would make 2000mm FL with 2um pixel size or sampling rate of 206.3 * 2 / 2000 = 0.2063"/px.

We need magnification that will turn that angular size into 1' or 60" for 20/20 vision.

so we have 60" / 0.2063 = ~x290

That is maximum needed magnification for 200mm of aperture. Similarly for 4" telescope - we get x145 as maximum needed.

Other criteria like Dawes limit or Rayleigh criterion give much lower magnifications still.

With the change of visual acuity above number changes as well. You will often hear that people use much higher magnifications than this - say x200-x300 for 4" telescope. I can't explain that phenomenon.

I do know that I enjoyed magnifications of x1 per mm of aperture (x100 for 4", x200 for 8") since before I knew about above and I probably have slightly better vision than 20/20 - so in my case above theory matches reality.

[dweller25]

My findings are about the same as @vlaiv’s calculations.

90% of my observing is planetary.

I find maximum useful magnification is 1.5x the aperture of the telescope in mm’s.

So the 100mm refractor would ideally be x150, but I often go to x180 !

The OO 1/10th wave 200mm F/6 Newt would ideally be x300, but I rarely go past x250 !

Magnifications beyond the “ideal” show a bigger image but do not show any more detail.

However, UK seeing conditions can often limit this to around x250 or less sometimes.

Exit pupil also plays a part.

 

Edited April 6 by dweller25

[Flame Nebula]

1 hour ago, scarp15 said:

I have 'pushed the envelope', in terms of high power, with my Orion Optics VX8L (200mm) dobsonian to 343x / 0.58mm exit pupil, using a 3.5mm Delos. An evening focused upon Lunar observing under very steady atmospheric conditions. The Newtonian is their standard F6 / 1200mm, though supplied with 1/10 PV wavefront grade mirrors, that is perhaps in terms of contrast and clarity an asset, in stable circumstances at such high mags.  

Thanks Scarp15. 

Do you usually view high mag without a Barlow? Just wondering if a 7mm, with Barlow would have matched the view. 

[Mr Spock]

It all depends on seeing conditions and target. In excellent seeing conditions my 12" f5 is sharp at x461 (3.3mm eyepiece) on the moon. On Jupiter, only x190 (8mm).

[Flame Nebula]

1 hour ago, vlaiv said:

You are likely to hear many different accounts on what works for people - and there is a reason for it.

I'm going to tell you what is maximum useful magnification based on objective criteria (and it varies from person to person), but there is no way to account for subjective criteria (at least not that I know of).

There is something called visual acuity of observer and it varies from a person to person:

https://en.wikipedia.org/wiki/Visual_acuity

There is a table on above wiki page - that lists MAR value. That is minimum angle of resolution. Person with 20/20 vision will have MAR of 1 arc minute. Person will be able to see the contrast change that is this small in the image.

On the other hand - there are several criteria of what the resolving power of telescope is compared to the aperture size. I'm going to use the "strictest" one - sampling rate. We are going to equate MAR of person with the size of a single pixel when critically sampling for given aperture at 400nm of light (lower bound of visible light) - that is absolutely the "worst case" scenario or maximum magnification that we can justify to be able to see all the detail that telescope offers.

We can do calculation for 200mm of aperture and it goes like this:

Let's say we have F/10 system with 2um pixel size (for critical sampling at 400nm we need F/ratio to be x5 of pixel size). That would make 2000mm FL with 2um pixel size or sampling rate of 206.3 * 2 / 2000 = 0.2063"/px.

We need magnification that will turn that angular size into 1' or 60" for 20/20 vision.

so we have 60" / 0.2063 = ~x290

That is maximum needed magnification for 200mm of aperture. Similarly for 4" telescope - we get x145 as maximum needed.

Other criteria like Dawes limit or Rayleigh criterion give much lower magnifications still.

With the change of visual acuity above number changes as well. You will often hear that people use much higher magnifications than this - say x200-x300 for 4" telescope. I can't explain that phenomenon.

I do know that I enjoyed magnifications of x1 per mm of aperture (x100 for 4", x200 for 8") since before I knew about above and I probably have slightly better vision than 20/20 - so in my case above theory matches reality.

Hi vlaiv, 

Thanks for this. So, if I understand what you are saying, 290x multiplication is needed to magnify the best resolution that the 200mm scope can offer, to a level where someone with excellent eyesight can discern that detail? 

And if you can't discern 1', then you would need to push the magnification higher, aperture and seeing permitted? 

Or something like this? 🤔

[Piero]

In my truss dobsons I pull up the light shroud about 3" from the mirror box. I find this to eliminate the formation of air boundary layer above the primary mirror as this cools down to ambient temperature. I also leave the fan behind the back of the primary mirror on, but at reduced speed.

It's quite interesting to notice how much "poor seeing" is actually very local.

