Best image possible with 70mm refractor

[heathgravett]

Hi All

Just new to astronomy after buying the "Cosmos" book. I have a celestron Astromaster 70 Refractor scope with 20mm, 10mm and 6mm eyepieces plus filters.

Currently observing Saturn but a little disappointed with the image I am getting. Even with the 6mm eyepiece, Saturn is just a smallish circle with a line through it. Image is sharp and it's still good to know I'm looking at Saturn.

By my calcs I'm using 150 X magnification and my telescope has a maximum useful power of 165 X so at the limit of good viewing. I was led to believe that I'd be able to see colour and ring formation but looking at what I get, you'd need 400 X magnification for that. Have I been misled and what I am seeing is the best I'll get or am I doing something wrong here?

Cheers

[Liz]

Originally Posted by heathgravett

Hi All

Just new to astronomy after buying the "Cosmos" book. I have a celestron Astromaster 70 Refractor scope with 20mm, 10mm and 6mm eyepieces plus filters.

Currently observing Saturn but a little disappointed with the image I am getting. Even with the 6mm eyepiece, Saturn is just a smallish circle with a line through it. Image is sharp and it's still good to know I'm looking at Saturn.

By my calcs I'm using 150 X magnification and my telescope has a maximum useful power of 165 X so at the limit of good viewing. I was led to believe that I'd be able to see colour and ring formation but looking at what I get, you'd need 400 X magnification for that. Have I been misled and what I am seeing is the best I'll get or am I doing something wrong here?

Cheers

Hi Heath to IIS .
I have also ordered 'Cosmos', and awaiting delivery.
Yes, I think you were lead astray a bit .... you have only a smallish scope, and you can see Saturn and its rings ... which is still great. Alas, you wont see too much detail. High mag doesnt give the best views in a small scope. I have a 10" and still have problems with high mag.
The rings of Saturn are pretty edge on at the moment, but will slowly open over the next few years. At least you can see the rings ... the line.
AM sure others will pop on with their point of view.
We advise those buying a scope to go for a small Dobsonian 6", or 8" if you can afford it. They are about $400 ... maybe cheaper.
Your scope will still give great views of Moon.

[torana68 (Roger)]

sounds sort of typical, without seeing it person that is, have a play with this:
http://www.telescope-simulator.com/i...d=45&Itemid=57

[heathgravett]

Thanks to you both.

The rings certainly are straight on at the moment. I guess when they open up it will show a little more detail.

And the site you gave was great - pretty accurate really - for simulated view.

Can I add a Barlow x2 to my current configuration to get a doubling in size or does that push the magnification beyond the useful mag - Unsure as it seems to double the focal length of the telescope rather than the eyepiece (or am I wrong). I'm guessing it pushes the telescope beyond what it can do?

It has also been suggested that assuming you have a very clear night (so exceptional viewing), the useful mag can be pushed to as much as 100 power rather than 60. Is this possible / worth the investment in a 3mm eyepiece or will this be a waste of money as I'll only get a sharp image once in a blue moon (or orange as it hit the ocean at 6.30am yesterday morning in WA!!)

Cheers

[astro744]

Clear skies doesn't always mean good seeing in fact sometimes the opposite. Good seeing requires steady air so look for little or no twinkling of the stars. When a could front passes the air is very clear but seeing is appalling.

You could try a good 2x Barlow with your 10mm eyepiece to get 180x which may be OK in exceptional seeing, but note the image brightness will be low. Image brightness is directly related to exit pupil which is the diameter of the cone of light entering your eye. Exit pupil = eyepiece focal length / telescope focal ratio.

In your case for a 10mm eyepiece, Magnification = 900/10=90, exit pupil = 10/12.86=0.78mm. The diameter of the cone of light entering your eye is 0.78mm (small). Add a 2x Barlow and this diameter is halved (0.39mm). An exit pupil of under 0.25mm is generally a little too small to see detail properly on any telescope.

A 2x Barlow doubles your telescope focal length or halves your eyepiece focal length.

Colour is visible especially on Saturn although the colours are subtle and Saturn is various shades of yellow. 150x is more than enough power for Saturn and Jupiter and yes, the planets are small in a telescope. A larger telescope at 150x will simply show you a brighter image which will make detail easier to spot.

Magnifications of between 120x and 240x are common for observing planets with telescopes from 60mm to 300mm. The larger telescopes will allow for brighter images at the higher powers but seeing will still be the limiting factor.

The 165x max limit for your telescope is not a hard and fast rule but generally only high quality apochromatic refractors can push these limits without the image breaking down although the image brighness will still be low for the smaller apertures.

Pushing the magnification past a practical limit gives a soft and fuzzy image with no extra detail being visible. It is often better to observe a smaller but sharper and brighter image.

[dannat (Daniel)]

the barlows i have seen ship with the small celestron scopes are not kind to image quality, though often the image depends on the sky just as much as the scope

[Stu Ward]

The first image you see, i understand can be a little underwhelming, but i think its important to note.

Saturn by Astronomical terms is a very small object in the sky and it in relation to its size it is very far away (up to 1.5 billion KM ) depending on its orbit of the sun in relation to ours.

It also has a very low Albedo which makes it very bright (almost white )
But with practice you can start to pick out details, averted vision really helps, maybe also try filters.

[heathgravett]

Thanks again to all

I am still very pleased to see Saturn given, as you state, it is 1.5bn k's away. Still very cool.http://www.iceinspace.com.au/forum/i...cons/icon6.gif

It certainly is very bright in my scope and hence no detail is seen. I have filters but they make no difference, ie the green makes it green, the orange makes it orange etc. Is there a filter that would help with the detail given my image is so small? I have the moon filter which would darken the image down.

Cheers

[Terry B]

The amount you can magnify the image is determined by 2 factors.
1. The diameter of the scope.
2. The local "seeing" or air turbulance

Resolution of a scope is proportional to the diameter of the lens and the wavelength of the light. This becomes more important with smaller scopes like the one you have. With larger scopes the theoretical resolution of the scope is better than the local seeing will allow so the limiting factor with magnification is the "seeing".
At a certain size the scope becomes small enough that the scopes resolution is the limiting factor rather than the seeing. I forget what the actual value is and it will vary depending on the local seeing.
An 80mm scope will be limited by it's size rather than the seeing mostly. A 200mm scope will be seeing limited mostly.

A bigger scope will give you a brighter image but also will have the ability to resolve more detail when the seeing is good.

[Robh (Rob)]

Originally Posted by heathgravett

It certainly is very bright in my scope and hence no detail is seen. I have filters but they make no difference, ie the green makes it green, the orange makes it orange etc. Is there a filter that would help with the detail given my image is so small? I have the moon filter which would darken the image down.

Cheers

As the others have mentioned, detail will be dependent on aperture size and seeing conditions but it will also depend on the quality of the eyepieces used. You say that Saturn is very bright. Is this with the 6mm EP? What sort of EP is the 6mm? A better EP might give you sharper images and better contrast under good seeing. Can you get together with someone who has some better EPs to try out on your scope?
The EPs supplied with most scopes are pretty ordinary and often abysmal. Most people upgrade their EPs for this reason. If you then upgrade the scope at some later time, you can still use the new EPs.

Regards, Rob.