Beginner with ST120

[DaveCon]

Hello, 

I am totally new to any star gazing involving a telescope but have recently got more interested, so my wife bought me a StarTravel 120.

As well as the supplied 10mm and 25mm EP, I also have the 3.6mm by Skywatcher and a Celestron Omni 2x Barlow. 

As the astronomy is becoming a bit of a family hobby, we have a range of things we would like to see between us -

Saturn's rings
Jupiter + moons
Moon
Orion nebula
Andromeda galaxy

As I understand it, the ST120 should be ok for the nebula and galaxy (EP depending) but less so for the moon. How about the planets?

Basically (other than buying another telescope, which is definitely on the cards in the future) what sort of advice can anyone offer regarding the use of the ST120 and the best accessories to be looking for?

Cheers

 

[PaulM]

Welcome to SGL

I cannot advise on what to expect from the ST120 but so you know Saturn and Jupiter are not visible at the moment from the UK (assuming that's where you are located within) but Orion and Andromeda are very well placed at the moment for viewing

Clear skies

[vlaiv]

Hi and welcome to SGL.

ST120 is not very suited for high power observing such as Moon and the planets, but you can make it work with some tweak.

This scope type is known as fast achromat, having ratio of aperture to focal length at F/5, or 120/600 and you can divide each with 120 to get 1/5 - which is written as F/5 to emphasize it's focal ratio. Anything below F/6 is considered fast, F/6-F/9 as moderate and above about F/9 as slow. Achromatic refractors suffer what is known as chromatic aberration - not all wavelengths of light come to focus at the same point, and this creates color fringing around bright targets, usually violet / blue type of halo around planets and limb of the Moon. Faster the achromat - worse this aberration is. Besides creating false color - it is blurring the image and reducing contrast.

With the Moon and the planets, you want to use high powers to get decent view, and that means range of x100-200.  With focal length of 600mm, this means 6mm eyepieces and below.

Suitable configurations of your eyepieces for this will be: 10mm and x2 barlow which will give you 5mm effective FL and magnification of x120. 3.5mm one will give you x171 - this will probably not be suitable for your scope.

In order to observe moon and planets without impact of CA you need to make your scope slow, and only way to do it is to decrease aperture - by use of what is know as aperture mask. It is just a "lens cap" with smaller opening in the middle. Lens cap that you have with your scope already has such opening - one of 50mm so you can use that one for the time being until you fashion another one with larger opening. You can make one out of cardboard quite easily, or plastic one if you want something more durable.

Maximum power that you can effectively use will be 2x aperture in mm, so for 50mm opening in your lens cap, you should limit your power to about x100. You can use higher power than this, but all you are going to get is larger, dimmer blurry image without additional detail (this is due to physics of light so we can't do anything about that).

Ideally you want 50-60mm aperture mask and appropriate eyepiece (that being 6mm or 5mm to give you x100 or x120 - that will minimize your CA and give decent views. If you want to go higher magnification than this, you can create larger aperture mask (for example 70mm) and use appropriate eyepiece (for example 4.5mm or 9mm and x2 barlow - for x133 power) but you will need to use some sort of filter to suppress chromatic aberration. Look at minus violet filters or Baader contrast booster filter. These will cost you a bit. You can also use simple #8 yellow planetary filter as the cheapest option, but it will color your image with yellow cast (all CA suppression filters will impact some cast to the image, but some, like Baader contrast booster, tend to keep colors more neutral).

For start you can use your lens cap opening (put lens cap on, but remove small "plug" in the middle) and 10mm eyepiece (with or without barlow) to observe Moon. That would be a good start. You can also try 10mm eyepiece with or without barlow and clear aperture (no lens cap on) to see how much your view will be impacted by chromatic aberration - some people are bothered by it more and some less.

Hope this helps.

[Peter Drew]

The ST120 will do everything on your list, some better than others. It will give great views of star clusters, nebulae and galaxies that are within its light grasp. As already said, it's not the ideal telescope for the high magnification that planets require for the best views but it will show you Saturn's ring and Jupiter's moons when the planets are next visible and plenty of detail on our Moon. Lots to look forward to.    ?

[Carbon Brush]

Adding to Peter's excellent advice.

Andromeda and the Orion nebula are best with low magnification.

I used to own an ST102 (the smaller version of your scope) and for the ££ spent it gave superb views.
Only showing limits when compared to scopes costing much more.

Enjoy the scope and keep asking questions. SGL members are happy to help.

David.

[DaveCon]

Thanks for the comments so far chaps. 

I have found the Orion nebula and M31 so far, but I am not sure if I have managed to get the best view possible from the telescope. 

The nebula was a clearly discernible grey cloud but without much detail other than 3 bright stars (10mm EP), and M31 was nothing much than a small smudge with no detail of any sort. I am aware that they won't look anything like the really amazing images seen in books etc, but I was expecting a little more from the galaxy.  Can anyone suggest anything?

[Astro Imp]

What you see will be greatly influenced by the quality of your skies. The darker your skies, the less light pollution the more you will see.

[DaveCon]

Our skies are not bad (North Northumberland coast), get a little bit LP on the horizon to the north but excellent to the west and very little on south and east. 

[rl]

One of the hardest things  for a newcomer to the hobby is managing expectations!

