DSO AP: Frac, Newt or Mak?

[Planetesimal]

I had to ask, loving the bizarrely exclusive contractions and acronyms!

Serious question though. I consider my current set-up a learner rig. In the future I think I'll want a separate observing 'scope and an imaging set-up. Probably a 10" Dob for the observing (going up two sizes to get a bit of wow-factor while staying just about practical), and something smaller on an EQ mount with motors etc. for imaging...

Some of the refractors I've seen are very beautiful objects in themselves, but I'm not interested in buying an ornament! From experience, which of the aforementioned 'scope types do people tend to use for prime focus astrophotography? In other words, how much do I need to start saving!

[michael.h.f.wilkinson]

Why not use the phrase Cat, to avoid insulting SCT owners?

A fast frac is the instrument of choice for many, as it avoids the collimation issues of fast newts (which can produce really nice images, especially with a coma corrector). Cats are generally considered too slow, unless you use a focal reducer. Modified Dall-Kirkhams (also cats) are very nice, but very costly.

[Planetesimal]

Thanks Michael. So, assuming a frac is the way to go for AP, I have two follow-up questions. 1) how important is aperture vs. the practical considerations of stability etc, e.g. is a 150mm better than a 90mm, and 2) what constitutes "fast" in frac terms - is it f5 and below like with a newtonian?

[michael.h.f.wilkinson]

In DSO AP, fast F-ratio beats aperture: A 150mm F/8 is beaten by a 90mm F/6 on all extended objects. A 150 F/6 would be nicer, but has a long focal length which requires a better mount. I have an APM 80mm F/6 triplet which I bought both as a wide-field travel scope, and as a fast imaging scope (now I need a better mount and camera). When I get my imaging rig in order, I intend to add a 0.8x reducer to reach F/4.8. This is common in fracs: They are slower than imaging newts, but with a focal reducer they get close. The Skywatcher Equinox 80ED (F/6.25) is used a lot. The cheaper Evostar 80mm F/7.5 can also be used, preferably with a reducer.

[rowan46]

The only thing forgotten. it probably doesn't need saying but I will say it anyway it needs to be an apo not just a refractor

[x6gas]

The only thing forgotten. it probably doesn't need saying but I will say it anyway it needs to be an apo not just a refractor

Not necesssarily! If you go down the mono camera with filters then you DON'T actually need an apo so long as you are prepared to refocus for each filter.

What should be said - and is said on here often enough but bears repeating - is to have a good enough mount for whatever 'scope you have. Small fracs are lighter and so tend to need a less heavy duty mount. They are also less susceptible to interference from wind than, for example, a large Newt (which can become sails!)

The other - and in my opinion oft overlooked - issue to consider is the focal length of the scope. DSOs are often quite widefield so you may find that you don't need much more than, say 600mm focal length. Depends on the target, but I find this website: http://www.12dstring.me.uk/fov.htm very useful and I'd advise you to try out some different focal lengths for some of the targets you'd like to image so that you can get a focal length that suits the type of DSO you want to image...

I have an SCT, a couple of 'fracs and a couple of Newts and for DSO imaging I would recommend a ~90mm aperture, ~600mm focal length frac as a general purpose scope. Anything f/7 or faster will be fine; a triplet apo if you can afford it, but do consider a non-apo and filters as an alternative.

[rowan46]

My mistake for some reason I thought i read dslr

[x6gas]

My mistake for some reason I thought i read dslr

Although you can still use filters with a DSLR!

Sorry - I'm in pedantic mode tonight! Best ignore me!

[Planetesimal]

Given that I'm in the beginners' section here, I'm going to have to admit at this point that I don't know what "apo" means! Sounds like I ought to find out...

[pvaz]

Given that I'm in the beginners' section here, I'm going to have to admit at this point that I don't know what "apo" means! Sounds like I ought to find out...

Non APO refractors make diferent colors focus at diferent points and cause a problem known as color aberration (to keep with the acronyms, CA ). This can be avoided with better corrected (Apochromatic) refractors. They are a fair bit more expensive then achromat refractors.

[pvaz]

BTW the cheapest most popular setup for AP, is a Skywatcher ED80 refractor and an HEQ5 mount. Just to save you some research.

You can average the gear used if you browse the deep sky imaging section. Most people post the specs for their shots and most have their gear listed in the signature.

[crazyjedi]

The above advice is good - and is in fact what I am hoping to get for my own AP set up (may be when I get my cash together)

[Tim]

What do you want to image, and what camera are you planning to use.

Widefield or detailed, galaxies or nebulae, lunar or planets? OSC camera or mono and LRGB/narrowband?

The answer to these questions gives you the answer to your query.

Every scope is a compromise of one sort or another, so it also depends on what you are willing to compromise. Decide what is most important to you, convenience, quality, portability, speed, etc, and make an informed choice.

If I could only have one scope for astroimaging, then it would be an SCT. If I could have two, then it would be an SCT and a refractor. If I could have three, then an SCT, 'frac and a Mak-newt.

