A telescope's Limiting Magnitude

[GeeKay]

I apologise for this somewhat obvious question, but does a telescope's limiting magnitude increase when using time-lapse photography - this compared to real-time viewing?

[swansont]

There's nothing special about time-lapse. The amount of light you gather depends on the exposure time. Taking multiple pictures doesn't change this.

[EdEarl]

The primary limits on telescopes are bandwidth, resolution and brightness. Magnification is a secondary property. Image resolution or pixel size depends on the maximum distance between two points on the mirrors. Binocular telescopes have better resolution for total reflector area than a single mirror, because two mirrors can be placed some distance apart, making maximum distance between two points on the two mirrors relatively far. It is relatively easy to increase magnification until one pixel becomes fuzzy (resolution limit).

 

A radio telescope called the Square Kilometer Array (SKA) is being built on two continents, Australia and (south) Africa, which will give the SKA very good resolution. Electronics are good enough to do that for a radio telescope, but not good enough for other kinds, for example optical telescopes, which for now must be close to each other.

[GeeKay]

I think I may have phrased the question wrongly. When I wrote 'time-lapse' photography, I really meant (for want of a better term) keeping the camera 'shutter' open for a extended periods of time. This question was prompted in part from hearing about professional astronomers, such as those at the Keck, deploying this means to rack up the magnitudes - say from (approx) +22 mag to around the +29/30 mark?

 

Also, I can't help but notice that photos of dim DSOs reveal an awful lot more (i.e. fainter) detail than what one would expect to see through the eyepiece of a telescope of the same aperture.

 

The reason why I'm asking this question is because I'm thinking of trying my hand at some astrophotography. It would mean buying another mount, however, plus a slow-motion drive. Plus there's the added issue of my scope's aperture, which were I ever to encounter a truly dark sky (sigh) would give me a limiting factor of around +13 mag, this assuming very good seeing conditions, of course. This is okay for me, but I do wonder if the added expense will be worth it if I only replicate via the camera what I see through the tube?

 

I guess my real question is this: if a star's brightness is measured in magnitudes - and I accept that's pretty much cast in stone as far as aperture is concerned - where do all the fainter stars, or that added structure in a galaxy, globular cluster etc, come from as a result of (say) an extended exposure taken by a camera? Same telescope, but more photons, I should have thought. So what does this mean exactly?

[EdEarl]

Longer exposures increase the number of things you can see. One of the best places in the continental US are in the Davis Mountains, where I understand amateur astronomers gather yearly. See: https://texasstarparty.org/welcome/

 

The Hubble took about a three week exposure of an empty dot in the sky to get the Hubble Deep Field and Extreme Deep Field photos. Before taking the exposure, people questioned whether it would be a worthwhile endeavor.

[GeeKay]

Yes, I dream of being under inky skies. . . or even clearer skies, come to that. I intend to get away from the perennially cloud-bound UK to enjoy an extended spell of stargazing in Southern Spain later this year. I also hope to visit Australia before I shuffle off this mortal coil and explore a full third of the southern heavens I've never seen before - and all this by means of a day's drive out from Alice Springs? Meanwhile, Texas? Now there's a thought. . .

 

Re. the issue about how astrophotography affects a scope's light-gathering potential: the link below offers some useful info here.

 

http://www.madawaskahighlandsobservatory.com/Astronomers-WFT.html