Help with image circle using reducers?

[libraryman]

Hi all,  i image with my trusty Vixen 102s native focal length f/8.8, i use a 0.8x reducer which brings it down to f/7.2...just in the upper range of acceptable for imaging with my DSLR.

I really want to try a 0.7 x reducer or even a 0.63 x reducer to bring the focal length to a more acceptable 630mm   but i do not understand how these reducers would effect the  image circle and vignetting.

The Vixen was supplied originally  with a reducer to bring it down to f/5.5 but the image was vignetted, these original vixen reducers are now obsolete.

the best i can find now is probably the   Optec NGUW 0.7XL              

Would the money just be better spent on a faster refractor ? The Vixen optics are very good.

any advise appreciated.

Ray 

[Leveye]

The spacing must be just right for a reducer to work properly. Imaging will have a different spacing than just viewing.

[libraryman]

Hi, yes i appreciate the spacing question, the use of the reducer is just for imaging.

Thanks Ray

[ollypenrice]

I don't think there is any way for non opticians to divine in advance the likely performance of an unknown scope-reducer combination. Usually what people do is scour the net for someone who has already tried the combination in question. Failing that, you try it yourself, ideally with borrowed reducer to start with!

The faster the reduction the harder it is to sustain a decent sized image circle.

When you say the Vixen has good optics I'm sure you're right, but in part this quality will be a product of the slow F ratio. It is an old design, probably predating CCD with its deep penetration into the short wavelengths on the edge of the visible spectrum. I think you'll be asking an awful lot of it at F6.3 though... 

Olly

[libraryman]

Cheers Olly, a wise and thought provoking reply, I feel that has helped me somewhat.

Ray

[lensman57]

Hi all,  i image with my trusty Vixen 102s native focal length f/8.8, i use a 0.8x reducer which brings it down to f/7.2...just in the upper range of acceptable for imaging with my DSLR.

I really want to try a 0.7 x reducer or even a 0.63 x reducer to bring the focal length to a more acceptable 630mm   but i do not understand how these reducers would effect the  image circle and vignetting.

The Vixen was supplied originally  with a reducer to bring it down to f/5.5 but the image was vignetted, these original vixen reducers are now obsolete.

the best i can find now is probably the   Optec NGUW 0.7XL              

Would the money just be better spent on a faster refractor ? The Vixen optics are very good.

any advise appreciated.

Ray 

What Olly says makes sense and  as a general rule unless your scope and the FF/FR were designed for each other and with imaging in mind you'd be well advised to assume that at best the combination would just barely cover an APS size sensor unless it was a TAK FSQ  85 or 106 or something similar. The Celestron / Meade F3.3 FF/FRs for example would just about cover a 1/4" format chip, that is SPC 900 size chip, the F6.3 was a little better up to 2/3" format. My SW 0.85 FF/FR covers a DSLR sensor but with vignetting noticable from 2/3 of the way towards the edges.

A.G

[libraryman]

Hi and thanks for the reply, yes i currently use a TV 4087 0.8x  reducer, it gives me a reasonable field without too much Vignetting on the DSLR sensor and i get round stars all the way across the image.

However, imaging at f/7 + has its drawbacks and i wanted to try and get faster imaging and a wider field without the expense of Tak or TV scopes..

Many thanks

Ray 

[lensman57]

Hi and thanks for the reply, yes i currently use a TV 4087 0.8x  reducer, it gives me a reasonable field without too much Vignetting on the DSLR sensor and i get round stars all the way across the image.

However, imaging at f/7 + has its drawbacks and i wanted to try and get faster imaging and a wider field without the expense of Tak or TV scopes..

Many thanks

Ray 

Hi Ray,

I totally understand what you are saying but  if you use an FF/FR to obtain a wider field of view then it is fine but I am beginning to doubt if they do much to speed things up if at all. Thinking about it logically an FF/FR will not increase the size of the aperture to allow more photons in, the indicated reduction in the F number is therefore solely a byproduct the apparent reduction in the FL.

Perhaps you could consider the new SW Esprit range of scopes in the long run, seem to have a natively fast F ratios or even a WO GT 81 @ F5.9.

