aperture for astrophotography

[neil groves]

I have been doing a lot of thinking about this and am I correct in saying that a larger aperture will let more light through to the camera, a smaller aperture can be used but the required exposure will be longer?

is this why stacking is popular among users of 80mm tubes? I thought an 8" tube would be a brighter image?

Neil.

[Gib007]

For astrophotography, focal ratio is pretty much everything. So, a large 11" telescope that's f/10 would be dead slow compared to a 3" that's f/5, as an example. Then there's the focal length and your camera's CCD sensor that define your field of view and image scale but ultimately you want fast focal ratios (f/6 and below is good for astrophotography).

[baggywrinkle]

One of the reasons why people like the short focal length refractors is that they are fast and additionaly have a wide field of view so larger diffuse nebula and DSO can be more easily captured.

[Gib007]

To add to my post, when searching for a suitable imaging telescope, it helps to look at programs like CCDCalc, where you can add your camera and then telescopes to check out the resulting field of view and image scale numerically as well as against deep space targets. Of course, one should then restrict choice of telescopes to those with either fast focal ratios (e.g. f/6 and below) or those that can be combined with good focal reducers to make them fast. Think about optics too - refractors, Newtonian reflectors, Ritchey-Chretien, etc. Always keep in mind: Focal ratio, focal ratio, focal ratio.

[ollypenrice]

Good advice above.

Slow f ratios give a low level of light intensity at the chip and fast f ratios a high light intensity. That is plain and simple, if counter intuitive.

What also matters is the sampling rate, the number of arcseconds of sky imaged by each pixel. Obviously a small pixel gets less light than a big one in the same telescope so will be slower. However, it has the potential to resolve more detail provided the sky's seeing (freedom from turbulence) will allow it.

You need to beware, though; a short fast telescope will produce a brighter but smaller image than a slower bigger one.You cannot just zoom in on the smaller image and expect it to be better than the larger one. This potential error is called the F ratio myth and is the subject of much anger and confusion!

Olly

[Gib007]

You need to beware, though; a short fast telescope will produce a brighter but smaller image than a slower bigger one.You cannot just zoom in on the smaller image and expect it to be better than the larger one. This potential error is called the F ratio myth and is the subject of much anger and confusion!

Olly

This is excellent advice. It really pays off to check out what you will see with a camera-telescope combination in the likes of CCDCalc.

[IanL]

Try this for estimating the fields of view of different camera and scope combinations (has most current cameras and scopes, and happy to add any that are missing if people let me know). It will also give you sampling in arcseconds per pixel.

http://www.blackwaterskies.co.uk/p/imagingtoolbox.html

It doesn't yet tell you f ratio of the scope itself, but I will add it in the next version.

[dph1nm]

You need to beware, though; a short fast telescope will produce a brighter but smaller image than a slower bigger one.You cannot just zoom in on the smaller image and expect it to be better than the larger one. This potential error is called the F ratio myth and is the subject of much anger and confusion!

The corollary of this is that you can 'zoom out' an image taken with a slower scope and produce an image just as bright as one taken with a snall f-ratio. It just won't cover as large an area of sky.

Nigel

[ollypenrice]

The corollary of this is that you can 'zoom out' an image taken with a slower scope and produce an image just as bright as one taken with a snall f-ratio. It just won't cover as large an area of sky.

Nigel

Indeed so.

Olly

[neil groves]

That should keep me busy for a while Ian....thanks.

Neil.

[NickK]

Look for Dawes Limit - it basically gives the limiting arcseconds for a given aperture (note this differs slightly for a central obstruction) and wavelength. So If you go below the limit you will start seeing blurring.

I use a 105mm aperture 670mm fl refractor at f/6.38. This gives me 1.67 arc sec/5.45um pixel. If I add a 2x giving 1340mm fl, it means I now have 0.88 arc sec/pixel. However the scope's dawes limit is 1.10. It doesn't cause me a problem - the only thing is that the image is slightly softer with the 2x. Which processing can help with (ie: )

Larger aperture means that you can get better detail because you can get a lower dawes limit. It doesn't mean you have to stick to it.. A small aperture will have a higher dawes limit, so it will blur when you use smaller pixels that result in a arcsec/pixel below it.

[neil groves]

Thanks Nick, I have so much to learn lol

Neil.