Which focal reducer?

[rockinrome]

Hello all and thanks for looking - not posted for a while...

Hopefully a quick question to answer.

I have a 6" Ritchey Chrétien and wish to focally reduce at most 0.5x.

I already have a 0.6x reducer/flattener and tried this. All OK, but has some vignetting on the edges. So my question is two-fold I guess:
(1) Is that reducer OK seen as it is also a flattener?
(2) Is the vignetting due to (1) or simply just the way the light falls on the sensor?

I did looke into a simple 0.5x reducer - would that work (https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-2-focal-reducer.html?gclid=CjwKCAjw-vmkBhBMEiwAlrMeF1d9yhWga3xDRvB1zOQlknp0PMXz5nsb-AP9kOOHMpqVeunYdsX6LBoCbeQQAvD_BwE)
Or do I need more like this: https://www.firstlightoptics.com/reducersflatteners/astro-essentials-075x-reducer-for-stellalyra-gso-ritchey-chretien-ota.html

Thanks again.

Clear skies.
Matthew

 

 

[Elp]

Does your existing reducer introduce coma/elongated stars at the outer field of view? It's a general consensus you shouldn't reduce more than around 0.8, there's a reason most good reducers do not exceed 0.75x.

What are you trying to achieve? You ask is it okay, only you can decide this depending on what your intended use is.

Vignette is due to the illuminated imaging circle on the sensor of the camera, if it's close to your sensor size diagonal length it'll be more apparent, lenses due to their shape and construction generally all suffer from some sort of vignette at the edges, it's easily corrected with flats or via post processing software.

[rockinrome]

3 minutes ago, Elp said:

Does your existing reducer introduce coma/elongated stars at the outer field of view? It's a general consensus you shouldn't reduce more than around 0.8, there's a reason most good reducers do not exceed 0.75x.

What are you trying to achieve? You ask is it okay, only you can decide this depending on what your intended use is.

Vignette is due to the illuminated imaging circle on the sensor of the camera, if it's close to your sensor size diagonal length it'll be more apparent, lenses due to their shape and construction generally all suffer from some sort of vignette at the edges, it's easily corrected with flats or via post processing software.

Hi Elp, thanks for the reply.

Yes, I do get slightly elongated stars with the 0.6x.
My goal was to get a faster system to reduce exposure times and to get a better match for my camera (533C).

Thank you.

[Elp]

To be honest, from my experience, reducers only help so much with regard to speed. Aperture makes more of a difference. And you have to change your train of thought that regardless, for a good image you'll be imaging for 10-20 hours at least.

Edited July 3, 2023 by Elp

[vlaiv]

11 minutes ago, rockinrome said:

My goal was to get a faster system to reduce exposure times and to get a better match for my camera (533C).

Better match in which way?

[rockinrome]

1 minute ago, vlaiv said:

Better match in which way?

My arc seconds per pixel.

[rockinrome]

9 minutes ago, Elp said:

To be honest, from my experience, reducers only help so much with regard to speed. Aperture makes more of a difference. And you have to change your train of thought that regardless, for a good image you'll be imaging for 10-20 hours at least.

Thank you for the information. Appreciated.

[vlaiv]

1 minute ago, rockinrome said:

My arc seconds per pixel.

Why don't you just bin the data? That will act as x2 reducer as far as arc seconds per pixel is concerned.

It won't widen you field of view though, but not sure if you are interested in that

[rockinrome]

2 minutes ago, vlaiv said:

Why don't you just bin the data? That will act as x2 reducer as far as arc seconds per pixel is concerned.

It won't widen you field of view though, but not sure if you are interested in that

Yeh. I have tried that route and that is great, but as you point out does not widen my view (not that that is a problem) ..... but yeh, deffo a route to think about.... Thanks.

[vlaiv]

2 minutes ago, rockinrome said:

Yeh. I have tried that route and that is great, but as you point out does not widen my view (not that that is a problem) ..... but yeh, deffo a route to think about.... Thanks.

Reducer is worth using if you want to widen the FOV somewhat. However - you can't really go to arbitrary reduction factor with it.

Maybe best way to think about it in first iteration would be not in terms of focal length reduction but rather as "sensor size increase".

Say you want to use x0.6 reducer on your scope. Your sensor is 16mm diagonal as is. With x0.6 reducer - you will have 16mm / 0.6 = 26.666mm sensor size. Almost APS-C size, and at that size - several things start to happen. First - edge of the field astigmatism starts to show, second - depending on type of reducer - you will start to get vignetting. Pretty much all the things that you already noticed.

You can certainly use x0.5 reducer on this scope - but in that case, I'd look into sensor below 10mm in diagonal. In fact - for best results, stick to 4/3 sensor size - approximately 22-23mm in diagonal. This would mean 16/22 = ~0.72 in your case.

 

 

[rockinrome]

3 minutes ago, vlaiv said:

Reducer is worth using if you want to widen the FOV somewhat. However - you can't really go to arbitrary reduction factor with it.

Maybe best way to think about it in first iteration would be not in terms of focal length reduction but rather as "sensor size increase".

Say you want to use x0.6 reducer on your scope. Your sensor is 16mm diagonal as is. With x0.6 reducer - you will have 16mm / 0.6 = 26.666mm sensor size. Almost APS-C size, and at that size - several things start to happen. First - edge of the field astigmatism starts to show, second - depending on type of reducer - you will start to get vignetting. Pretty much all the things that you already noticed.

You can certainly use x0.5 reducer on this scope - but in that case, I'd look into sensor below 10mm in diagonal. In fact - for best results, stick to 4/3 sensor size - approximately 22-23mm in diagonal. This would mean 16/22 = ~0.72 in your case.

 

 

That is great information - thank you very much for your time.

[ollypenrice]

It's paradoxical, but I usually have two or three focal lengths on the go and find the following to be true of what I actually do with each:

Super-wide field (short FL) - usually make mosaics.

Medium field (medium FL) - sometimes make mosaics.

Narrow FOV (long FL) - never make mosaics and usually crop out the object of interest before posting.

This is not based on what the gear can do, it is not a recommendation, it is simply an observation of what I end up doing. With a wide field I want wider, with a medium field I sometimes want wider and with a narrower field I want narrower.

I only post this observation because you might, quite possibly, find the same - in which case you might prefer binning over a reducer. I wouldn't buy any reducer which has not been shown to work with your scope.

Olly

[rockinrome]

5 minutes ago, ollypenrice said:

It's paradoxical, but I usually have two or three focal lengths on the go and find the following to be true of what I actually do with each:

Super-wide field (short FL) - usually make mosaics.

Medium field (medium FL) - sometimes make mosaics.

Narrow FOV (long FL) - never make mosaics and usually crop out the object of interest before posting.

This is not based on what the gear can do, it is not a recommendation, it is simply an observation of what I end up doing. With a wide field I want wider, with a medium field I sometimes want wider and with a narrower field I want narrower.

I only post this observation because you might, quite possibly, find the same - in which case you might prefer binning over a reducer. I wouldn't buy any reducer which has not been shown to work with your scope.

Olly

Thanks Olly, sound advice.
That certainly gives me food for thought before I start spending (or not).