It is currently Sun Nov 19, 2017 3:03 am


Post new topic Reply to topic  [ 59 posts ]  Go to page 1, 2, 3, 4  Next
Author Message
 Post subject: Magnification
PostPosted: Fri Feb 03, 2012 3:53 pm 
Offline

Joined: Fri Feb 03, 2012 3:31 pm
Posts: 17
Location: Long Ditton, Surrey, UK
Hi everyone!

I am new to the SPA and wanted to ask a question-

I have noticed that many astronomers, even for planetary observing seem to avoid using magnifications anywhere near the highest useful magnification of their telescope e.g:

On the planetary  observing section, one person has posted a sketch of mars using a 300mm newtonian, but he used a magnification of just 170x, nowhere near the highest useful magnification. The seeing was AII - III.

Wouldn't it just be easier to magnify to around 600x and take full advantage of the telescope aperture, on such a difficult to see target?

Happy Stargazing!


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Fri Feb 03, 2012 4:15 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
Welcome,

The higher the magnification the dimmer the image.
I.e. the surface brightness is low, but more importantly the contrast is low.
On planets everything seen is due to contrast.
The best observers with the best eyes use the lowest magnification that will show all the detail.
The full resolution of a scope is produced with a magnification of 14 per inch.
Observers like me with poorer eyes prefer two or three times that.
My preferred magnification on a 317mm good scope was 265x. I also used around 400x as well.
But the best planetary observers use 190x on Saturn and Jupiter with a 305mm.
Mars can take higher magnification, I suppose because it is nearer the Sun and the surface brightness is higher. Here 400x is often used.

For double stars really high magnifications can help even 600x

I used 1100x to observe Jupiter's moons discs, but only as a test of the optics.

The atmosphere in most countries does not allow high magnifications and small scopes say 114mm magnifying 625x are just a trap for the unwary.
150x is the more sensible limit or possibly 200x for a really good one.

regards, David


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Fri Feb 03, 2012 6:04 pm 
Offline

Joined: Fri Feb 03, 2012 3:31 pm
Posts: 17
Location: Long Ditton, Surrey, UK
Hi,

Thanks for the informative advice.

 I am interested to buy a Vixen  NLV eyepiece for planetary viewing -( I have a skywatcher 305mm dobsonian). I feel this series is what I need as I have a bad astigmatism and I would benefit from the 20mm eye relief. 

The series includes 2.5, 4, 5, 6, 9 mm focal lengths. Which of these do you think is best for general planetary viewing, (mostly mars) with my slightly weak eyesight?

If not, is there anything else you can recommend?

Much appreciated,

Regards Umar


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Fri Feb 03, 2012 6:22 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
What is the focal length or focal ratio of the 305mm Dobsonian?
At high power you may be able to observe without glasses.
Do you know the approx. amount of astigmatism you have?
Do you have astigmatism in both eyes?
Could you use your less dominant eye if it has less astigmatism?

If money were no object Televue have I think dioptre correction lenses that fit on their very good eyepieces.

The cheapest option is to use your glasses with long eye relief eyepieces.
Their are so many eyepieces available now but the ones you suggest are good if a bit narrow field.

Is it a driven Dobsonian or if not can you track at high magnification. Is it smooth?

I would suggest a magnification of 250x approx or maybe 230x for a single eyepiece and say another of 300x for Mars if it is not driven.
A wide field eyepiece might also be good but expensive.

Regards, David


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Fri Feb 03, 2012 6:42 pm 
Offline

Joined: Fri Feb 03, 2012 3:31 pm
Posts: 17
Location: Long Ditton, Surrey, UK
Some details:

Telescope: Skywatcher Skyliner 300p flextube (305mm GOTO dobsonian)
Focal length: 1,500mm
Focal ratio: f/4.9

I am not quite sure what the numbers mean on my eye subscription, but I have them if they help!

I think the astigmatism is in both eyes, and bad enough to make it necessery to wear glasses always while viewing.

Would these details help? I guess the narrow field of view would only be a problem if the GOTO would not be in use? So you think its best not to exceed 300x for planetery viewing?

Regards, Umar


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Fri Feb 03, 2012 7:06 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
I would go for a 6mm first, then 5mm and 4mm in the Vixen also if you can afford it.
There are some zoom planetary eyepieces from Televue, high quality but narrow field, expensive and maybe Hyperion and others.

If you start off with 250x with experience you may be able to go for 300x and 375x.

Some Dobsonian users happily use up to 600x undriven but they are experienced.

The 2.5mm would give 600x but you would rarely use it.

Your GoTo would probably not find anything at 600x although it might track it once found for a limited time.

Regards, David


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Fri Feb 03, 2012 8:08 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
You could ask if you buy a set of three eyepieces 6mm, 5mm and 4mm, whether they will be cheaper, its winter Sale time.

David


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 9:18 am 
Offline

Joined: Thu Sep 04, 2008 3:36 pm
Posts: 309
Location: Gt Dunmow, Essex
Hello and welcome.

My favoured magnification for Jupiter and Saturn is about 160x. On a good night with steady seeing, I can sometimes push this to around 270x.

Planetary observation is scuppered by atmospheric turbulence, of which we unfortunately have plenty in the UK. At higher powers, not only do you magnify the image, but you also magnify the atmospheric distortion, leading to an image that is dim, wobbly and out of focus.

It's a common question we get at public star parties. People see big telescopes and one of the first questions is "How big a magnification can you get with that?" They are often surprised when I tell them that most of my viewing is done at 60-80x, going up to around 200x on the planets. Large apertures are about resolution, not magnification.

