ed80 ds pro f7.5 speed

[iwols]

hi all just wondered what is similar to this scope but quicker and time wise for data what does it mean cheers

[Uplooker]

Can I ask why you have posted your question using white text on a white background??????????

I must be missing something but this seems to becoming more and more common. It makes it difficult or impossible to read posts

[iwols]

sorry ian its a copy and paste problem

[LightBucket]

It's when it's copied from a page that shows it as black text on white background, then when you paste here it converts the text to white automatically....I have done it myself

[LightBucket]

To answer part of the question, being quicker time wise for data, means having a lower f ratio scope, the lower the more light is collected, the lower the f number the larger the FOV so more light gathered, so no need to image for so long....in a nutshell.  

[Racey]

6 minutes ago, LightBucket said:

It's when it's copied from a page that shows it as black text on white background, then when you paste here it converts the text to white automatically....I have done it myself

The paste as plain text option removes any formatting I believe...

[ronin]

Not that many scopes really, the ED 80 has been around a while, not sure if originally it was a WO scope and the lens was rehoused by Skywatcher - there is a lot of mixing of components. Maybe go search out the WO Zenithstar 80 for the fun of it. Also if really want to WO Megrez 80.

The "closest" I think you will get is the Primaluce 80mm which I think is a WO scope clone. PL seem to have taken on the role of picking up some WO designs and manufacturing them still. Shame that WO didn't do it.

Will say that as the scopes get faster then the glass gets pushed to it's limit and eventually you get some CA appering. Both are doublets, even if one component is defined as ED glass. Look at it like this if you want a worst case: Both are achromats that use a fancy glass.

[ollypenrice]

The situation is actually quite complicated, requiring lots of different factors to be considered simultaneously. If you really want to understand what is happening you have to take a bit of time over this. People giving you four word answers will give you wrong answers. What does 'fast' mean? That is dead easy to explain.

Now, does this mean that F5 is 4x faster than F10? Alas no. In a sense it is, in a sense it isn't. That is for the next stage of the conversation!

Olly

[newbie alert]

A smaller f number the less imaging time to get the same results so less time imaging for the same data..

You can get the f ratio lower using a focal reducer..

[alacant]

20 minutes ago, newbie alert said:

get the f ratio lower using a focal reducer..

But you're still collecting the same amount of light, so all it does is give you a wider field of view. AFAICT the only way to get the same data faster is to use a bigger aperture. To get roughly the same data as an 80ed faster, you could use a 150mm f4. Then you'd really see how dim f7.5 was! Just my €0.02 but HTH.

[iwols]

hi all just saw the Skywatcher Esprit 100 ED f/5.5  how would this compare to my ed80

[newbie alert]

20 hours ago, alacant said:

But you're still collecting the same amount of light, so all it does is give you a wider field of view. AFAICT the only way to get the same data faster is to use a bigger aperture. To get roughly the same data as an 80ed faster, you could use a 150mm f4. Then you'd really see how dim f7.5 was! Just my €0.02 but HTH.

Yes I agree but the reason to stack the image is to make it brighter..if it was just about aperture then everyone would just use sct's..  but unfortunately they're all native f10 and to get it to perform a bit better on dso you need to make it faster using a reducer at f6.3.. also a lot of imagers use fast ratio frac/newts etc for that very reason

[laser_jock99]

On 18/05/2017 at 11:10, iwols said:

hi all just wondered what is similar to this scope but quicker and time wise for data what does it mean cheers

The GSO 150mm F4 is the 'fast' 600mm alternative:

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4762_GSO-6--Imaging-Newton---150-mm-Oeffnung-f-4---2--MONORAIL.html

They are an 'interesting' little scope, I have one- but maybe better for the more experienced imager.

GSO 150mm F4

With the right 0.7x coma corrector, you can turn the 150mm F4 scope  into an ultra fast F2.9 scope.........

[ollypenrice]

22 hours ago, newbie alert said:

A smaller f number the less imaging time to get the same results so less time imaging for the same data..

You can get the f ratio lower using a focal reducer..

Alas, totally incorrect. Sorry but this is a beginner mistake. You need to understand the concept of 'object photons' and also, in modern imaging, the business of signal to noise. You can get a lower F ratio by using a focal reducer but you cannot get more object photons that way.

Olly

[newbie alert]

Sorry  Olly,yes I'm a newbie so making a few schoolboy errors on the way.. but I never said that you would get more photons..

So if the f ratio is dismissive why do imagers use the fastest f number scopes they can?

It's all a bit confusing in the newbie world 

[ollypenrice]

44 minutes ago, newbie alert said:

Sorry  Olly,yes I'm a newbie so making a few schoolboy errors on the way.. but I never said that you would get more photons..

