Thinking of getting a SW ST 120

[Paul73]

Hi Guys

I'm looking for a little Frac that will complement 10" Dob. Ie. Nice wide fields, act as an all round grab & go and be small enough to take on holiday (in a car full of family kit).

I can't really afford a little ED which would be favourite, but this scope was reccommended in a previous thread as a a good all round alternative.

I am a bit worried about the Chromatic Abraision from this type / size of scope. Does anyone have any thoughts / experience /advice /alternatives.

Thanks

Paul

[xtreemchaos]

i dont worry about ca its there with big fracs it like dif spikes with refecs its just one of them things,you can make it min with the right filers..good luck with your choice mate clear skys..charl

[laudropb]

Hi Paul. I am going through the same process as you. I was lucky enough to get a view through the SW ST 20 the other night. The CA was very obvious on bright objects. I found it very off-putting when looking at some doubles. Otherwise the views were very sharp and had good contrast. It all depends on how much the CA bothers you. Some people do not mind it, but it's not for me.

[Moonshane]

I previously had a 120mm F5 skywatcher and really bought it mainly for white light solar with a Lunt wedge. It excelled at that although there was some distortion at the edge of field (field curvature?) when the solar disc went to the edge of field.

In use at night, it was as you'd expect. Brilliant for wide fields, great for all but the brightest doubles and OK on planets and moon - I also found the CA a real distraction. I used it happily to about 100-120x or so.

I now have a 80mm triplet which I bought for a further £80 and I find this as good for solar, better on doubles and planets/moon and although I have not yet tried it in truth on fainter stuff, I presume it will not match the 120.

For a scope away from home, are you really going to be looking at moon and planets? After all you can do that anywhere.  I'd have thought that although to some extent you will of course look at solar system objects, it's going to be most useful for fainter/wider stuff at places darker from home. On this basis I'd not recommend it as a singular scope but for the purposes you describe, especially the holidays away, it would be a good choice I think.

[damnut]

Hi There,

I have the ST120 as a big finder on my skytee and my grab & go scope of choice. As others have mentioned there is some CA if you look at brighter objects. However the widefield views are excellent. The other thing is that I would not treat a small apo like I treat this scope. It is bullet proof, travels on the back seat, roofbox or boot. Impervious to sand, snow & dog slobber, put it on an az4 and you are good to go.!!

[Paul73]

Thanks guys.

Does quality of eyepiece help the CA?

Paul

[damnut]

Hi Paul,

It does to an extent, but it only applies to bright stuf!. However out of curiousity I tried a fringe killer and a semi apo filter - don't bother! they just gave a sickly colour to everything...(and they are expensive). But I only use the big ep's at home. If I am on hol I don't mind dropping a Moonfish or a zoom, I would mind dropping an axiom in an unknown site! (or 3' of snow  )

[xtreemchaos]

i find a simple moon filter helps loads with planets ...charl.

[Paul73]

i find a simple moon filter helps loads with planets ...charl.

Cool. I've Got a decent one of those kicking around somewhere.

Could I get a clean colourful split of Izar under normal conditions without finding my view wrecked with CA?

Paul

[xtreemchaos]

 i dont now about splitting izar il give it a go with my ar127l a soon as i get clear skys and let you know cos if its good with the 127 it should be better with 120...charl...

[Moonshane]

Possibly might split Izar. The issue would possibly be magnification required. I find it's best around 150x which the 120mm f5 might struggle with. Don't think I ever tried when I had mine. It's an easy split in the 80mm apo.

[Moonshane]

On the whole I'd think a 100mm f10 Tal or similar would be a better option if you have interest in higher magnification targets.

[John]

On the whole I'd think a 100mm f10 Tal or similar would be a better option if you have interest in higher magnification targets.

Agreed. Izar is relativly easy with the TAL 100 as is Delta Cygni. I've split down to 1.4 arc seconds so far with the rather old TAL 100 that I picked up recently. I've been using 159x for that which is out of the comfort zone for the F/5 achromats I reckon.

[damnut]

To rule out a grab & go scope for splitting a double star seems extreme. The grab & go is always a compromise. Izar is a 1.4 arc seconds double , the Dawes limit for this scope is .97 arc seconds. I'll have a look tonight if clear. If you are not looking for something portable as above look for a longer focal length instrument.

