Telescope shopping!

[AngelLestat]

from 4 min to 30 there is a big difference. Of course is not impossible, I just said very hard. It also matters how small is your sky view (second-min-degree arc), If we are using our mount just to place our DSR camera with a 50mm lens, then it is easier to get long exposures.

For example, we already did the room building where it will be located the telescope, it only need the dome.

But to place the telescope, we can not use the same floor, we need to use extra iron beams to the walls, to not transmit vibes into the telescope.

therefore any tracking equipment handmade, needs to be robust enough to avoid any miss movement under the exposure. The most important is to possitioning the mount to the perfect north-south respectively.

Post software edition helps a lot.

[PakledHostage]

The problem with short exposures is that every exposure adds readout noise which can be quite bad if you are using a relatively cheap camera (including low end DSLRs). However, it is definitely better to use shorter exposures if you don't have good tracking.

While we're on the topic, I wonder if someone here with some expertise in optics and CCDs could comment on this thread: Longer exposure & lower ISO or shorter exposure & higher ISO - what gives better results when photographing stars?

The answer given by jrista (the first answer) seems extremely thorough and knowledgeable but as often happens with our own threads on this forum, others jump in with their own comments that seem to contradict those of the guy who most seems to know what he's talking about. He goes into a fair amount of detail about photon shot noise and read noise.

What I'm most curious about is the paragraph just above the first figure. He says these things without giving any explanation of the reasons behind them:

In the diagram below, the linear X axis represents each ISO setting, and the logarithmic Y axis represents charge level in electrons (e-). Red and green lines are drawn for each ISO setting, with red representing read noise, and green representing saturation point. The dynamic range is effectively the ratio between saturation point and read noise (green over red). For ISO 100, the saturation point is also the literal maximum photodiode charge level (FWC, or full well capacity). The blue bars represent the signal, and the darker part of the blue bar represents the intrinsic noise in that signal (photon shot noise, which is the square root of the signal.)

[K^2]

I don't know much about photography specifically, but signal/noise is mostly just a question of statistics. Still, take it with a grain of salt.

Short exposure with high ISO reduces SNR due to shot noise. That's pretty obvious. But if you are going to stack a minute worth of short exposure images or do one minute-long exposure, then it doesn't matter. Stacking averages out shot noise exactly the same way that long exposure would. Ditto read noise. You can run into problems with quantization noise, but that's what ISO setting is for. So if you are going to stack, you'll get the same SNR either way.

So it comes down to whether you plan to take just one shot or stack a bunch of images. With a single shot, go for as long of an exposure as you can manage. Adjust ISO to get contrast you want. If you are going to stack, then go for short, high ISO takes. That will get you same level of noise in the final image, but you'll get fewer problems due to movement/vibration.

[indroth]

Ditto read noise.

If I am not mistaken, the SNR of N 1 minute exposures considering only read noise is worse than 1 N minute exposure by a factor of sqrt(N).