I did once use a 6mm eyepiece with a 2x Barlow on Saturn, giving me a ridiculous 600x. Saturn was huge, but the view was terrible! :D

Rachel

_________________
Celestron C8-S XLT
CG5 mount, dual axis motor driven
Imaging Source DFK21AF04.AS camera
North Essex Astronomical Society


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 10:29 am 
Offline

Joined: Mon Mar 12, 2007 8:05 pm
Posts: 236
Location: Cardiff
On my 4" refractor, I get great views of Jupiter at 200X.
On my 8" reflector the view at 200X is not as detailed.
On my 5" Mak 200X is quite nice, but the refractor beats it.
The mak has a FL of 1500mm. the other two are 1000mm.

I find that planets like Jupiter are too bright for wide aperture newts.
Maybe a neutral density filter would solve this?

For deep sky stuff, the newt is king. Stunning views with superb detail.
This is why I have 3 telescopes.
One for planets, one for deep sky and the other as an in-between jack of all trades.

_________________
I once came last in an astronomy competition.
I was awarded a constellation prize


Skywatcher Explorer 200 HEQ5
Skywatcher Skymax 127 SupaTrak
Celestron C4-R CG-4 mount


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 1:04 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
The reason I suggested 230x or 250x and also 300x for Mars and maybe 375x is because of the astigmatism and maybe less than perfect eyes.
On good nights on say a grass surface these magnifications are reasonable.
Those with not so good eyes often prefer somewhat higher powers.

I presume the scope comes with lower power eyepieces for deep sky work.

It is after all quite a large 12inch scope and if the optics are good the limit will be the Seeing conditions.

If an observer is prepared to look at the planets at 03.00 UT about then the Seeing is often good enough to comfortably handle the above magnifications.

Also planetary observers often use colour filters to bring out details, so neutral density filters are not usually used.

For me with slight astigmatism I don't use glasses for observing.
But on Saturn I see more detail by tilting my head 90 degrees.
I use my slight astigmatism to enhance the detail I see by rotating my eyes as necessary.
The banding on Saturn and Jupiter is in one direction so this trick works.

Regards, David


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 2:22 pm 
Offline

Joined: Fri Feb 03, 2012 3:31 pm
Posts: 17
Location: Long Ditton, Surrey, UK
It looks like I need to sacrifice a little contrast and detail due to my less than perfect eyes. Does  250x (6mm) seem like a good compromise?

If the seeing is awful, then I have a 25mm super plossl (recommended as the first thing to be replaced by Astronomy Now when they reviewed my scope) and a 3x barlow to fall back on, giving me 180x.

I guess I wont know what suits me until I buy it, but how does the 6mm Vixen NLV sound as the first port of call?

Umar


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 2:37 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
The 6mm Vixen is a very good first choice.

I have just looked at some my old Association of Lunar and Planetary Observers Journals at Mars drawings.
These top observers use a wide variety of magnifications.
One observer habitually uses 250x on a 300mm scope.
A French observer uses 700x on a 400mm scope.
A 225mm Maksutov user uses 250x, 300x and 390x.

So each observer finds his or her own favoured magnifications.

I preferred 265 x and 360x on a 317mm for Saturn and Jupiter and sometimes more on Mars.
When Mars was very close I was staggered by the amount of detail I once saw. Totally amazing, but the Seeing must have been near perfect.

So yes try the 6mm and I am sure that is a good choice.
You can then judge if you want or need an additional eyepiece.

Regards, David


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 5:13 pm 
Offline

Joined: Sun Apr 27, 2008 2:59 pm
Posts: 198
Location: Cambs
Also have astigmatism, get long eye relief eyepieces and wear your glases. There are a lot of them out there, and for a 305mm scope I cannot see why you would consider the budget plossl's.

A 6mm plossl will give you about 4mm of eye relief if you get a plossl, probably not enough.

Try something like the BST Explorers or TMB planetarys. They both supply 15+ mm of eye relief on their eyepieces.

The BST's come in 5mm and 8mm at the lower end.
The TMB's come in more options down to about 3.2mm
Seems generally that people find the BST's a little better.
However eyepieces are a personel item.

Strange thing about this Max Mag = 2x Dia is I read of it many times, and see that most who try for it find they do not get it. Also few years ago people said max mag = 1.5x Dia. I suspect that many find 1.5x is much more realistic.


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 5:24 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
The Vixen 6mm NLV eyepiece discussed above is advertised as 20mm eye relief. Made in Japan.
Probably high quality. 7 elements, 4 groups.
Field a bit narrow at 45 degrees advertised

First light optics sell them as do others.

Regards, David

As to highest power it depends on the scope.
A very high quality refractor say 100mm can easily take 300x and the best 400x. I.e. 3x to 4x per mm.

A large reflector maybe 50x per inch. or 2x per mm.
Any really high class scope up to say 300mm should be able to take 2x per mm. if the Seeing allows.
In actual use I would agree that a 300mm is usually not used above 450x.
However, top 150mm Maksutovs have been used well at 500x.


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Feb 04, 2012 5:37 pm 
Offline

Joined: Mon Jan 18, 2010 9:25 am
Posts: 5117
The Williams Optics SPL do a 6mm advertised 7 elements in 4 groups.
20mm eye relief.
These have an advertised 55 degree field.
£69 FLO.

Perhaps someone can comment on the image quality.

David

The TMB 6mm seems O.K.
type 1 or 2 ?
Seems low price but not many about?
15mm eye relief, 58 degree field .


Top
 Profile Send private message  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 59 posts ]  Go to page 1, 2, 3, 4  Next

All times are UTC


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group