Which Apo Refractor for imaging?

[poltabs]

Ok I have decided its time I stop messing about and buy an APO refractor for imaging.

Its whole life will be used for imaging using a DSLR.

Can anyone recommend any particularily good APO refractors for this type of use?

Would prefer a triplet, FPL53 glass etc etc, budget about the

£500-600 mark, second hand rather than new as I would be struggling a bit with that budget new.

Cheers

Kevin

[MartinB]

I assume you want a refractor for convenience, ease of handling and possibly slightly enhanced contrast compared with other designs. If this is the case the next thing to think about is the FOV you want to work with. DSLRs have large chips. Shorter focal lengths will be great for large targets such as M31 but many other targets might not be shown to there best advantage. If I had to pick one scope for that chip size I would probably go for a focal length of 800-1000, this will generally put you in the sweet spot for optimum resolution/sampling rate.

You will need to think about whether you want to go for an astrograph with an inherently flat field (expensive) or use a seperate field flattener..

Then comes optical quality and, unless you have a few grand to drop then that is all about compromise. I don't think you should dismiss doublets and don't get too hung up about the glass type.

Personally I have found my ED120 to be an excellent performer and you may be able to find one at the top end of your price range. If that is too large a scope there is no end of choice. I think it is best to avoid going for a sub F6 scope since, unless it is a very high quality ££££ piece of kit, colour correction will suffer.

[poltabs]

Thanks for the reply Martin, I have a Nexstar 8 GPS for visual and some imaging and an Astrotrac for the widefield stuff, I have decent lenses for the DSLR upto 300mm so defo would like a longer focal length.

I think a field flattener would be the only option in my price range.

The W/O scopes are superb quality although the F\L will be a bit short. The 120 ED you mentioned sounds interesting, are they still made?

Cheers

Kevin

[Ben Ritchie]

What mount do you have?

[johninderby]

There is the brand new TS Triplet FPL-53 APO 80/480 which works out at about £530.00 plus shipping with the current exchange rates. I've had mine for a few weeks and it is a great little scope. Gave me some great views of Jupiter the other night. They do a field flattener for it that isn't too expensive.

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3881_TS-Triplet-FPL53-Apo-80-480mm---f-6---kein-stoerender-Farbfehler.html

It shows just how good the latest Chinese optics are getting. While I think a TMB 80mm triplet is a little bit better (and I've owned one) the TS triplet is surprisingly close to it in performance. I bought mine mainly for use as a portable grab'n'go setup. The Meade triplet has been around for a little while but it's only an FPL-51 equivalent.

There is the Orion EDT80 which has the same optics at £599.00, but that includes a diagonal.

John

BTW There's a nice image of the Rosette Nebula on the bottom of the Telescope Service web-page

[MartinB]

Latest version is the Skywatcher Evostar My earlier version (gold tube) has the same optics but a one speed crayford (I'm not trying to sell it btw!).

Ben will have some useful points to make about mounting.

[Ben Ritchie]

While I think a TMB 80mm triplet is a little bit better (and I've owned one) the TS triplet is surprisingly close to it in performance.

That's interesting - my TMB80/480 was one of the best visual 'scopes i've ever used, and to this day I regret parting with it. For imaging it required the TeleVue TRF-2008 0.8x reducer/flattener (more £££) but was an extremely effective 'scope with it.

Ben will have some useful points to make about mounting.

I'm in complete agreement about the merits of the 800-1000mm focal length range, but it is significantly harder to mount well than a shorter-FL refractor (both in terms of weight - and distribution of weight, which is at both ends for a refractor - and the demands of tracking at the higher resolution). So if it's a more basic mount then i'd be tempted to go for shorter focal lengths just for ease of use.

While I agree that there are many targets that really require longer focal lengths, with the weather in this country it'll be a long time before you run out of targets suitable for a 500mm focal length refractor and a (modified) DSLR!

[johninderby]

That's interesting - my TMB80/480 was one of the best visual 'scopes i've ever used, and to this day I regret parting with it.

It has surprised me. I was looking for a grab'n'go scope along the lines of a Megrez 72, but then I came across the TS 80/480 that is about the same size and weight (the OTA weight is wrong on the TS site) and so I decided to go for it even though it was a couple of hundred quid more, but it is a full 80mm triplet and uses the proper O'Hara FPL-53 glass. I expected it to be good, but not as good as it's turning out to be. It's exceeded my expectations.

If you compared this scope and the TMB side by side the TMB would have the edge in sharpness and contrast, and of course absolutely zero CA, and there would be no question that it was the better scope, but the difference wouldn't be that much, perhaps 10% better. And when you take the price into consideration........

John

[poltabs]

The scope will be fitted to either the Astrotrac or piggy backed onto the NS8.

Am maybe thinking 600-800mm might be the best FL as I have upto 300mm and a 2000mm (1334mm with a .6 reducer) already so its to add to the existing setup rather than replace any part.

I saw the TS range of scopes and they look interesting, what is the build quality like - on par with WO?

Cheers

Kevin

[johninderby]

The build quality on the TS 80/480 is very good. The focuser is about the same quality as the WO and the OTA has a very smooth gloss finish with nicely anodised black parts. The mounting foot is removeable so you can use rings if you want. As to which looks better it depends on whether you prefer the Megrez's pebbly finish or the TS's gloss. They're both very well made scopes.

John

[poltabs]

How about a WO Megrez 90?