What are the pitfalls of combining a scope with a non-purpose designed reducer?

[Giles_B]

I have an f7.8 Altair Wave 125EDF which I combine with the recommended 0.8 reducer for astrophotography with a ZWO 1600mm - this gives me a focal ratio of 6.24

Today a Altair Lightwave 0.6 reducer has popped up on ABS, and I'm tempted to buy it at the price so I have another focal length (a fairly fast f4.6) to play with. But I'm concerned the results might be disappointing - I expect that there will be some vignetting due to the increased reduction factor (is this what you call it?) but will this be a lot given my current image train, and are there any other problems I should be alert to?

I'd be grateful for thoughts and advice.

[Clarkey]

Normally I would say "try it and see", but obviously this won't help! The standard 0.8 reducer is designed for full frame use, so would think that the 0.6 should work on the 1600. I suspect if you are a pixel peeper, the stars might not be perfect - but it should be OK.

 

Might help...

 

[Giles_B]

hmmm, that's tempting. Not sure if I'd obsessed with round stars - maybe if I'd noticed them... I suppose I could always create synthetic stars on Siril if it really got to me.

Is there any formula that will give me my image size based on my aperture and focal ratio?

[Clarkey]

32 minutes ago, Giles_B said:

Is there any formula that will give me my image size based on my aperture and focal ratio?

I guess you can use the filter size calculator. This does not have 0.6 reducer - but close enough. The 0.6 reducer has 38mm aperture which suggests it would be OK. The 75mm is a guess at the distance from the glass. I suspect you will get some vignetting, but for a 1600 sensor this suggests it would be OK. (I would take a punt - but don't take my word for it).

[Giles_B]

Thanks - yes, definitely moving in the direction of a punt!

[Elp]

If it does fit your focuser I'd assume there'd be some tinkering with the backspacing, and you may not get round stars at the edge. My apex 0.65x I've never got round stars even with a 183 sensor (13 x 9) but it doesn't bother me that much, the wide field is extremely useful.

Edited April 29 by Elp

[Giles_B]

Focuser should do it - the reducer is designed to fit a 2" and has a T2 connector, and requires 55mm of backspacing thereafter - so all eminently doable. Square stars we can fix , but my worry was that the image circle being much too small to be useful, but it sounds like this won't be a dramatic problem - or at least it's worth a go to find out.

Edited April 29 by Giles_B

[Adam J]

I generally think that 0.6x reducers only work really well on small chip cameras like a IMX533 or smaller. 

Adam

[Giles_B]

Just now, Adam J said:

I generally think that 0.6x reducers only work really well on small chip cameras like a IMX533 or smaller. 

Adam

Can you explain why - because of the small image circle? square stars? pixel size? Or just personal preference because of all of the above?

[Elp]

As long as the imaging circle is large enough for the sensor it should be fine. Vignette you sort out with flats.

[Adam J]

18 minutes ago, Giles_B said:

Can you explain why - because of the small image circle? square stars? pixel size? Or just personal preference because of all of the above?

Its a complex thing to put into words but in the end what you are doing with a corrector is correcting the field curvature as a function of distance from the optical axis. The amount of correction needed changes with distance from the optical axis, so with a faster corrector you have a steeper correction "gradient" (not the right term but I am having a go here), you need to compensate for that gradient more precisely, That means that you need a better match between the original profile of the field to be corrected and the correctors profile itself. So you will need more precision back focus, but also any miss match in that gradient is more significant. On top of this you are also going to increase vignetting with a faster corrector. The larger the chip the worse it all gets.  It also gets more sensitive to tilt in the corrector relative to the principle optical axis. 

That is the best i can explain it.  In essence 0.8x is the standard for a reason, its a design balance. 

Adam

Edited April 29 by Adam J

[ollypenrice]

The only reason to use a reducer is to increase workable field of view. If it's not going to do that, it's not worth bothering.

If you would like to swap more signal for less resolution you can resample the image downwards before processing. Beware the F ratio myth when it comes to reducers.

Olly

[Adam J]

42 minutes ago, ollypenrice said:

The only reason to use a reducer is to increase workable field of view. If it's not going to do that, it's not worth bothering.

If you would like to swap more signal for less resolution you can resample the image downwards before processing. Beware the F ratio myth when it comes to reducers.

Olly

I disagree you are exchanging Resolution for both FOV and Reduced time to reach a given SNR. As a Rasa owner its exactly the same thing as your scope is doing only it is preconfigured to do that.

Resampling is worse because you exchange resolution for speed without also gaining FOV.  Can only do both with a reducer. 

Adam, 

Edited April 29 by Adam J

[ollypenrice]

28 minutes ago, Adam J said:

I disagree you are exchanging Resolution for both FOV and Reduced time to reach a given SNR. As a Rasa owner its exactly the same thing as your scope is doing only it is preconfigured to do that.

Resampling is worse because you exchange resolution for speed without also gaining FOV.  Can only do both with a reducer. 

Adam, 

I think it's all about maximizing the useful object photons.  If you want object photons from a wider object then the focal reducer makes perfect sense because it captures them when the narrower FOV doesn't and would require a time-consuming mosaic.

But if the object fits on the chip with and without the reducer, the reducer brings in precisely zero new photons. What matters now is what you do with them.