Plug your scope data in to this :---

http://www.stargazing.net/naa/scopemath.htm

[spaceboy]

An ST120 for wide field will do a good job as long as what your looking at doesn't have a very bright star amongst it. Filters do work to a degree but you still need to be accustomed to CA as it doesn't eliminate CA only reduces it. It is also worth noting that although it is a refractor at f/5 edge performance will suffer in a similar way as a fast newt.

[ollypenrice]

I previously had a 120mm F5 skywatcher and really bought it mainly for white light solar with a Lunt wedge. It excelled at that although there was some distortion at the edge of field (field curvature?) when the solar disc went to the edge of field.

In use at night, it was as you'd expect. Brilliant for wide fields, great for all but the brightest doubles and OK on planets and moon - I also found the CA a real distraction. I used it happily to about 100-120x or so.

I now have a 80mm triplet which I bought for a further £80 and I find this as good for solar, better on doubles and planets/moon and although I have not yet tried it in truth on fainter stuff, I presume it will not match the 120.

For a scope away from home, are you really going to be looking at moon and planets? After all you can do that anywhere.  I'd have thought that although to some extent you will of course look at solar system objects, it's going to be most useful for fainter/wider stuff at places darker from home. On this basis I'd not recommend it as a singular scope but for the purposes you describe, especially the holidays away, it would be a good choice I think.

This is exactly my view of the situation.

Olly

[Paul73]

Moonshane & Olly

You are dead right. For planets I'd be looking to my dob from home.

I was trying to get to; at what point does a star become bright enough for the CA to become an issue? I just picked Izar as a mid range bright double which, given some tidyish optics, can be split happily at 150x. I guess that I need to start thinking lower mags!

Paul

[Moonshane]

You may be better getting a couple of folk that still have a 120 f5 to have a look and see what's what. I cannot remember using my old one on izar but liked the aperture so much I sold it to part fund a 120ed. This splits this sort of double with some ease.

[Moonshane]

P's don't get me wrong. I really liked it for what it did well and the price Is low I just wanted ca free views myself

[John]

I was trying to get to; at what point does a star become bright enough for the CA to become an issue? I just picked Izar as a mid range bright double which, given some tidyish optics, can be split happily at 150x. I guess that I need to start thinking lower mags!

Paul

I have the feeling that CA bothers some people more than others. I've seen reviews of the 120mm F/5's that state that the observer found the CA bothersome and distracting at anything above medium magnifications and others where the CA was noted but was not considered a significant issue. As the levels of CA are largely determined by the achromat objective design and specification (ie; aperture and focal ratio) and are probably not going to vary massively example to example, I can only assume that some people find it more of an issue than others.

I guess the only way to find where you are on it is to have a look for yourself if possible.

As a way of indicating the relative levels of CA that achromats of varying specifications are likely to produce this table seems broadly accurate from my experience:

The manufacturers state that these F/5 achromats were designed primarily for low to medium power wide field observing so I guess they should know !

[Paul73]

As many wise astronomers have said "it's a game of compomises". What I want and what I can afford are two very different things.

I know that there will be some CA, but the question is, how intrusive? Very subjective.....

Sounds like if you steer clear of the close-ups everyone likes them. As you up the mag, the CA becomes more pronounced. Where the acceptability line is drawn depends purely on the individual.

Paul

[damnut]

If it would be your only scope no. If you accept that there will be a level of CA then prepare for it to be your most used scope. On an az4 it is a setup in a minute.

If you take to a dark site or on hol, then you will not be stressing the scope with high mag. Else pack your dob, or an 80mm apo that will split doubles  . A mak that will be excellent on planets. A grab & go is a compromise full stop. 

[F15Rules]

For similar portability and better optics I'd go for a Talk 100RS. It will lose out on a few fainter objects (but it's all relative as neither is truly a "deep sky" scope).

In all other respects the Tal is better IMHO.

You will also find on planets that various (quite cheap to buy) coloured glass filters can bring out details and suppress the worst of the CA (which is much less than the ST anyway).

Using 2" EPs in the Tals' pretty good crayford will will also allow fairly wide field visual views too. Its quite light weight and easy to transport. You could remove the dewshield and focuser for transport quite easily.

As it happens a very nice 100RS has just gone up for sale this morning for £160 on UKABS (nothing to do with me BTW:-).

Good luck!

Dave

[damnut]

I would agree except the op wants a small refractor. The 100rs is f10.

[F15Rules]

..but most of the above posts are referring to an ST120 F5, hardly a little scope :-)

The Tal would be lighter if anything than the ST120.

Dave