Atik 460 Ed80 and focal reducer

[andyo]

Hi all 

I have been discussing the spacing of the sky ed80 focal reducer in another thread and it has got me thinking about my own set up.

I have a skywatcher ed80pro with an Atik 460ex and presently use the skywatcher focal reducer/flattener for this scope. This was great for when I used an Atik 314L+ with the smaller chip as it gave me a much needed wider field of view and so in my opinion the pro's outweighed the cons. However now with the bigger chip of the 460ex I am wondering whether this is still the case. I am interested in peoples views on this and if anyone uses the ed80 with the 460ex without flattener or reducer and how they find it.

Also as a side issue a question I am wondering about is does a reducer truly change a scopes F ratio so it is faster as it doesn't significantly change its focal length or apperture 

[kirkster501]

I had that combination of SW ED80 and their matching FR/FF with the 460ex and it worked perfectly.  The SW ED80 is a fabulous scope that punches well above its weight.  I still have mine.

[ollypenrice]

I don't know if you'll get a good flat field without the flattener.

To answer your question about what flatteners do, yes they do reduce the focal length and they do speed up the F ratio. However, this cannot be treated by the same F ratio rules as govern camera lenses because in camera lenses you add aperture (more light) to reduce F ratio/exposure times while keeping focal length the same. You do not do this with a reducer, you change the F ratio by changing the focal length without adding aperture. Does this speed up the imaging process? Sometimes yes, sometimes no.

A small object which will fit on the chip will send exactly the same number of photons to your camera with or without the reducer. There is no new light. What changes is that the reducer puts the light onto fewer pixels, 'filling' them faster at the expense of making a smaller and less detailed image of the object. But you could do this by resampling your unreduced image downwards to the size of the reducer image and get pretty much the same effect. (This does assume that you can expose for long enough to get above the read noise. If you can't then the reducer will help you.)

However, on large targets which the reducer gives you the sky coverage to image, the faster F ratio of the reducer will give you a good S/N ratio more quickly than you'd get without. There is, if you like, 'new light' brought in by the outer part of the object which the unreduced system couldn't 'see.'

Professional astronomers will tell you every time that to capture faint objects you need aperture. 

Olly

[andyo]

2 hours ago, ollypenrice said:

I don't know if you'll get a good flat field without the flattener.

To answer your question about what flatteners do, yes they do reduce the focal length and they do speed up the F ratio. However, this cannot be treated by the same F ratio rules as govern camera lenses because in camera lenses you add aperture (more light) to reduce F ratio/exposure times while keeping focal length the same. You do not do this with a reducer, you change the F ratio by changing the focal length without adding aperture. Does this speed up the imaging process? Sometimes yes, sometimes no.

A small object which will fit on the chip will send exactly the same number of photons to your camera with or without the reducer. There is no new light. What changes is that the reducer puts the light onto fewer pixels, 'filling' them faster at the expense of making a smaller and less detailed image of the object. But you could do this by resampling your unreduced image downwards to the size of the reducer image and get pretty much the same effect. (This does assume that you can expose for long enough to get above the read noise. If you can't then the reducer will help you.)

However, on large targets which the reducer gives you the sky coverage to image, the faster F ratio of the reducer will give you a good S/N ratio more quickly than you'd get without. There is, if you like, 'new light' brought in by the outer part of the object which the unreduced system couldn't 'see.'

Professional astronomers will tell you every time that to capture faint objects you need aperture. 

Olly

Thanks olly  for a really good comprehensive explanation, I did think if I was going to get rid of the reducer I would probably have to get a flattener but one that didn't also reduce the field of view. I did also think would I get a decent flat field without one altogether. Now I am thinking maybe a scope with the flattener built in with a shorter focal length to give a faster F ratio and wider field. I am finding using a reducer is limiting in respect of tilt which I believe is affecting my images,I am not able to add a tilt adjuster to the optical train due to the spacing required and I have no other way of fine tuning the system.

[Uranium235]

You might want to think about getting a good prime camera lens if you want to take on the really big stuff, its just a shame Samyang dont make a 200mm f2. If it was anything like their 135mm, I'd be all over it.

The only problem with that setup is getting a filter into the mix, but it can be done with the right adaptors. Also, you need a location to take advantage of the speed, the dark site where I go allows me to expose much longer than what I can at home. But, having said that, I did (just about) manage some dust from the back garden a while ago:

With the smaller pixels your camera has, you'd get a bit more resolution out of it as well.

[andyo]

15 hours ago, Uranium235 said:

You might want to think about getting a good prime camera lens if you want to take on the really big stuff, its just a shame Samyang dont make a 200mm f2. If it was anything like their 135mm, I'd be all over it.

The only problem with that setup is getting a filter into the mix, but it can be done with the right adaptors. Also, you need a location to take advantage of the speed, the dark site where I go allows me to expose much longer than what I can at home. But, having said that, I did (just about) manage some dust from the back garden a while ago:

With the smaller pixels your camera has, you'd get a bit more resolution out of it as well.

Thanks Rob

That is a great shot. Gives me something else to mull over but I am swayed towards an astrograph however it is early days and I will be looking at many options,Not sure how the camera lens idea would work with a filter wheel and how it would all be mounted and guided etc without a complete reworking of my present set up