Olly

 

[Giles_B]

10 minutes ago, ollypenrice said:

I think it's all about maximizing the useful object photons.  If you want object photons from a wider object then the focal reducer makes perfect sense because it captures them when the narrower FOV doesn't and would require a time-consuming mosaic.

But if the object fits on the chip with and without the reducer, the reducer brings in precisely zero new photons. What matters now is what you do with them.

Olly

 

That's a helpful exchange for me. Both points of view seem to make sense, even if you disagree. My primary objective is to increase the FOV, but it's interesting to think about increasing the SNR - I'll be interested to see if I get less noisy images as a result, although I'm unlikely to spend enough time looking at the same object with different set ups to prove this one way or another.

[Elp]

From my experience imaging Thor's Helmet at three different focal lengths I get the impression a shorter focal length (hence more signal concentrated across fewer pixels/area) gives the perception of better (brighter) signal given exposure times are equal. My Z61 (370mm) data was definitely brighter than the Starfield 102 (714mm) or C6 at 1045mm in the strong signal parts of the nebula.

Edited April 29 by Elp

[ollypenrice]

50 minutes ago, Giles_B said:

That's a helpful exchange for me. Both points of view seem to make sense, even if you disagree. My primary objective is to increase the FOV, but it's interesting to think about increasing the SNR - I'll be interested to see if I get less noisy images as a result, although I'm unlikely to spend enough time looking at the same object with different set ups to prove this one way or another.

If your primary objective is to increase FOV then you would be making proper use of a focal reducer.

If you want a better SNR at a given focal length then you can either increase the aperture (by changing the scope, since Aperture Increasers do not exist ) or you can put the object photons you have onto fewer 'Effective Pixels' by binning or resampling.  If you are over sampled, doing the latter comes at zero cost in terms of resolution.

57 minutes ago, Elp said:

From my experience imaging Thor's Helmet at three different focal lengths I get the impression a shorter focal length (hence more signal concentrated across fewer pixels/area) gives the perception of better (brighter) signal given exposure times are equal. My Z61 (370mm) data was definitely brighter than the Starfield 102 (714mm) or C6 at 1045mm in the strong signal parts of the nebula.

But try taking your 3 focal length images at the linear stage and resample the two longer focal lengths till Thor's Helmet is the same size as it is in the shortest, then compare them as you stretch them.  This is a meaningful comparison and cuts to the heart of the matter.

The reason that talk of F ratio doesn't dissolve into nonsense in the daytime photography world is that, in that world, the pixel size and focal length are constant and only the aperture varies. 

Olly

[Giles_B]

2 hours ago, ollypenrice said:

If your primary objective is to increase FOV then you would be making proper use of a focal reducer.

If you want a better SNR at a given focal length then you can either increase the aperture (by changing the scope, since Aperture Increasers do not exist ) or you can put the object photons you have onto fewer 'Effective Pixels' by binning or resampling.  If you are over sampled, doing the latter comes at zero cost in terms of resolution.

But try taking your 3 focal length images at the linear stage and resample the two longer focal lengths till Thor's Helmet is the same size as it is in the shortest, then compare them as you stretch them.  This is a meaningful comparison and cuts to the heart of the matter.

The reason that talk of F ratio doesn't dissolve into nonsense in the daytime photography world is that, in that world, the pixel size and focal length are constant and only the aperture varies. 

Olly

Thanks Olly.

So, my main object since about February has been to take images of galaxies. I've been doing this at 0.8x because my understanding was that f7.8 would make my targets too dim, even though the FOV would be better for objects of the size of M101 etc.

However from what you say, I've been misled and as a rule should always be guided by the FOV and not get hung up on the focal ratio - ultimately I'll get the same photons and a better image at the higher f ratios.

Scope is getting a clean at Optrep at the mo (filthy objective , long story) - but it sounds like I have a new project when it comes back!

 

[ollypenrice]

9 hours ago, Giles_B said:

Thanks Olly.

So, my main object since about February has been to take images of galaxies. I've been doing this at 0.8x because my understanding was that f7.8 would make my targets too dim, even though the FOV would be better for objects of the size of M101 etc.

However from what you say, I've been misled and as a rule should always be guided by the FOV and not get hung up on the focal ratio - ultimately I'll get the same photons and a better image at the higher f ratios.

Scope is getting a clean at Optrep at the mo (filthy objective , long story) - but it sounds like I have a new project when it comes back!

 

Yes, this is my view. A given size of objective collects a given amount of light and a given amount of information in a certain time. To make a large image of a galaxy requires more information than is needed for a small image of a galaxy, so it needs more time. There are various ways of controlling the image size, a focal reducer just being one of them.

You'll soon find that, when imaging close to the limit of the seeing, you'll need a lot more time to make your image presentable at full size than at 66% of full size. For a given objective size, the thing to remember is that there is no short cut to get round this, even if focal reducers seem to suggest that there is.

Olly

[Elp]

On 29/04/2024 at 20:04, ollypenrice said:

try taking your 3 focal length images at the linear stage and resample the two longer focal lengths till Thor's Helmet is the same size as it is in the shortest, then compare them as you stretch them.  This is a meaningful comparison and cuts to the heart of the matter.

Probably not done completely accurately but I scaled the linear C6 reduced image down to the same (or similar) as the Z61 before stretching both, it looks closer but the Z61 data still looks slightly better to me, could be sky transparency or something (C6 also doesn't look well collimated, actually I think it was the guiding):

 

Edited May 2 by Elp