Refractor advice?

[Seamaster]

Hi, I am looking a a refractor on a non GoTo mount.

I want to look at planets but also some DSO's.

I am what you may call (amongst other things!) an opportunist or spontaneous viewer with no interest at present in photography.

I'd also like some degree of portability although they all come apart for transport I suppose.

I want to know the pros and cons over for example...

Skywatcher "Startravel" 102 short tube f6? over the same scope in a 120 aperture?

I have been advised by a reputable retailer that the 120 short tube "Startravel" will produce bad chromatic aberration whereas the 102 version will produce less with negligible reduction in performance?

Is that true?

Also there is the longer tube "Evostar" 102 or 120 does the same logic apply to them?

For £350 would you go for a 102 Evostar over a 120 Startravel? (Budget driven)

Would the 102 Startravel be any good for planets and the higher mags required!

You get the idea?

Any advice welcome but please remember I don't want to be messing with collimation!

Anyone got any of these refractors I can perhaps have a look through to see if I can live with the chromatic aberration I keep hearing about!

Thanks

Neil

Derby

Edited March 8, 2015 by Seamaster

[Perkil8r]

I think the st120 has poor objective lenses if memory serves, not heard of those issues with the evo though. I would say the advice is fair. The evo will be better for planets but the St better for dso's because of the faster optics.

[+Graham]

For your budget you might be better off going for a second hand ED80 

[Guest knocker]

I agree with Graham the ED80 is a supper scope, I just got one my self BUT if you can find one second hand good luck with that I was looking for over a month in the end I got a new one from FLO. So if I was you save up the extra £85 and get a new ED80. one, its far superior in the way of both build and glass and two, going on what sort of star gazer you said you are it will last you a life time.

 

You will of course need a mount

Edited March 8, 2015 by knocker

[Werisit]

Hi Neil. I have no experience of the ST 120 but as I've said on your other post I have a ST102. I can't see any reason why the optics on the 120 would give worse CA than the 102. They both have about the same focal ratio (more on that later). At additional cost, you could add a Baader semi apo filter. Maybe have a  look at the reviews. The fringe killer certainly helps on the 102. I went for the fringe killer  rather than semi apo because it has less effect on image brightness. It does give a yellow tinge but I find I get used to it and don't notice it after a few minutes. I believe the semi apo gives more natural colours.

 

The evostar 120 has a focal length of 1000mm (f 8.33). The higher focal ratio should produce less CA and would also give higher mag for a given eye piece so would be better for planets. As for DSO's, my 120ED has a fl of 900mm which isn't too different and I can find quite a few faint fuzzies (no go to) with that.

 

The ED optics are in a different class to the ST and evostar. Still a doublet but exotic glass. I think the only CA I've ever seen in the 120ED was caused by atmospherics and was only very minor. On that note, maybe, as Graham suggested, an 80ED would fit the bill. Small, light, almost no cool down time and will go on a light weight mount. Imho, the quality of the glass is more important than aperture.

Plenty of head scratching for you! 

[Tweedledee]

From what you have said here and in other threads about your opportunist quick look style of stargazing, I don't think you'd notice a massive amount of difference in the views through the different scopes. If you are planning longer observing sessions with proper cool down times, then you would have time to notice the quality improvements. As said many times before, buying scopes is always full of compromises. Some people are less bothered than others about CA. A longer focal ratio will show less chromatic aberration than a shorter one. A longer focal length will generally provide better views of planets as higher powers are easier to achieve and CA is minimised. A larger aperture should show slightly fainter objects and give better views of DSO's. The higher quality scopes with crisper star images and minimal CA cost more.

In defence of the ST120 which I own and use, I have seen some fine views of DSO's and rich wide star fields through it, but with fainter fuzzier objects and generally lower powers used the CA is much less noticable. I can't say I've had good views of planets through it, since brighter objects do show CA more and the short focal length does not easily allow high powers (needed to show detail) to be used very well. For the ridiculously cheap price I paid secondhand for my ST120, it definitely has some bang for the buck, and is a robust scope.

I would say that you'd be better off with a longer focal scope, up to a quality you can afford. The longer focal length will automatically mean less CA and should give good higher powered views of the planets and moon, yet will still be excellent for DSO's. The longer focal length would be a better all rounder.

I'd be happy to let you have a look through my ST120, but the test would only be of use if trying side by side with a better quality scope. And such a test might also help me make a decision about a better refractor.