So if the f ratio is dismissive why do imagers use the fastest f number scopes they can?

It's all a bit confusing in the newbie world 

No worries. In order to think clearly about this you have to compare like with like, which means comparing images taken at the same focal length. Now in the camera world that is always the case, since a camera lens has a certain focal length (even a zoom once set) and it has an aperture. The focal length is not variable but the aperture is. Open the aperture with the diaphragm and you get more light and shorter exposures. Nobody will argue with that.

But if you reduce the focal length without increasing the aperture you get no new photons (and so no increase in speed) from an object which fitted on your chip previously. You do concentrate those photons onto fewer pixels, but this may or may not be important. As long as your signal is strong enough to get over the read noise, you're in.

If the reducer increases your FOV to include photons you do want then it brings in new photons and reduces exposure time at the cost of resolution.

In a nutshell the situation is target-specific. This is a much discussed and much misunderstood topic but I think I have got my head around it - which does not mean that I have!!!

Olly

PS I failed to answer your question, 'why do imagers use the fastest f number scopes they can?'

They don't. At least, this one certainly doesn't! I boost speed by using two F5 scopes in parallel. This is, by definition, worth one F stop over a single scope. Why not find a scope of the same focal length and twice the light grasp? Because fast scopes are difficult scopes. They have very steep light cones making them ultra sensitive to focus, tilt and collimation. As an impaging provider I want kit that just does it, night after night. I feel that I get that from two premium F5 rafractors working, effectively as an F4 system. In this game the devil is in the detail.

[jambouk]

I love these threads; poor Olly must be fed up of typing this stuff out every few weeks

[LightBucket]

19 minutes ago, jambouk said:

I love these threads; poor Olly must be fed up of typing this stuff out every few weeks

He should keep it in a text file and just cut and paste.....

[wimvb]

8 hours ago, LightBucket said:

He should keep it in a text file and just cut and paste.....

... and get white text on a white background?

[wimvb]

12 hours ago, newbie alert said:

Yes I agree but the reason to stack the image is to make it brighter.

Alas, no. Since stacking involves taking the average or median of a number of pixels, the result will not be an increase of brightness, or pixel value, but a decrease in noise. Stacking increases snr. It also removes outliers, such as data from bad pixels or satelite/plane trails.

The increased snr (lower noise) allows for more aggressive processing, revealing fainter signal.

[ollypenrice]

10 hours ago, jambouk said:

I love these threads; poor Olly must be fed up of typing this stuff out every few weeks

Heheh, I don't mind at all.

55 minutes ago, wimvb said:

Alas, no. Since stacking involves taking the average or median of a number of pixels, the result will not be an increase of brightness, or pixel value, but a decrease in noise. Stacking increases snr. It also removes outliers, such as data from bad pixels or satelite/plane trails.

The increased snr (lower noise) allows for more aggressive processing, revealing fainter signal.

I was tempted to make the same point. However, in a sense we do stack in order to make the image brighter or, more precisely, to make the possible image brighter. We can only stretch to some user-defined point close to the final noise floor of the stack. (Exactly where is a subjective judgement.) At this point we have to stop stretching. However, if we have four times as much data we can stretch much harder and the key faint details can be rendered more brightly. So there is a relationship between signal to noise and signal which can be stretched into visibility. Essentially I think we agree.

Olly

[wimvb]

23 minutes ago, ollypenrice said:

Heheh, I don't mind at all.

I was tempted to make the same point. However, in a sense we do stack in order to make the image brighter or, more precisely, to make the possible image brighter. We can only stretch to some user-defined point close to the final noise floor of the stack. (Exactly where is a subjective judgement.) At this point we have to stop stretching. However, if we have four times as much data we can stretch much harder and the key faint details can be rendered more brightly. So there is a relationship between signal to noise and signal which can be stretched into visibility. Essentially I think we agree.

Olly

Exactly. I had added to my earlier reply (but then deleted), that if the stacking process would increase brightness, we would eventually end up with white, bloated stars or an entirely white image. AfaIk, the only instance where stacking is used to make an (entire) image brighter, is in video astronomy, where images are added during capture to increase signal.

[LightBucket]

1 hour ago, wimvb said:

... and get white text on a white background?

Yes, that was kinda the joke.... ok wasn't that funny...

[wimvb]

 

4 minutes ago, LightBucket said:

Yes, that was kinda the joke.... ok wasn't that funny...

Sure it was. (The  was implied in my response)

And now you made me wonder if I should add another  to this response.

Okay, here goes.

Cheers,