refractor aperature - f/ratio question

[HanoverFist]

I've been reading up a lot lately on buying a new refractor as my first scope. Astrophotography of DSO's will be my primary use. Can someone explain how the various apertures compare please? I guess what I'm asking is about field of view and such. Will an 80mm suffice for most DSO? Also, does the f/ratio just impact exposure time?

While we're at it...if you had $3000 to spend on a rig ( mount, tube, diagonal, autoguider, etc) what would be on your shopping list? At the moment I'm looking at the Orion Atlas EQ-G mount with maybe an Orion ED80T CF refractor ($800). I've been researching a scope in the 100mm range though too.

Thanks for reading.

p.s. I should point out I'll be shooting with a Canon T2i - I think you folks call it a 550D over there.

Edit: I have also been looking at the Explorer Scientific 102mm f/7 Air-Spaced Triplet ED Apochromatic Refractor Telescope ($1300)

[ronin]

The f number is one aspect that determines the exposure time. With the proviso that the brightness of the object is more relevant.

The f number means the same exposure time for the same object if the scope is 100mm f/5, 80mm f/5 or 60mm f/5. What changes is the size of the image. The 100mm f/5 producing a bigger image on the sensor then the 60mm f/5.

Many use an 80mm refractor for imaging. The physical length means the it is more tolerant of whatever tracking errors are present.

If you bought a 100mm f/5 then the mount would have to track a little better. Eventually the scope is just too long to get long exposures.

I guess that an 80mm is the best compromise, reasonable size image and more tolerant of tracking errors.

[brantuk]

Take a look at these: Altair Astro 115mm f805 EDT APO with 3" Field Flattener :: Refractor telescopes :: Telescopes :: Altair Astro .

Great package and superb performance imaging and visual -and leaves a lot of budget for the mount and accessories - mines on neq6 pro

[rocketandroll]

Hi mate

Well.... personally, with that sort of budget, I'd be aiming for two scopes.

The trouble is.... people talk about DSO photography like it's a single task to be achieved with a single 'ideal' tool. The trouble is, DSO's vary SO much in their size and brightness that you really need an array of equipment to best capture all of them.

Everyone recomends an 80mm or so apo as a good first scope and it is good for some stuff. The trouble is, with a DSLR (and er-go a huge chip) you will end up with a huge star field with a small DSO in the middle of it for all but the largest DSO's. I can only think of about four or five DSO's that are about the right size for a DSLR/80mm Apo combo.

To some extent it's focal length and focal ratio you want to worry about, not so much aperture. The focal length is a better guage of the size of the object on the chip in the finished image.

Personally, I went against everyone's advice and bought a 1m focal length apo for my first scope, and had a good five months of fun doing unguided DSO imaging with it with no problems.

I'm now heading rapidly towards my 1 year anniversary of getting in to this hobby and am settling on a more ideal set of scopes... which are: A WO Megrez 72 with FF2 reducer which gives a focal length of about 380mm at f4.8 - this is ideal for the really BIG DSO's, the NA Nebula, IC1396 in it's entirity, M31, the Gamma Cygni nebula and some of the other large, lovely areas of nebula/star field in the Milkyway.

A 100 - 115mm aperture apo with a focal reducer giving a focal length about 700 - 800mm and at f5 - f6 (yet to be acquired, probably buying one in the new year). This will suit smaller nebulae and some of the larger galaxies like M33, Sculptor etc... also good for open clusters.

Finally, a 1.2m focal length f6 Newtonian (on order) which I will be using for the smallest objects, plnetary nebulae, globular clusters and most galaxies.

Along with my Skywatcher HEQ5 mount, an ST80 guidescope and SX Lodestar guide camera that should cover me for next year.

I am guessing you are in the US? All that from scratch is probably more like $4000 or so... but for $3000 you can certainly get a good HEQ5 sized EQ mount, a nice apo refractor in the 300 - 500mm focal length range and a nice refractor in the 700 - 800mm range and the bits you need to get guiding. To be honest, that will almost certainly serve you well for your first year.

To summarise: a faster focal ratio really IS a good thing, going from f7.5 to f6 effectively halves the length of exposure required to get the same image. If you either select a good apo that is already f6 or faster, or pick a slower one which, when matched with a focal reducer gives the right sort of focal length and speed, you'll be on to a winner. Personally, I'd go with a short focal length fast scope (like the Meg72 or similar) for the big stuff, and a 100mm+ refractor at 700mm+ fl for the smaller stuff.... and take it from there.

It's all the little things you forget about like mounting bars, guidescope rings, eyepieces for getting the scope aligned, star diagonals, extenders and reducers that really eat up all of that sort of budget :-)

Have fun :-)

Ben

[HanoverFist]

Thanks, that's exactly the sort of info I was looking for.

So if I understand this correctly...

I have been looking at the Explore Scientific 102mm f/7 Air-Spaced Triplet

ED Apochromatic Refractor. It has a 102 mm aperture, 714mm focal length and a f-ratio of f/7. If I use an Orion 0.8x focal reducer on it that will give me a f-ratio of f/5.6. I would mount that on an Atlas EQ-G.

Would that would be an acceptable setup to start with? I'd rather work back towards an 80mm than be frustrated with tiny DSO's in a wide starfield.

Open to suggestions on scopes.

[ollypenrice]

With a DSLR F ratio is very important because you can't just go for longer subs as you can with cooled CCD cameras. F5.6 would be good, therefore.

Focal length determines what will be framed on the sky. There is no problem finding targets for short focal lengths. I certainly take more images at 450mm in the small apo than I do at 980mm in the big one - but having the choice isn't a hardship!

Aperture, in itself, is not directly important in imaging. Think directly about

F ratio for signal to noise/speed and FL for what will be framed. To put it another way, at a given FL aperture will increase resolution because of its effect on the Dawes limit of resolution and because it will pull in more light in the time. The latter effect is far more important than the former in imaging. With deep data you can sharpen at the processing stage. A theoretician might call this 'false resolution' but no-one looking at your picture is going to do so!

Olly

[JoLo]

Ben and Olly hit it on the nose. I purchased an Atlas about six months ago, and use the AT65 (400 FL, f6.5) and the AT106 (800 FL, f6.5). This has been a wonderful setup to learn on, and gives me lots of flexibility as Ben alludes to....I also can mount my camera on the setup with 20mm widefield, and 70-200 lenses, which increases your options ten fold.

The Atlas has been a magnificent mount for me, easy to learn on with the short refractors, i was able to get some good images from the get go.....learning all the ins and outs of processing has been a bigger challenge for me. I purchased the Atlas with a goal in mind about a year out of getting a 8" imaging Newt or an RC. The mount will still be able to handle this, and I can get some of the smaller / fainter DSOs that currently are small with the 106 (I use a Canon Rebel for imaging).

I ended up spending about $4000 US, but all the little things (reducer, field flattener, rings, etc) add up quickly...... Can't go wrong with the Atlas for entry level short FL refractor, and room to grow in the future.