As it is I have those and a couple of newts as well, but find that my SCT's present me with the best compromise for the sort of targets I personally enjoy photographing. For wider field photo's then a Mak-newt is very rewarding, but the weight and size is the compromise. For star parties and travelling I use a small refractor. If I had the money then I would like to add an expensive quadruplet refractor to my stable for properly colour corrected images with a OSC camera.

Most people start with a refractor when imaging I suppose though, and that will provide reasonable results, reasonably easily.

Hope that helps a bit ? :s

Tim

[Planetesimal]

Thanks Tim - I think my main interest would be in galaxies / nebulae, rather than wide field. Planets are great but there's really only Jupiter and occasionally Saturn whereas there are almost unlimited targets when it comes to the galaxies / nebulae (not too fussed about open clusters!)

So, what is it about SCTs that puts them ahead of fracs for AP? I imagine it's harder to find a fast frac...

[pvaz]

Thanks Tim - I think my main interest would be in galaxies / nebulae, rather than wide field. Planets are great but there's really only Jupiter and occasionally Saturn whereas there are almost unlimited targets when it comes to the galaxies / nebulae (not too fussed about open clusters!)

So, what is it about SCTs that puts them ahead of fracs for AP? I imagine it's harder to find a fast frac...

It's the other way around. The lowest the focal ratio, the fastest it is for a camera to get exposure. Say if you have an f/7 frac and take a 5 min sub with it, you would need to make a 20 min exposure with the f/11 SCT to get the same amount of light. Thus the name "fast" for scopes (or lenses) with low focal ratios.

In practice, everytime you move up one fstop you half the mount of light you get.

For more details:

http://en.wikipedia.org/wiki/F-number#Stops.2C_f-stop_conventions.2C_and_exposure

SCTs are better for planetary imaging. Planets are bright so you won't need long exposure and the added focal lenght lets you achive a good magnification for planets without using many barlows. The extra apperture (per €) also gives it better resolution at high magnifications.

[LukeSkywatcher]

Given that I'm in the beginners' section here, I'm going to have to admit at this point that I don't know what "apo" means! Sounds like I ought to find out...

Glad you asked. Ive been here a couple of yrs and never bothered to ask or research it.

[cantharis]

Apochromatic - objective lens is a triplet which brings three colours to a focus at the same point. Of course they are the most expensive type, especially in larger aperture sizes. Many people compromise and use a ED (extremely low dispersion glass) doublet - not quite as well corrected as a true apochromatic, but a lot cheaper.

[earth titan]

Apochromatic - objective lens is a triplet which brings three colours to a focus at the same point. Of course they are the most expensive type, especially in larger aperture sizes. Many people compromise and use a ED (extremely low dispersion glass) doublet - not quite as well corrected as a true apochromatic, but a lot cheaper.

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.

[earth titan]

So, what is it about SCTs that puts them ahead of fracs for AP? I imagine it's harder to find a fast frac...

You can use SCTs for deep sky AP, but you need to use a focal reducer. By dropping the f ratio to f6 you get a 'faster' scope (about the same as most francs). There are issues over guiding etc. with an SCT but people do image with them and very successfully.

The attraction of a frac for imaging is the 'out of the box' simplicity. On the basis 'if it can go wrong it will', the SCT will be more of a challenge.

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

[Planetesimal]

Thanks all, sounds like SW ED80 on HEQ5 will most likely be the way to go. Looks like it's not that easy to get hold of though - unavailable from FLO at the moment. Which is good because I can't afford a new scope right now so time to save up!

[cantharis]

FLO aren´t the only suppliers - recently cancelled an order to them because I found a supplier who DOES have the stuff I want.

[cantharis]

Oh [removed word] another typo they won´t allow me to correct because I don´t have 2 million posts.

[cantharis]

Oh [removed word] another typo they won´t allow me to correct because I don´t have 2 million posts.

[Planetesimal]

Hey Cantharis - I didn't mean to imply FLO were the only suppliers, just illustrating that it might take a bit more work to get hold of the 'scope at a good price. I'll definitely look on the second hand market anyway!

Also, I don't think a limit of 250 posts is unreasonable. Doesn't impact my enjoyment of the forums anyway!

[michael.h.f.wilkinson]

It's the other way around. The lowest the focal ratio, the fastest it is for a camera to get exposure. Say if you have an f/7 frac and take a 5 min sub with it, you would need to make a 20 min exposure with the f/11 SCT to get the same amount of light. Thus the name "fast" for scopes (or lenses) with low focal ratios.

In practice, everytime you move up one fstop you half the mount of light you get.

For more details:

http://en.wikipedia....2C_and_exposure

SCTs are better for planetary imaging. Planets are bright so you won't need long exposure and the added focal lenght lets you achive a good magnification for planets without using many barlows. The extra apperture (per €) also gives it better resolution at high magnifications.

Not quite correct: and F/7 scope at 5 minute subs gives the same exposure per unit of surface area as an F/10 scope (more common in SCTs) in 10 minutes, or an F/11 scope in 12.3 minutes. Still a significant difference. An F/5 scope would need just 2.5 minutes. If we keep the aperture fixed, the size of the image is of course reduced.