DSO AP: Frac, Newt or Mak?

[pvaz]

Yeah, just checked and its only a little over an fstop from f/7 to f/11. For some reason I thought it was closer to 2 full stops.

[inspectapart]

And manufacturers aren't all that honest about their descriptions. Some use words such as 'semi-APO' or ED APO, but to be an APO it must be a triplet.

You can generally tell though by a careful examination of the price tag:-)

Typed by me on my fone, using fumms... Excuse eny speling errurs.

I don`t see why a scope has to be a triplet to be classed as APO! I have a Vixen 102fs fluorite which is a pure fluorite doublet, just like a Takahashi doublet, and I can definately say that my scope is an APOCROMATIC telescope. In fact, probably one of the best 4" Apo`s you can buy!

Bob.

[ollypenrice]

I'm afraid I disagree with one earlier post quite strongly. For imaging you DO need an apo. There is a lot more to it than refocusing for each colour filter. Even within one filter you will get chromatic abberration, notably in blue. And then, when you combine the three colours, the individual focus of each colour means that each was shot at a slightly different focal length so when you recombine them you reintroduce the CA unless you use some pretty fancy software. So I thnk that in refractors it has to be apo or nothing.

Interesting that Tim, a highly respected imager, would go for an SCT as his only imaging scope. I'm pretty keen myself and an SCT would be my last choice and a refractor my first. Which only goes to show that asking astronomers for advice is like herding cats!! Good luck to you!

WIdefield? Nebulae are generally widefield objects though planetary nebulae are usually small. Galaxies, other than M31, M33 and M101, are small and need long focal lengths. (around 2 metres and up). You can't have both in one scope. I image at 328, 450, 980 and 2,380mm. The two short FLs and the long one are by far the most productive. The short FLs make guiding easy on ordinary mounts. The long FL runs on a huge premium mount (not mine) and I would not want to use it on anything less.

I'd say that to learn DS imaging, which is hard enough at the best of times, give yourslef a break and start with a small refractor. This was with an 85mm, albeit a good one. http://stargazerslounge.com/topic/165771-at-the-risk-of-boring-you/

Olly.

[x6gas]

I'm afraid I disagree with one earlier post quite strongly. For imaging you DO need an apo. There is a lot more to it than refocusing for each colour filter. Even within one filter you will get chromatic abberration, notably in blue. And then, when you combine the three colours, the individual focus of each colour means that each was shot at a slightly different focal length so when you recombine them you reintroduce the CA unless you use some pretty fancy software. So I thnk that in refractors it has to be apo or nothing.

Olly.

I stand corrected and defer to Olly's superior knowledge.

[laser_jock99]

I don`t see why a scope has to be a triplet to be classed as APO! I have a Vixen 102fs fluorite which is a pure fluorite doublet, just like a Takahashi doublet, and I can definately say that my scope is an APOCROMATIC telescope. In fact, probably one of the best 4" Apo`s you can buy!

Bob.

Technically a Newtonian is 'apochromatic' as well- there are no lenses just first surface mirrors so all wavelengths of light come to focus at the same point. An important often ignored by refractor buffs. Critically this may be important if you are planning to use a full spectrum camera (UV through to near IR) like do. Even the best triplets will struggle with this as they are optimised for visual use.

The practical outcome of this is that I can use the full spectrum camera on a Newtonian without worrying about UV haloes & Infra-red haloes you might otherwise get with a lens system. The downside is that corrective optics (eg coma correctors) are often only optimised for visble light.

[ollypenrice]

I don`t see why a scope has to be a triplet to be classed as APO! I have a Vixen 102fs fluorite which is a pure fluorite doublet, just like a Takahashi doublet, and I can definately say that my scope is an APOCROMATIC telescope. In fact, probably one of the best 4" Apo`s you can buy!

Bob.

Well... plenty of doublets are effectively apochromatic in visual use but they are unlikely to be truly apochromatic in imaging unless on the slow side. The Sky 90 certainly isn't, it shows softness in the blue channel. So does the Tak FS60 - big time. The slower the F ratio the more a doublet can be accurately corrected. Mind you, there are triplets out there which struggle in blue. I had a Meade 127 triplet which always bloated the blue stars and even our TEC140, which is absolutely exquisite visually, does let a bit of blue escape pure focus on the chip. The most apochromatic apo I've ever imaged with is the Tak FSQ85. It is even tighter on colour control than the 106, I think. The TV quadruplets are also remarkable.

That said, you are right that the FL Vixen is a killer apo, especially in visual use. Have you imaged with it, Bob? I haven't, though I've seen an ED in imaging action.

The only way to image without any glass throuput at all would probably be in an RC at native FL. A bit slow though.

Olly

[Planetesimal]

How much should I expect to need to save up for an apo 85mm? Are we talking 4 figures here?

I should probably just get a small, fast newt

[pvaz]

How much should I expect to need to save up for an apo 85mm? Are we talking 4 figures here?

I should probably just get a small, fast newt

As you're just starting, a sturdy mount (HEQ5) and a decent doublet should be more then enough. You can see excelent pictures with this setup. Your next addon should be a guidding setup as it will make a bigger impact on the final results, then a triplet. A small fast newt will require perfect collimation. Usually people end up buying expensive collimation aids for that.

Once you know how much time and effort it takes to make an image and if you find it's worth stepping up to a tripplet, you can easily resell the ED80. If you get a used one, you'll probably get all your money back. Olly's advice is great, but keep in mind he runs a lodge in South France where he makes a profit from tourists wanting to spend some relaxing days enjoying the country by day and stargazing at night, so whatever he spends on astro gear is a profitable investment and he probably gets a lot more clear nights there, then you will in the UK.

[Planetesimal]

Okay then, on balance, as a beginner, I think I need NEQ6 pro synscan, SW ed80, a canon DSLR and a copy of "every photon counts". Actually I intend to read the book and pay a visit to my local AS before spending a penny on the equipment - the mount is probably a no-brainer though!

[michael.h.f.wilkinson]

My 80mm triplet with flattener was just shy of 4 figures in euros (848 euros to be precise). They have gone up in price a bit since then (by 30 euros or so)