Are any of these telescopes good for Astrophography?

[vlaiv]

1 minute ago, alacant said:

So the illusion that f4 is brighter than f5 actually isn't an illusion at all. F4 is brighter than f5. Nice:)

Cheers

Yes, however, that does not mean that F/4 is faster than F/5.

Many people think F/4 is faster than F/5 and hence the term speed.

F/4 is faster than F/5 if you keep pixel size the same.

Two things can happen when we switch between F/4 and F/5 if we keep pixel size the same:

- If we switch from F/4 to F/5 by using aperture stop - we are limiting amount of photons and this is why F/5 is slower - less photons reach sensor

- If we switch from F/4 to F/5 by increasing focal length - we are increasing working resolution - we spread light over more pixels (since pixels are the same size).

This also means that two F/5 scopes will have same speed even if they have different apertures - if you keep pixel size the same. As aperture increases, so does focal length - and hence working resolution. Increase in photons is exactly the same as increase in spread of these photons so 4" F/5 scope will be of same speed as 6" F/5 scope - if you, of course, keep pixel size the same.

This is another thing that reinforces F/ratio myth. F/ratio myth is not the myth if you keep the camera same (as is case in daytime photography when talking about F/stops).

On the other hand - this means that F/5 scope can be faster than F/4 scope - if you change pixel size or in particular - if you can set working resolution by either changing physical size of pixels (using different camera) or changing logical size of pixels (using binning to increase size).

For this reason it is best to compare "aperture at resolution" of two systems rather than F/numbers - as first takes camera into account, while second does not.

[alacant]

1 hour ago, vlaiv said:

this means that F/5 scope can be faster than F/4 scope if you change pixel size 

I want to capture M1 using my DSLR using:

1. The 2000mm f10 telescope at La Palma.

2.  My 200mm f4.

No change of pixel size. You're saying my telescope is faster?

[vlaiv]

1 hour ago, alacant said:

I want to capture M1 using my DSLR using:

1. The 2000mm f10 telescope at La Palma.

2.  My 200mm f4.

No change of pixel size. You're saying my telescope is faster?

Yes.

We are talking 20000mm of focal length here.

Let's say you have DSLR with 4.3µm pixel size - each pixel will cover only 0.04" of the sky.

In fact, each pixel at La Palma will capture 0.04" x 0.04 = 0.0016 arc seconds squared of sky (surface)

200mm F/4 will sample with same pixel size at 1.11"/px, or it will cover 1.11" x 1.11" = 1.2321 arc seconds squared of the sky

Single pixel of your F/4 will cover x770 more sky than at La Palma. This means that your single pixel will receive all those photons that would fall on 770 pixels at La Palma

Now, 2000mm of aperture is x100 larger gathering area than 200mm of aperture (x10 by diameter, x100 by surface).

This means that telescope at La Palma will gather x100 more photons than your scope, but it will spread those photons over 770 pixels.

Let's say that M1 from little patch of its surface shines with 10 photons on your telescope and all these 10 photons end up on one pixel.

La Palma scope will gather 1000 photons in the same time - but they will be spread over 770 pixels, so each pixel will gather  ~1.3 photons or x7.7 less than your scope.

There is simple way to get slow telescope to be faster than your telescope - change pixel size.

Say you are using 200mm F/4 telescope again, and you have 4.3µm pixel size and 10 photons fall on your aperture per your single pixel in exposure - your pixel has 10e.

I'm going to use 100mm F/10 telescope. Now, my telescope is both smaller aperture and slower than your scope, how could it possibly be faster?

I'm going to use 12.9µm pixel size with my telescope.

If your telescope gathers 10e per exposure - that means that 1.2321"^2 of the sky produces 10 photons on 200mm aperture. 1"^2 will produce  10/1.2321 photons on 200mm aperture - that is ~8.116 photons per 200mm or 8.116/4 per 100mm and that is ~2.0291 photons per arc second squared per 100mm of aperture.

What is my sampling resolution? I use 1000mm FL and 12.9µm pixel size and that gives 2.66"/px or 2.66" x 2.66" = 7.0756"^2 of sky is covered by my pixel

Now, my 100mm of aperture gathers 2.0291 photons per exposure per 1"^2 and 7.0756 of arc second squared end up in single pixel in my setup - that means that my pixel gathers 2.0291 x 7.0756 = ~14.3571e

There you go - your pixel gathered 10e and my pixel gathered 14.3571e - my setup is faster by 40%, regardless of the fact that I have four times less aperture and my system is F/10 vs yours that is F/4

How about that?

Again, it is aperture at resolution that defines speed. If you decrease aperture  and want to get fast system - decrease resolution more. In above case aperture was halved (thus x4 less photons were captured) but I increased pixel size by factor of x3 to offset for that and slight change in focal length (from 800mm to 1000mm).

 

 

[alacant]

33 minutes ago, vlaiv said:

How about that?

Wow.

Thanks vlaiv. That's amazing. You're gonna have to give me time to digest it all!

[planetman83]

On 31/12/2020 at 16:33, AlanP_ said:

I think i'll just order that cable to be on the safe side. 

I've messaged telescop-express about the dovetail, hopefully they can clear things up for me.

Thanks for your help again!

Did we finally learn if the dovetail is included?

[AlanP_]

On 22/01/2021 at 23:00, planetman83 said:

Did we finally learn if the dovetail is included?

It was! They told me it wasnt and I ended up buying one for 45 pounds from FLO. The one I bought was a lot bigger though, so it will make balancing a bit easier. However, I would have rathered not spend the extra money

[planetman83]

1 hour ago, AlanP_ said:

It was! They told me it wasnt and I ended up buying one for 45 pounds from FLO. The one I bought was a lot bigger though, so it will make balancing a bit easier. However, I would have rathered not spend the extra money

So, did you use the scope for astrophotography? Please share your impressions.

[AlanP_]

8 hours ago, planetman83 said:

So, did you use the scope for astrophotography? Please share your impressions.

Unfortunately my mount is still stuck in the depot! I'm in Ireland and deliveries are held up a lot due to Brexit atm. I paid customs and everything, just waiting for the courier. I wouldn't mind, but theres been 4 crystal clear nights this week and lots of clouds forecasted for next! Hopefully it does arrive this week though. I'll certainly share my progress!