[Flame Nebula]

1 hour ago, dweller25 said:

My findings are about the same as @vlaiv’s calculations.

90% of my observing is planetary.

I find maximum useful magnification is 1.5x the aperture of the telescope in mm’s.

So the 100mm refractor would ideally be x150, but I often go to x180 !

The OO 1/10th wave 200mm F/6 Newt would ideally be x300, but I rarely go past x250 !

Magnifications beyond the “ideal” show a bigger image but do not show any more detail.

However, UK seeing conditions can often limit this to around x250 or less sometimes.

Exit pupil also plays a part.

 

Hi Dweller, 

Yes, interestingly, I used to think my sharpness maxed out at 35 per inch with my 80ed, but only 30 per inch with the 127mm mak. 30-35 per inch SEEMS to be common( unless super apo. 😁) 

[vlaiv]

5 minutes ago, Flame Nebula said:

Hi vlaiv, 

Thanks for this. So, if I understand what you are saying, 290x multiplication is needed to magnify the best resolution that the 200mm scope can offer, to a level where someone with excellent eyesight can discern that detail? 

And if you can't discern 1', then you would need to push the magnification higher, aperture and seeing permitted? 

Or something like this? 🤔

Yep - with exception of:

20/20 vision is not really excellent eyesight - more "most common" or "median" or something like that. Exceptional eyesight would be 20/10 or 20/8.

If you have poorer eyesight than that, but note that even if you wear eyeglasses - you might not have poor eyesight when using the telescope as focusing can compensate for certain diopter thus some people can observe without glasses, then you'll probably need a bit more magnification to clearly see all the detail.

It's a bit like small letters. Some people are fine with reading the small letters and other people need glasses / magnifying glass to read the text.

[scarp15]

31 minutes ago, Flame Nebula said:

Thanks Scarp15. 

Do you usually view high mag without a Barlow? Just wondering if a 7mm, with Barlow would have matched the view. 

Yes I do not use a Barlow, preferring instead to switch between an incremental range of (Delos and DeLite) high power, parfocal eyepieces, that retain comfortable eye placement, a reasonable relaxed exit pupil and adequate true field of view.  

[Earl]

I always aimed well below maximum magnification, and prefered 32, 25 and 15 mm eyepieces.

Now my nystagmus makes it all pretty pointless however now, i do need to try my null zone to see if it works with an eyepiece, (I have a feeling it wont as its looking up)

Edited April 6 by Earl

[ollypenrice]

38 minutes ago, Piero said:

In my truss dobsons I pull up the light shroud about 3" from the mirror box. I find this to eliminate the formation of air boundary layer above the primary mirror as this cools down to ambient temperature. I also leave the fan behind the back of the primary mirror on, but at reduced speed.

It's quite interesting to notice how much "poor seeing" is actually very local.

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

[Earl]

5 minutes 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

Your 20" gone?

 

[Flame Nebula]

17 minutes ago, scarp15 said:

Yes I do not use a Barlow, preferring instead to switch between an incremental range of (Delos and DeLite) high power, parfocal eyepieces, that retain comfortable eye placement, a reasonable relaxed exit pupil and adequate true field of view.  

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? 

[Flame Nebula]

14 minutes 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

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. 

Edited April 6 by Flame Nebula

[dweller25]

There is a 5mm Pentax XW in the for sale section 🙂

[dweller25]

15 minutes 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

I fitted a fan on the back of the Newt and pulled air out, it immediately removed the boundary layer 👍

[Flame Nebula]

3 minutes ago, dweller25 said:

There is a 5mm Pentax XW in the for sale section 🙂

Thanks Dweller, 

I was thinking of something even shorter. Say 3-4mm, but thanks for letting me know 👍

[dweller25]

There is also a 7mm Pentax XW, which barlowed will give x285 🙂

[Mr Spock]

Don't forget the Svbony 3-8 

[Flame Nebula]

5 minutes ago, Mr Spock said:

Don't forget the Svbony 3-8 

I wonder if baader do a short zoom. Their 8-24mm is very good. I guess I could use the 2x Barlow to get 4mm. But, to get to the 290 mark, I'd need more. 

[John]

11 minutes ago, Flame Nebula said:

I wonder if baader do a short zoom. Their 8-24mm is very good. I guess I could use the 2x Barlow to get 4mm. But, to get to the 290 mark, I'd need more. 

Baader do a barlow lens that is expressly designed for use with their 8-24mm zoom:

Baader 2.25x Barlow for Zoom Hyperion | First Light Optics

[John]

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.

[Flame Nebula]

7 minutes ago, John said:

Baader do a barlow lens that is expressly designed for use with their 8-24mm zoom:

Baader 2.25x Barlow for Zoom Hyperion | First Light Optics

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 

Edited April 6 by Flame Nebula