Regards,

A.G

[ollypenrice]

Hi Ray,

I totally understand what you are saying but  if you use an FF/FR to obtain a wider field of view then it is fine but I am beginning to doubt if they do much to speed things up if at all. Thinking about it logically an FF/FR will not increase the size of the aperture to allow more photons in, the indicated reduction in the F number is therefore solely a byproduct the apparent reduction in the FL.

Perhaps you could consider the new SW Esprit range of scopes in the long run, seem to have a natively fast F ratios or even a WO GT 81 @ F5.9.

Regards,

A.G

If you consider the new (ie focally reduced) image as a whole, on a target which wouldn't fit in the unreduced version, then I think we can be sure that the reducer does speed things up according to the square of the F ratio. If you use a reducer on a target that would already fit on the unreduced image, though, then there is no real gain because there are no new 'object photons.' (I usually use the example of M42 at native and M42 plus Running Man with reducer. Where does the reducer find more photons from the same aperture? It finds them in the Running Man.)

Such, at least, is my thinking.

A surprising number of scope/reducer options have been tried on the net so a search is often fruitful. However, I think that beating your AP 4087 reducer would take some doing. It's a very well regarded bit of glass.

Olly

[libraryman]

Thanks olly, i have always assumed that the speed gain of a f/r & scope combination was independent of any new photons, I.e. the gain is always achieved irrespective of anything else?

I ask the question because I don't fully understand the complexitys! But I appreciate your view point.

I will be looking at another option to get the combination I want, 100mm is as small an optic that I want to use, the Esprits appeal, I'll have a good scout around!

Regards

Ray

[ollypenrice]

Thanks olly, i have always assumed that the speed gain of a f/r & scope combination was independent of any new photons, I.e. the gain is always achieved irrespective of anything else?

I ask the question because I don't fully understand the complexitys! But I appreciate your view point.

I will be looking at another option to get the combination I want, 100mm is as small an optic that I want to use, the Esprits appeal, I'll have a good scout around!

Regards

Ray

It can't be independent of new photons. What a reducer does on a small target is put the available photons onto fewer pixels, 'filling' them faster at the expense of resolution. But then, you could just shoot at native and present the image at a smaller size. I've never beaten myself up over which is better because the real answer is obvious; shoot at native until you have enough light!

Olly

[lensman57]

If you consider the new (ie focally reduced) image as a whole, on a target which wouldn't fit in the unreduced version, then I think we can be sure that the reducer does speed things up according to the square of the F ratio. If you use a reducer on a target that would already fit on the unreduced image, though, then there is no real gain because there are no new 'object photons.' (I usually use the example of M42 at native and M42 plus Running Man with reducer. Where does the reducer find more photons from the same aperture? It finds them in the Running Man.)

Such, at least, is my thinking.

A surprising number of scope/reducer options have been tried on the net so a search is often fruitful. However, I think that beating your AP 4087 reducer would take some doing. It's a very well regarded bit of glass.

Olly

HI Olly,

I have to confess that you are actually the one whom got me thinking about all this FF/FR buisness. The way that I  have thought about is that if the FF/FR would indeed reduce the FL in a tangible physical way then the focus point would have been shifted inwards by a factor equivalent to reduction and therefore reducing the length of the scope but in practice this does not happen. In my limited experience I find that the focus point shifts a little inwards but no where near enough as to make significant difference. Much as the increas in FOV is easily visible and measurable the so called increase in the speed of the capture is not and as yet I am to be convinced of this rather " virtual " phenomena.

Regards,

A.G

[Uranium235]

I think there may be some confusion as to where the extra "speed" is coming from.

I agree that per object there are no new photons - but the bonuses are coming in the form of a better S/N ratio instead. More signal + less noise = better performance (thats how I see it anyway)

[ollypenrice]

I think it's simple. If the object fits on your chip at native FL then shoot at native FL. Shoot enough data. If you want a wider field (with something other than background sky in it) then shoot at a faster F ratio/wider field and get the benefits. But if there is nothing of interest in the wider field don't kid yourslef into thinking that the faster F ratio is really doing anything useful. I really don't see how it can be doing anything useful.

As I say, I think it's simple but I'm pretty simple myself so you have been warned!

Olly

[lensman57]

I think it's simple. If the object fits on your chip at native FL then shoot at native FL. Shoot enough data. If you want a wider field (with something other than background sky in it) then shoot at a faster F ratio/wider field and get the benefits. But if there is nothing of interest in the wider field don't kid yourslef into thinking that the faster F ratio is really doing anything useful. I really don't see how it can be doing anything useful.

