How long

[shropshire lad]

Sorry more newbie questions (trying to get things right in my head while waiting for some clear nights)

How long Can I expect to expose a section of sky using my Canon 80D Dslr and 300mm lens without getting startrails ... 

1 Using just the tripod.

2 Using AZ-GTI mount in az mode after setting alignment using polaris and sidereal tracking.

3 Using mount in eq mode after polaris alignment and sidereal tracking.

Lets assume that my polar alignment is pretty close.

Thanks guys/gals 

And will this depend on what part of the sky I am imaging.

 

[Nik271]

To get an accurate answer for 1 and 2 you also need to specify the declination of the target, since objects close to the celestial north/south move much less than those on the equator. There is a guy on youtube who did a in depth video on this. I will dig it out in another post.

For 3 the answer is easy: in theory with perfect polar alignment and  perfect mount forever. In practice you are limited by the mount, how close to the weight limit your gear is, how windy, clouds. and again the declination of the target. People get anything between 2 and 5 minutes I think, but with just 300mm focal length maybe its  possible to do even better. If you are stacking multiple photos then you actually don't need very long exposures beyond a certain time, I would say taking multiple 2 min exposures should be good enough for most targets.

 

[Nik271]

Here is the video, I found it very informative 

 

[Alien 13]

I tend to just use the 500 rule as a starting point i.e. MAX exposure length but modify it by 1.6 (Canon APSC crop factor) for non full frame cameras so I get 500/1.6 = 312.5, so my goto rule of thumb is 300/lens focal length.

Alan

[vlaiv]

It also depends on what is the resolution that you want your image looked at.

Let's take your camera - 80D as a starting point. It has 6000x4000 sensor with 3.7µm pixels. With 300mm lens - you'll be working at 2.54"/px. Let's leave aside the fact that I think that sort of resolution is too high for camera lens, if one looks at 100% zoom (screen pixel == image pixel) they will be looking at your image at 2.54"/px

Say you post your image online, on facebook or here on SGL. Majority of the people will not see it at full resolution and odds are you'll even scale it down to help upload it (reduce size).

They will be viewing it at something like 1500 x 1000 at most. That is no longer 2.54"/px - but rather 10.16"/px. Any perceived error will be reduced x4.

To answer your question number 3. I've found that I can push AzGti without guiding at that sort of resolution ~10"/px for up to a minute. Longer than that and there is trailing due to periodic error. With guiding you'll probably extend that to multiple minutes, however, due to mount itself - I would never use it to image at resolutions of about 2.5"/px. You need to be at twice lower resolution at least - something like 4"-5"/px to be comfortable with this mount. Even when guiding. It is in fact widefield mount.