Good luck sorting out the compromises.
 

[philjay]

St120 = fast, decent apeture, higher ca than its slower evostar kin. Good for dso, not great on planets and lunar but its passable it depends 8n what you want. Good portability short tube makes for a very portable reasonabke apeture scope

Evostar 120 = slower, decent apeture, ca slightly better due to longer fl controlling ca more effectively. Ok on dso but better than the st120 8n planets and lunar. Longer tube so less portable, this is actually a big telescope.

Same applies to the 102 scopes but you are getting less apeture

The optics on all these scopes are achromats which have been built to a budget so dont expect apo performance. They are great for general viewing and beginners

[Seamaster]

Thanks, can I ask a couple (well a few actually) of questions?

I have been told by one scope retailer that I will not see any detail at all on planets with the 120 ST (Startravel) just (quote) "featureless bright discs"

Is that true?

I understand that the SW ST is not the best for planets but surely it will reveal some detail such as the banding of Jupiter and the rings (and the division?) of Saturn?

Another retailer told me that a 102 ST will produce less CA than the 120.

Is that true? If it is will I notice a significant difference in performance such as what I can see and useable magnification from a 120 to a 102 ST?

Will a Dielectric diagonal help with CA?

Will a Fringe killer help with CA and what (if any) are the draw backs of using one? )

I have been told they may reduce the image brightness and sharpness...but...would a Dialectric diagonal compensate for that by increasing light to the eye?

Does a Barlow help with CA and how? (And are there any down sides to that?)

If I have a 120 ST and use all of the above will that make for a noticeably better, clearer, more crisp and detailed view through the eyepiece?

Sorry to ask so many questions but I seem to be getting many conflicting and confusing opinions from retailers or perhaps it's just me getting confused?

If anyone close to Derby has a ST I can take a peep through that would be appreciated?

Thanks.

Edited March 11, 2015 by Seamaster

[Perkil8r]

You will, or should, be able to see banding with an ST120 or 102, so that I would say is not entirely true. I refer you to my previous comment regarding the quality of the optics in the 120, the 102 has slightly better optics I am led to understand.

 

The rest I cannot really help with.

 

I do have a 120ST at work that if you cam to a meet, and let me know before hand, I could get out and let you have a look through.

[Seamaster]

Thanks Mike, can I ask why the 102 would have "better" optics than the 120?

I presume the aperture size is not the only difference then?

Are you suggesting the 102 would be almost as good with less CA?

Or...would I lose more from a smaller aperture on the 102 than from slightly more CA on the 120?

[+Bino-viewer]

Neil.....if you're free sometime, pop round to mine and you can take a look through my 132mm.

12mm extra isn't really a lot of extra aperture, and i'm surprised that dealer told you that.

I can see loads of detail on Jupiter on a good night.

 

Remember seeing is  all important where planetary observation is concerned.

 

FWIW, i've never even seen the Cassini division on Saturn. 

In fact i've not observed it for a couple of years as its so low in the sky to be even bothered with and will be for the next 10-15 years.

 

I'll PM you with my details.....

[Perkil8r]

Since both are mass made, the figure of the optics in the 102 are better than those of the 120 from what I have read. For instance the st80 is renowned for being a particularly good figure for the money. It purely means they got the lenses right and did a good job on them whereas on the st120 they did not do such a good job.

 

Aside from focal length, which I don't know off hand what they are for both, you are not likely to note much difference on planets between the two other than better CA on the 102. For DSO's though the light will be much less bright in a 102 compared to the 120

 

Edit:

Much less may be a little misleading, it will not be huge as such but you will not see as many objects in the 102 due to less light grasp.

Edited March 12, 2015 by Perkil8r

[Perkil8r]

If you have a look about halfway down this page: http://en.wikipedia.org/wiki/Chromatic_aberration you will see about doublets. In simple terms the two lenses in the 102 are better matched than in the 120, hence CA is less severe with the 102.

[philjay]

No problems with the questions this is what its all about ask away

 

Folks above have answered  most and heres my 2 penneth worth

 

 

 

I have been told by one scope retailer that I will not see any detail at all on planets with the 120 ST (Startravel) just (quote) "featureless bright discs"
Is that true? No its not true,

 

I think the retailer has not looked through one. Yes there is CA and yes its worse than a longer FL scope but Im afraid this advice is not correct. I restarted astro with an ST120 back in 98 and regularly viewed Jupiter belts, GRS, transits also Saturn detail on the disc, cassini division, Mars Syrtis Major, polar caps.