As I say, I think it's simple but I'm pretty simple myself so you have been warned!

Olly

Likewisw Olly.

Regards,

A.G

[ChrisLX200]

Before I bought mine I used a FOV indicator box in StarryNight to scan around prospect targets to see how well they would be framed. Whole sections of larger nebs (like the Veil) will fit on the chip without resorting to mosaic mode so that convinced me buying a reducer would make for a worthwhile addition to my kit. The hi-res 490EX should be up for the task too. So don't assume it will be better - check your targets against FOV.

ChrisH

[libraryman]

Some good points of view on this subject, glad I asked the question, my understanding of this was that a f/reducer gives a wider field and concentrates the subject onto fewer pixels, therefore capturing the available photons faster, however, I now realise that if one trys to then stretch the image you see the lower resolution.

With my f/r on the vixen, I can just get m31 to fill the sensor diagonally! Without the reducer, it does not fit.

As for the veil, again I can get the eastern veil on the sensor, however I thought that I was capturing these targets at f/7.2 rarther than the native F/8.8

Am I wrong to conclude this?

Ray

[ollypenrice]

Some good points of view on this subject, glad I asked the question, my understanding of this was that a f/reducer gives a wider field and concentrates the subject onto fewer pixels, therefore capturing the available photons faster, however, I now realise that if one trys to then stretch the image you see the lower resolution.

With my f/r on the vixen, I can just get m31 to fill the sensor diagonally! Without the reducer, it does not fit.

As for the veil, again I can get the eastern veil on the sensor, however I thought that I was capturing these targets at f/7.2 rarther than the native F/8.8

Am I wrong to conclude this?

Ray

No, you're right to conclude this, surely? The only controversy surrounds the 'F ratio myth' which only usefully applies, in my view, when consdiering discrete objects which will fit on the chip reduced and unreduced. The 'myth' itself wrongly asserts that you will capture the object faster with a reducer. What you will really do is simply make it smaller so it looks better. It's analogous with binning. Less noise and less resolution but no free lunch.

Now if you stick to the same focal length but lower the F ratio by increasing the aperture then, yes, you do get a real benefit (and an excellent lunch, but it won't be free!)

Olly

[michael.h.f.wilkinson]

Though the image quality of reducer/scope combinations is hard to guess, computing the image circle is fairly straightforward (at least to a good approximation or an upper bound). Simply measure the clear aperture of the front of the reducer and multiply with the reduction factor. This is the upper bound of the resulting image circle (in practice, the image circle may be a bit smaller). Thus, starting out from a (fairly optimistic) 48mm for a 2" reducer, at 0.8x you get an image circle of 38.4mm at most. Moving to 0.7x and 0.63x you get 33.6 and 30.24 respectively. Only if you use a reducer with a larger front aperture (2.5" and 3" are available, but you would need to have a big enough focuser), do you get bigger image circles.  

[libraryman]

Though the image quality of reducer/scope combinations is hard to guess, computing the image circle is fairly straightforward (at least to a good approximation or an upper bound). Simply measure the clear aperture of the front of the reducer and multiply with the reduction factor. This is the upper bound of the resulting image circle (in practice, the image circle may be a bit smaller). Thus, starting out from a (fairly optimistic) 48mm for a 2" reducer, at 0.8x you get an image circle of 38.4mm at most. Moving to 0.7x and 0.63x you get 33.6 and 30.24 respectively. Only if you use a reducer with a larger front aperture (2.5" and 3" are available, but you would need to have a big enough focuser), do you get bigger image circles.  

Thank you for this calculation, that really does help. i could ask loads of questions about this subject but I'll quit while I'm ahead. 

Ray

[libraryman]

No, you're right to conclude this, surely? The only controversy surrounds the 'F ratio myth' which only usefully applies, in my view, when consdiering discrete objects which will fit on the chip reduced and unreduced. The 'myth' itself wrongly asserts that you will capture the object faster with a reducer. What you will really do is simply make it smaller so it looks better. It's analogous with binning. Less noise and less resolution but no free lunch.

Now if you stick to the same focal length but lower the F ratio by increasing the aperture then, yes, you do get a real benefit (and an excellent lunch, but it won't be free!)

Olly

Thank you Olly...beginning to understand this now.