Aperture vs f/stop - Newbie Question

[Leedsgreen]

Sorry, posted this in the hints & tips section first. Think it should go here...

Dear All,

At the moment I'm having a go a imagine DSOs using a LX modified webcam and my 400mm SW Equinox 66 refractor (66mm aperture, f/6). My question is, is it the f/stop alone that determines how much light a scope can 'pull in' or the aperture as well?

What I mean is, will a 120mm f/6 scope will require less exposure time than a 66mm f/6 scope (to achieve the same image)? My sense is that the bigger aperture will have a big effect?

Thanks for any help,

Nick

[themos]

They can't "achieve the same image" with the same sensor. In the 120mm, the image will be 120/66 bigger.

[Leedsgreen]

Themos,

Ahh, good point. OK but would you expect a 500mm focal length 102mm aperture scope (i.e. f/5) to pull in much more light than a 400mm focal length 66mm aperture scope (i.e. f/6)? If so, by how much (i.e. using pi x radius squared, is it 2.4 x more)? Or would there be little difference?

Hope this makes sense? I guess what I'm trying to work out is if it is worth me buying a SW achromat 500mm refractor with 102mm aperture (i.e. Startravel 102). I know there will be some colour aberration compared to my APO scope but I'm after some more light capture for imaging DSOs (fairly cheaply, using a light enough scope that can fit on my mount).

Nick

[SteveP]

OK but would you expect a 500mm focal length 102mm aperture scope (i.e. f/5) to pull in much more light than a 400mm focal length 66mm aperture scope (i.e. f/6)? If so, by how much (i.e. using pi x radius squared, is it 2.4 x more)? Or would there be little difference?

Going back tbo your original question, given that the two examples you given have the same focal ratio (f/6) than both scopes will require a very similar exposure time. However, as Themos says you will be capturing different image 'sizes'. The relative magnification of those images will be determined by the focal length of each scope and not the aperture - everything else being the same

On the other hand, the light capture will be more or less as you say in your latest question, ie a function of the aperture of the two scopes. ie proportionate to the square of the respective apertures. Which is why aperture fever is so prevalent amongst DSO observers!

HTH

Steve

[themos]

If you managed to collect 100 photons in one pixel with a f/6 60mm, in the 120mm you will get (120/60)*(120/60)*100 (=400) photons from the same region but they will be spread out over (120/60)*(120/60) (=4) pixels so you still get 100 photons per pixel.

[Leedsgreen]

Themos and Steve,

Thanks for your detailed replies, which I think I'm almost getting! Whilst I appreciate that using the 500mm (102mm aperture) scope will give me a slightly larger image than using my 400mm (66mm aperture) scope (well, 500/400mm bigger) would it be fair to say that I would stand a better chance of recording more nebulosity for a given exposure time using the bigger scope?

I guess I'm trying to work out if the purcchae would make a difference. If there was hardly no change, then it wouldn't be worth it but if it really did improve what I could capture (for the same exposure) then it would. I realise I'm drifting into subjective experience here, so no probs if it's unanswerable but I would appreciate any further help if possible.

Thanks again,

Nick

[SteveP]

Nick

Yes, your 'bigger' scope will allow you to record more signal for the same exposure time since its focal ratio is faster - f/5 vs f/6

Steve

[themos]

Yes the f/5 creates a more intense photon "stream", but you may well record "less" nebulosity if the larger image means that some features no longer fit the frame. As you see, there's more than one variable to consider, sorry about that. Also, keep in mind that "fast" optics are more expensive to get right and not all f/5 optics are the same. You may get too much chromatic aberration (colours "leaking" out), or the stars might look bloated or misshapen near the edges of the frame, all kinds of issues.

[Leedsgreen]

Ahh, OK, thanks guys. It makes much more sense to me now and is much appreciated.

Nick

[PeterW]

For diffuse objects you want the 'fastest' scope you can get <f5 (Hyperstar can give f2!). For stars this doesn't apply as they are point sources, so you get fainter with longer exposures and larger exposures. For deep sky you want the fastest scope you can, with a focal length that gives you the required image scale for the CCD you have.