I've owned the ST120 and enjoyed it...it's optimized for wide field low magnification viewing....the image goes a bit soft at magnifications over 100 times. But that's enough to show the rings of Saturn with the Cassini Division and the cloud belts and moons of Jupiter, as will as all the deep sky objects on your list. Virtually all the Messier objects are within its range under the right conditions. 

The 3.6mm will be too much magnification unless your example is markedly better than the one I had! 5-6mm gives you 100 - 120 times which is a sensible top whack. 

Try reading the writing on a 5p piece at arm's length....that's the sort of detail image scale you're going to be looking at for the planets at *100. You can see stuff but it's still small. 

I'd stick with the stock eyepieces for a bit and see how you get on, but a good accessory in the future might be a good quality 20-30mm widefield eyepiece which will make the most of what the scope does best...low power, wide field. This might seem a duplication with the standard 25mm supplied but the improvement in image quality across the whole field can be worthwhile. If you're happy with the standard one, then fine. Most people find themselves getting more critical of image quality as they get more experience, and perfectionism quickly turns into a slippery and expensive slope! Some very expensive wide field eyepieces only serve to show up imperfections like field curvature..if you can try someone else's eyepieces first.

Virtually all deep sky objects look like faint bits of cotton wool ..you have to train yourself to pick out the limited detail visible....dark adaption is very important. Going to a dark site helps enormously with any scope. 

A lot of the fascination is in understanding what you're looking at rather than the immediate visual impact. Having a target list which you've previously researched a bit helps keep up the interest. 

Sorry for rambling on...stick with it!

RL

 

[John]

5 minutes ago, rl said:

....A lot of the fascination is in understanding what you're looking at rather than the immediate visual impact.....

 

Very, very true

[Ags]

I have an ST80 and it gives great views. I will always remember the stunning view it gave me of the Orion Nebula - it was like an astrophoto. I was using a 9.4mm eyepiece at the time. The secret of the great view was not the eyepiece or my skill (ha ha) as an observer, but simply the darkness of the site. Orion has a lot of structure to see - but that delicate structure is obliterated by city lights. 

On another dark night I looked at Andromeda with a cheap pair of 15x50 binoculars bought at a tourist trap in Spain. It was the best view I've ever had of the galaxy, including other times where I observed it from on top of a mountain with a 16 inch reflector! The reason is simply the huge object is best viewed at lower magnifications only possible in binoculars and small telescopes, so for Andromeda in particular (and M33 too) the ST120 is one of the best tools to use - from a dark site. Under urban light pollution you will only get the bright core, with hints of "wings" on either side on the very best nights.

For planets and the moon, you might want to consider getting a little Maksutov. For example I use a Skymax 102 and for a reasonable price it gives really good views of the Great Red Spot, Saturn's Rings, and detail on Mars (but not last year - all I could see was dust clouds... no fault of the scope). Stopping down the ST80 to 50mm gives an F8 scope that can reveal some planetary detail, but a mak is much better for the task.

[Cosmic Geoff]

With my Startravel 102mm, using a x2 Barlow lens seems to reduce the chromatic aberration, in effect changing it from a f5 achromat to a f10 achomat.  I used the stock Barlow that IIRC came with the telescope.  It also of course doubles the image scale and the magnification of any eyepiece used.

I am not a fan of stopping down a telescope. If you do that, you might as well have bought a smaller and cheaper telescope in the first place.?

[vlaiv]

2 hours ago, Cosmic Geoff said:

With my Startravel 102mm, using a x2 Barlow lens seems to reduce the chromatic aberration, in effect changing it from a f5 achromat to a f10 achomat.  I used the stock Barlow that IIRC came with the telescope.  It also of course doubles the image scale and the magnification of any eyepiece used.

I am not a fan of stopping down a telescope. If you do that, you might as well have bought a smaller and cheaper telescope in the first place.?

Barlow lens won't actually help with CA introduced at lens even if scope operates on apparent double focal length and hence slower F/ratio. What can happen (but is not guaranteed) is that you are in fact also stopping down scope - if your barlow has small clear aperture. This will not show as vignetting if stop is far away from focal plane and barlow is used short focal length eyepiece (almost always the case) - which has smaller field stop anyway, but what it will do is prevent light from outer parts of the lens to reach focal plane - most CA is introduced at the edges and almost none in the center (this is why stopping down scope works in the first place).

I think that you are looking at stopping down a scope in the wrong way - you can always stop down larger scope to make it slower, but you can never "widen" smaller scope to make it fast

 

[domstar]

Hi, I hope you enjoy your scope- you already have more aperture than me. To improve your views, try looking at the object for longer and you'll see more detail. Also, with the Orion Nebular and the Andromeda Galaxy, try using averted vision, that is, not looking directly at the object but looking around it. If you look directly at the Andromeda Galaxy it's disappointing but avert your gaze in a low power eyepiece under dark skies and it can be amazing. 

As Peter Drew says above, your scope will do everything on your list.

Have fun.

[dannybgoode]

On 08/01/2019 at 22:36, John said:

A lot of the fascination is in understanding what you're looking at rather than the immediate visual impact

Absolutely this. That smudge you saw that was M31 is over a trillion stars and the photons that have hit the back of your retina have travelled over 2.5 million light years. 

And you’ve seen it, from your garden. Isn’t that awesome? Blows my mind every time.