 

I understand that the SW ST is not the best for planets but surely it will reveal some detail such as the banding of Jupiter and the rings (and the division?) of Saturn?
See above

 

Another retailer told me that a 102 ST will produce less CA than the 120.
Is that true? If it is will I notice a significant difference in performance such as what I can see and useable magnification from a 120 to a 102 ST?
 

Hmm, as per Mikes answer above but with reservations, it all depends on the figuring, these are machine made optics to a budget and with some QA, the quality of these vary I know I have looked through many, in other words not all SW budget optics are equal. 

 

Will a Dielectric diagonal help with CA?

No, the CA is in the design and material of the objective, the diagonal just reflects that.

Will a Fringe killer help with CA and what (if any) are the draw backs of using one? )

Yes and Yes.

Yes it will help a little with CA.

These work by cutting out some of the wavelengths of light so you loose by the image not being as bright and some are tinted yellow/green. They are not the answer to CA but help a little if you can put up with them, I don't like them personally.

 

I have been told they may reduce the image brightness and sharpness. See above

 

..but...would a Dialectric diagonal compensate for that by increasing light to the eye?
A bit of extra light always helps but it wont be that much, I know Ive been there done it got the Tee Shirt

 

Does a Barlow help with CA and how? (And are there any down sides to that?)

No, it magnifies it

If I have a 120 ST and use all of the above will that make for a noticeably better, clearer, more crisp and detailed view through the eyepiece?

 

No sorry it doesn't work like that, we all wish it did as we wouldn't feel the need to spend thousands on Apos

 

Hope that helps Neil, these are all questions I have asked myself in the past and answered through practice with my ST120 way back when

 

I loved my ST120, it was as near to a general purpose grab and go scope as I could get at the time. Yes it had more CA than the Evostars but it was more practical and brought loads of DSOs into view. It gives average views of lunar and planetary but If you want pin sharp views with no ca then Im afraid you will have to look elsewhere and stretch the scope tube and budget
 

Edited March 13, 2015 by philjay

[Seamaster]

Thanks,some great advice from you guys!

Regarding mounts?

I don't want GoTo and that leaves me with either an EQ or an Alt Az mount.

I don't intend to use it for imaging/photography and unlikely to really need to track anything.

So I need a stable mount that won't shake every time a moth flys past or when I try tot week the focus, that I can use in a quick, simple and no fuss or set up way?

A lot of scopes in my budget seem to come on either an AZ3 Alt Az or an EQ3 mount.

Is an AZ3 any good for a ST102 or 120 or do I need an AZ4?

Can an EQ be used as if it were an Alt Az in other words as easily moved around and without having to align etc or will it be a bit if a faff compared to a simpler Alt Az mount?

Would a ST102 on an AZ4 be a good match or will the AZ3 be more than adequate?

I suppose I could look at a Mak but they are more cash for a SW Skymax 127 and over my budget on an AZ4 although not on an AZ3 (back to the question of it being stable or sturdy enough for the scope?)

Thanks

Neil.

Edited March 13, 2015 by Seamaster

[philjay]

All good mounts, I had my st120 on an eq3/2 was perfect for it, get an ra drive and you have a good set up

[Seamaster]

Phil, but doesn't the EQ mount take more setting up and isn't it harder to use as a simple "move the scope around wherever" mount like the AZ mounts?

I'd rather have a nice solid and simple AZ but which one?

Would the AZ3 be ok on an ST102 or 120 or a 102 / 127 Mak for that matter?

Edited March 13, 2015 by Seamaster

[philjay]

Set up is just point the ra axis somewhere near the north celestial pole, done in seconds. There is alot of buncum talked about eqs neding setting up, only if your imaging, for visual point it in the general direction of polaris and your fine for low to mid mag visuals.

If your planning on a 127 mak its field of view is much smaller than an st, f12 compared to f5. So if you dont like the extra 2 seconds setting up with an eq over an altaz then you will not get on with a mak.

[Tweedledee]

The AZ mounts are slightly quicker to set up than eq, as they don't need polar alignment. If you take the extra time to polar align an eq, then you have the ability to track your object by simply pushing only one axis, and better still with an RA motor drive keeping the object in view, but of course that means even more setting up to do.

