Rich Field Refractors??

[catburglar]

I currently have a WO Zenithstar 80 mounted on a Skywatcher AZ4 but would like a little more light grasp. Having recently read the S@N review of 6 inch achromatic refractors, I'm thinking of getting a 5 or 6 inch rich field refractor to go with this mount.

Does anyone have any experience of this type of scope in comparison to the ZS80 at magnifications up to around 150x? How good/bad is the chromatic aberration?

Secondly, would these be OK on the AZ4- I've heard that the 6 inch scopes are a bit of a handful?

[Peter Drew]

I use a 6" F5 Skywatcher refractor as a finder on a 16" SCT. The stallar images are excellent and there is no objectionable chromatic aberration at low power. 150X magnification is possible but the false colour is noticeable on bright objects and is not really the rich field end of the performance. The optical tube assembly is pretty heavy and very biased towards the objective end.

[catburglar]

Peter, thanks for this - for the most part I'd be using magnifications in the range 40-80x I guess, but occasionally higher say for globular clusters. I was hoping I'd still get a reasonably sharp image.

I don't suppose you have too many balance issues if you've got a mount that can manage a 16 inch scope- but I have read elsewhere that the 6 inch OTA is very front heavy- hence my concerns about the AZ4 and also why I'm considering a 5 inch which will still get substantial increase in light grasp over my current 80mm.

Thanks again

[ollypenrice]

I use a 6" F5 Skywatcher refractor as a finder...QUOTE]

That's showing off!!!

No, I agree with Peter's assessment. Our club had one once. The thing is, though, that as refractors get more aperture they become harder to colour-correct and have to be made with ever increasing focal length, getting away from the rich field idea. I can recommend an old Gemesis if you can find one. High order colour correction at f5/100mm.

Wouldn't a fast Newtonian suite your needs?

Olly

[catburglar]

I've got a flextube 250, but in short, I don't really get on with it- the diffraction spikes and coma are a lot more intrusive (to my eye at least) than I ever thought they would be. In the past I've had a look through a small achromat (probably st80), and certainly wasn't offended by false colour when viewing the moon. I recently acquired a ZS80 (which apparently isn't quite apochromatic) and love it. I suspect I may just be a little more tolerant of chromatic aberrations than the typical aberrations found in a newt.

I'd love a larger apochromatic refractor, but probably couldn't justify it to my better half at the minute.

Thanks for your input.

[RobH]

I've got one of the older Helios ST120 F5 refractors.....they are now the skywatcher ST120's but are the same scope.

For low power, widefield views it's great.

At high power, the CA can be intrusive, but at x25-x50 or so, it gives good views.

Cheers

Rob

[russ]

No, I agree with Peter's assessment. Our club had one once. The thing is, though, that as refractors get more aperture they become harder to colour-correct and have to be made with ever increasing focal length, getting away from the rich field idea. I can recommend an old Gemesis if you can find one. High order colour correction at f5/100mm.

Olly

I agree with what Olly says. Just got the 102 version and it's a great scope. Performs best between 20x and 70x. Things really fall apart above 100x. I found the 150 version to be a notch worse than the 102 or 120.

As for the AZ4 taking the weight, i don't know. I have the Orion VersaGO (rebadged AZ4) and the 102 is as solid as a rock but the 150 weighs a fair bit more. That said, the 150 is a short tube, so there is no lever arm to worry about.

[PeterW]

At medium to high powers the ST120 will purple haze the view, I use a yellow filter on the moon. I have the AZ3 mount, which I got a counter weight mod for, otherwise using it at high angles was an exercise in frustration. Low power should be fine though. There are some people in the US who have stripped and flocked the tube and use it with the 31mm Type 5 nagler as the ultimate wide field scope, so can't be all bad.

Cheers

PEterW

[catburglar]

My current favourite eyepieces with the ZS80 are 15mm SWA and 7mm Nirvana- these would give around 30x & 85x with an ST120.

I was also thinking of adding a 28mm Nirvana to give around 3.8 degree field at 22x.

[russ]

Your current eyepieces will work a dream with the ST120. And the AZ4 shouldn't have any problems with the weight of the 120.

I use a Vixen Lanthanum 2" 30mm 60deg with the 102 and the FOV is huge!

[Ogri]

Must admit I've gone a bit bonkers with enthusiasm for my ST120.

They are really great for terrestrial use as well. One of those cheap 20mm Erfle EP's for 30 x magnification, works beautifully for me for daylight use (still not managed to try that EP at night yet).

An 8mm Hyperion blows my socks off for night time views at 75 x mag, and it Barlows ok to 150 x mag too.

I don't find CA a problem, but I do steer well clear of bright things, due to my eyes.

Mine lives on an AZ3 mount, with the tube mounted as far forward in the rings as possible.

To me they are the best value 'scopes on the market (but as a beyond hope ST120 addict, I freely admit I am biased).

[catburglar]

Thanks for all the input so far- I guess I'm getting close to a decision point. Thanks to Olly & Ogri I have a real catch 22. I can't justify spending mega bucks on a large apo refractor at F/5 -6 but

• 
  I've got a 10 inch flex tube that I don't get on with, and will be sold once I've identified it's successor.

• 
  I've got a ZS 80 II which I love, but would like a bit more light grasp. This could be sold if I was sure I was getting the right replacement

Should I sell the dob and replace it with an ST120/150 or similar (keeping the ZS 80 for the times when I want a snappy high magnification session) or should I sell both the dob and the ZS80 and get a 100mm F/6 ish ED type scope- sacrificing a bit of light grasp, but keeping the pristine views and widefield option. I'm looking to keep the overall cash outlay modest.

Any more ideas/thoughts/opinions?