120mm Achro refractors

[nitram100]

Hi,

I've been thinking about buying a 120mm refractor, maybe the celestron omni 120mm, because they're so cheap and i've always wanted a refractor, but in all honesty, would it provide any real benefit over my 10" F4.8 Orion Optics?

[roundycat]

they are probably cheap because the price being charged is all they are worth. With an achromat you will likely get horrible blue fringing because they do not focus all colours at the same place. Try before you buy.

Dennis

[GazOC]

The 120mm f8.4s are pretty good scopes IMHO. Some false colour but nothing too bad if you have realistic expectations. The major benifit over a 10" would be ease of setup for quick looks.

Theres are 3 very good deals for these scopes on ABS at the moment.

[TheThing]

I've got a slightly smaller (and shorter) Achromat and I don't have any problems with colour fringing at all - maybe I'm just used to it?

For the money and for a quick grab and go, you can't really knock them.

[PeterW]

120mm f5 gives nice lots of blue fringing, but you can get rid of the worst of it with a minus violet filter. Short focal length will give you wide fields and as mentioned it is a lot more portable than the newtonian.

PEterW

[jgs001]

I've got an 80mm f5 achro (now used for light travel and guiding duties) but I've imaged using it with a SemiAPO filter and apart from blue halos around the largest and brighest stars it's really good. Visually, with the SemiAPO filter, I don't see any CA, and the SemiAPO is more natural colouring than the Contrast Booster or the other one whose name escapes me from Baader.

[GazOC]

"Fringe Killer", I have one and it gives everything quite a noticable yellow tinge.

[TheThing]

It really isn't the problem that it's made out to be. Seriously causes me no worries at all.

[jgs001]

That's the one Gaz... it's not as yellow as the CB... but more yellow than the SemiAPO..

A little off topic, but this thead gives an Idea of the differences in colour and how the CA correction of the filters works... The images in it were shot with the 450d through the ST80...

http://stargazerslounge.com/equipment-reviews/67599-baader-ca-filter-comparison.html

[John]

I've recently picked up a 127mm F9.4 refractor

It's a big old thing and has mounting needs to match. From the couple of nights viewing I've had with it so far, while it won't touch my 12" Lightbridge on DSO's, it's darn good on planets, lunar and double stars

The 120 / F8.3's are good too - and more compact and easier to mount than mine.

As Gaz says, go into one with realistic expectations the they are a lot of fun !.

John

[nitram100]

Theres are 3 very good deals for these scopes on ABS at the moment.

Thanks for the info guys, i've seen colour fringing before through a friends 120mm refractor and it didn't bother me at all.

Thanks for that link Gaz, i actually saw them last night and got me thinking about a refractor. I've asked the seller of the 55 quid scope for some pics as i want to see if it has the adjustable lens cell as i hear some later Helios models had these. If it does have the cell i'll nab it off him, assuming it hasn't already been sold!

[John]

I ...I've asked the seller of the 55 quid scope for some pics as i want to see if it has the adjustable lens cell as i hear some later Helios models had these. If it does have the cell i'll nab it off him, assuming it hasn't already been sold!

Bear in mind you will need to make a dew sheild for that one as it does not appear to have one. Not difficult though !.

I'd be tempted at that price even if it's not the adjustable cell type - you can always tweak the collimation by shimming the focusser slightly.

John

[FLO]

I am a huge fan of these 120mm f8i(sh) refracters. Most of my first year of astronomy was done using an Evostar 120 and Explorer 200p (black Helios versions). You can do a lot of good astronomy with either of them.

Also, John's filter comparison is worth reading, his results mirror our own tests. The Baader fringe-killer is best suited to the short-tube f5 achromats, the longer/slower versions are usually pretty well corrected so the more subtle Baader semi-apo or William Optics VR-1 filters can be used. After saying that, the small amount of CA never really bothered me so I happily went without a filter.

[albedo0.39]

I'm very interesting in getting a new "grab and go" scope for nights when I just cannot be asked to haul my scope out of it's place in the garage and like you had considered a 120mm acrho. When at SSP, I spoke with Steve from FLO who showed me the Skywatcher but I'm looking for an Az3 mount (Steve - as soon as you have worked out a price please PM me... - thanks)

Like you, I'm concerned about the false colour but according to Steve, provided you treat it sensibly and manage your own expectations, it'll deliver some nice views.

I'd be very interested to hear how you get on

[FLO]

Yes, the short-tube Startravel refracters are low-medium wide-field telescopes. So-long as you use them for that you will be pleased but push them too far and they will be quite kaleidoscopic!

[GazOC]

If you're looking at the AZ3 I find the 100mm f5 a better match up for it than the 120mm. The larger scope gets a little bottom heavy when viewing at higher altitudes.

[lunator]

I have the ST120 F5.

It is a great scope. There is colour on Moon but nothing that really bothers me.

Cheers

Ian

[GazOC]

I've just set up a 150mm f5 for tonights viewing, colourful on the planets but not too bad on most DSOs.

[albedo0.39]

Is it colourful in the centre of the field ? From my recall of physics, the only colour abberations are at the edges of the lens, where the light is bent the furthest.. perhaps someone with a scope would enlighten me ?

Thks

[GazOC]

Its colourful at the centre of the FOV. Its slighly worse at the edges but I've always put that down to other aberations kicking in as well.

[John]

The colour seems to fringe the object that you are viewing eg: the lunar limb, Jupiter's disk etc. On my F/9.4 scope it's only a very slight fringe on the brightest objects - not there (or at least not visable to me) on DSO's and stars fainter than mag 2ish.

As aperture increases it's harder to make well corrected achro objectives so an 80mm F/5 shows less colour than a 120mm F/5. To show no colour a 120mm achro would need to be somthing like an F/16 or so I think.

John

[ollypenrice]

I have had both the longer focal length 120 and 150 achromats and would consider them real bargains. I still have the six inch and for visual use it is far better than it has any business to be. On deep sky it is a match for our ten inch SCT and I thought the stellar images were usually a tad nicer. Planetary colour is not that bad.

You wouldn't image with one of these but visually they are great. When Maccholtz was around a visiting group from a London astrosoc made it their instrument of choice for the comet.

My experience of the fast ones, though, (were they f5?) was not encouraging. I couldn't really enjoy the view through those, but it's a personal thing.

Olly

[The Warthog]

My 105mm f/9.5 achro had a little fringe colour when I first got it. It was easy to remove that by putting a pale yellow filter in. I got a MV filter, and now I don't notice frigning at all.

[PeterW]

...AZ3 at higher angles.... use this mod and it's good to go right up to Dobsons hole.... has | AZ-3 Mount Mod

I tend to use a yellow filter for fringe-killing.... OK bit overkill, but allows me to really up the power 120x. I'll look into the Baader/WO options, but the f5 really needs some powerful filtering or you're in for 'purple haze'.

All the best

PEterW

[ploughc]

Hello everyone just reading the views on Achro refractors as I was thinking of getting one to go with my 6 " newtonian my question is do the group members think are the Tal scopes better than the Chinese ones also where could I get the 5" tal refractor. Thanks everyone PAT.