 

I didn't get on well with the AZ3 and ST120. I kept needing to re-adjust the tightness of the altitude axis bolt using a spanner whilst observing, which was such a faff. It was always either too stiff to move smoothly or would not hold the scope firmly enough when pointing nearer the zenith. When pointing higher the scope became slightly off balance due to the design of the mount. That is just my experience with the AZ3. Other AZ3 mounts may work better, or maybe I just didn't have it set up correctly.

 

Now I use the ST120 on an AZ4 and absolutely love it on that mount. The AZ4 can be had very cheaply secondhand and works perfectly for me. As with anything, you can spend loads of money and get much better quality and sturdier tackle, but I find that the ST120 on the AZ4 makes a nice budget grab n go setup.

 

As you can see below, I have a slight tendency to overload things ...

 

 

Edited March 13, 2015 by Tweedledee

[Guest]

I have an ST80 and Skymax 127. Horses for courses, as they say. I've got a 32mm Plossl and Antares focal reducer which gives a whopping 4 degree field of view with the ST80. I even did DSO photography using a compact digital camera afocally. I only stopped it when I bought my first DSLR. It has even been to Kiwiland where I mounted it on a camera tripod and I split Alpha Centauri.

 

The Skymax 127 is great but cannot go on 'planes unless someone can lend me a tripod at the other end.

 

There is a Skymax 90 that can fit on a camera tripod. I haven't got it but you should get more than 2 degrees field of view with a 32mm Plossl and focal reducer. If you are on a budget, I'd suggest it's worth looking at.

[philjay]

They look nice in the black diamond livery Pete. Mine was the all black Helios, I fitted a revelation crayford, super scope.

Sorry Neil, my tab ran out of charge so my last post finished abrubtly. Dont be scared of eq mounts, there are lots of ocd posts about polar aligning, if your doing grab and go visual all you need is the polar axis permanently set at 52ish degrees. Then when you set up you just stick the tripod on the ground with the ra axis pointing somewhere near polaris, this is fine for low mag stuff that the st scopes give you.

Plus you have the option of Ra drives if you want to upgrade, cant do that with an altaz. Plus an eq3 can be set in altaz, just stick the ra vertical. :-)

The mak has a smaller field of view, is great on planets and ok on dsos when you can find them. Things arent as easy to find in a mak. Plus a mak requires far more cool down time than an st so the views wont be good on planets for at least 45 to 60 mins, I know I used to have a 127, I now have a 180 which takes even longer to cool. A mak is a good scope but its not really a grab and go, its more if a grab, wait and go for lunar and planets

[Tweedledee]

They look nice in the black diamond livery Pete. Mine was the all black Helios, I fitted a revelation crayford, super scope.

Cheers Phil. Your focuser was probably worth as much as the ST120 But I'm sure it would be a nice addition to a short ratio frac.

[philjay]

Take a look here as well Neil

http://stargazerslounge.com/topic/239148-skywatcher-st-120mm/

[Tweedledee]

Take a look here as well Neil

http://stargazerslounge.com/topic/239148-skywatcher-st-120mm/

Cheers Phil.

Nice to hear some praise for its strong points, not just about the CA.

[Guest]

They look nice in the black diamond livery Pete. Mine was the all black Helios, I fitted a revelation crayford, super scope.

Sorry Neil, my tab ran out of charge so my last post finished abrubtly. Dont be scared of eq mounts, there are lots of ocd posts about polar aligning, if your doing grab and go visual all you need is the polar axis permanently set at 52ish degrees. Then when you set up you just stick the tripod on the ground with the ra axis pointing somewhere near polaris, this is fine for low mag stuff that the st scopes give you.

Plus you have the option of Ra drives if you want to upgrade, cant do that with an altaz. Plus an eq3 can be set in altaz, just stick the ra vertical. :-)

The mak has a smaller field of view, is great on planets and ok on dsos when you can find them. Things arent as easy to find in a mak. Plus a mak requires far more cool down time than an st so the views wont be good on planets for at least 45 to 60 mins, I know I used to have a 127, I now have a 180 which takes even longer to cool. A mak is a good scope but its not really a grab and go, its more if a grab, wait and go for lunar and planets

One thing I added to my Mak and one of the best £50 odd quid I ever spent was a 9x50 RA finder. I even saw M33 through it! OK, from a dark site. It helped me finish the Messier Catalogue.