PEterW

[themos]

Here's a handy chart for matching focal lengths, targets and sensors (in terms of their pixel pitch)

Webcams have pixels roughly the same size as entry level DSLRs, about 6 microns. For deep sky, that gives a focal length of about 600-700mm. For planetary, something like 3000mm. A webcam doesn't have many pixels, so for deep sky you would be constrained to objects about 500 x 2 = 1000 arc-seconds across (or 16 arc-minutes, half a full Moon).

[Leedsgreen]

Wow! Great chart. Thanks Themos (and Peter for your reply).

Nick

[ollypenrice]

But on the wider issue of getting slightly more speed for webcam DSOs my instinct would be not to put money in that direction. You will remain very limited. And if the fast scope is badly corrected a lot of the light will be too far out of focus to contribute much. How you would calculate the extent of that I would not like to guess at!

Olly

[themos]

From what I've seen about modded webcams, you are still limited by the 8-bit readout and significant thermal noise. The latter forces you to expose for about a minute or two, maximum, unless you also do the "amp-off" modification which reduces the heat output of surrounding components. You don't have much dynamic range in the first place (those 8 bits) so anything taken by thermal signal really bites.

[themos]

On the plus side, the small size of the webcam sensor means you will not be affected so much by optical aberrations.

[Leedsgreen]

Point taken, the modified webcam is good but clearly has its limitations. Fortunately, the on ei have is fan cooled, which means the temperature of the chip stays cooler (also has amp off mof if I need it). The thing is, with my cheap GOTO AZ mount I can only reach 20s before trailing becomes apparant anyway, so f/5 compared to f/6 should make a big difference.

Whilst I know achromats can cause colour abberrations, I was thinking of using a Baader high contrast filter that supposedly conteracts some of the effect. I know there are much better (and more expensive) solutions out there but the 400mm (80mm aperture) scope is only £80 plus £40 for the filter, isn't a big hit and perhaps one worth trying?

Cheers,

Nick

[jgs001]

Nick, something to bear in mind using CA correction filters, you will lose a lot of the blue spectrum... This came very clear to me, using a Semi APO (not as harsh as the Contrast Booster) filter on M42 and M45... M42 came out looking like it was shot with a modded SLR and M45 had very little nebula showing at all. I was using my ST80 clone on me Celestron version of that same mount.

[Leedsgreen]

jgs001,

Ahh, that's a very good point, thanks for that. Yes, seems a high contrast filter would counteract what I'm trying to achieve. In that case, how bad is the colour abberration with no filter? Does the image appear horribly out of focus or is there just a bit of colour fringing about each star? Given that I'm talking shoestring astronomy here, I'm not looking for perfection, but just a better chance of recording nebulosity with a cheap setup.

I presume a Baadeer Neodymium filter will help with light pollution but not block the blue spectrum nearly as much as a high contrast filter?

Thanks for any further help, much appreciated,

Nick

[jgs001]

Nick, it's hard to answer... I had a fair amount of blue fringing even with the Semi APO filter, but it let more light through. Best I can suggest is try it without and see how you go... You can always add a filter later. I can probably dig out a couple of examples I did with my ST80 clone if you want to see them.

[themos]

just a better chance of recording nebulosity with a cheap setup.

It can be done if you spend more of your time! If CA is a problem you can just capture more sets of images, one for each of R,G and B. For each colour, you focus using the corresponding filter. Then, in processing, you only keep from each set the colour channel that was used to focus and combine the separate stacks. Details here

[Leedsgreen]

Themos, thanks for that link, really interesting and useful (and will probably keep my out longer)!

John, yes please, would be useful to see any examples you have.

Nick

[jgs001]

Here you go Nick... These were shot with my SLR, Konus Vista 80s (ST80) mounted on my NexStar SLT

I didn't think/know about Themos trick at the time... Both were shot with the Semi APO filter

[Leedsgreen]

Wow! Great work John, thanks for sharing. It amazing what you have achieved with an AZ mount. What is the longest exposure time you are able to achieve with that set up?

Nick

[jgs001]

Thanks Nick. 2 minutes low in the east, but I was losing 50% of my frames to do it.