6SE Celestron Focal Reducer Help

[daveangie0110]

Hi All

Can anyone give me some advice and help i am a complete novice as you can tell !

I have a Celestron 6SE and I am thinking of getting a Celestron focal reducer so I can try to take some short exposure shots of DSO with my DSLR and ZWOASI camera, i have watched some youtube videos and they seem very good.

After reading some forums they say you have to have 105mm between the focal reducer and the camera sensor, why is this ?

I done a rough measurement i connected my T-Ring and my DSLR to my scope and i should be there or there abouts of 100 to 105mm with a focal reducer fitted.

Are they worth the £100 price tag ?

Doses a focal reducer make that much difference to your images ?

With my mount i am limited to 25-30 sec exposures max, will a focal reducer mean i could do longer exposures ?

Can i use the focal reducer with my ZWOASI camera ?

Many thanks for your help

[knobby]

It will roughly double the amount of light getting to the sensor similar to doubling your exposure time.

The spacing is where the light path is at its flattest ie stars at the edge of picture are sharpest . The further away you get from optimum the worse the edges get .

Worth it ...

Great question ?

I have one , with it I can fit the moon in one frame , without it I can't

[Davey-T]

They come up pretty regularly S/Hand, interchangeable with the Meade one.

Dave

[ollypenrice]

They widen the field of view.

They shorten the focal length so tracking becomes more tolerant.

They do or they don't reduce exposure time, though. If you are photographing something that will fit on the chip at F10, they don't. They just make it smaller on the chip, and brighter, but you could do that in software with an unreduced image. If you are imaging something that fills the chip with the reducer then you'll get a good image much faster. Exp time in this case goes as the square of the F ratio so 6.3x6.3 = 39.69 (eg seconds) while 10X10 = 100 (eg seconds).  Quite a difference.

They also flatten the field, cleaning up distortions towards the edge.

Why do they have to have the chip at a certain distance? To be honest I'm not sure, but this is true of all 'add on' reducers and flatteners. Only quadruplet/quintuplet refractors with fixed rear elements are exempt. You just focus and that's that. The good news with the Meade-Celestron reducer flattener is that it is very tolerant. Some require sub mm perfection. Not these.

Sorry but I don't know your camera at all so can't help on fitting.

Olly

[rwg]

If you change the distance between the focal reducer and the chip then you change the reduction ratio (in the same way that you can increase the magnification of a barlow by putting an extension tube between it and the eyepiece). I expect that the best results in terms of a flat field at the sensor will be obtained at the specified distance. This will be more important for the larger sensor on the DSLR than the small sensor of the ZWO camera.

You'll be able to connect your ZWO camera as follows

telescope -> reducer -> visual back -> 1.25" nosepiece -> camera

The visual back came with your SCT and is the bit you put the diagonal into. The 1.25" nosepiece came with your ZWO camera.

cheers,

Robin

[daveangie0110]

Thanks all, so if I was to mainly use my dslr with the focal reducer will it be worth the £100 ?

[ollypenrice]

Thanks all, so if I was to mainly use my dslr with the focal reducer will it be worth the £100 ?

Yes, without doubt. 

Olly

[lensman57]

Hi All

Can anyone give me some advice and help i am a complete novice as you can tell !

I have a Celestron 6SE and I am thinking of getting a Celestron focal reducer so I can try to take some short exposure shots of DSO with my DSLR and ZWOASI camera, i have watched some youtube videos and they seem very good.

After reading some forums they say you have to have 105mm between the focal reducer and the camera sensor, why is this ?

I done a rough measurement i connected my T-Ring and my DSLR to my scope and i should be there or there abouts of 100 to 105mm with a focal reducer fitted.

Are they worth the £100 price tag ?

Doses a focal reducer make that much difference to your images ?

With my mount i am limited to 25-30 sec exposures max, will a focal reducer mean i could do longer exposures ?

Can i use the focal reducer with my ZWOASI camera ?

Many thanks for your help

This is my own opinion but I think that this reduction of exposure by using a FF/FR is a myth. It does reduce the FL of the scope by whatever factor it is designed to do but it will not bring in more light through the aperture of the scope so your FOV will be wider but whether this translates into shorter exposures I wouldn't bet on. Yes it is worth the money if you are into imaging.

A.G

[knobby]

This is my own opinion but I think that this reduction of exposure by using a FF/FR is a myth. It does reduce the FL of the scope by whatever factor it is designed to do but it will not bring in more light through the aperture of the scope so your FOV will be wider but whether this translates into shorter exposures I wouldn't bet on. Yes it is worth the money if you are into imaging.

A.G

It always makes me wonder too ... How van an extra piece of glass in the light path allow more light through

[andrew s]

This is my own opinion but I think that this reduction of exposure by using a FF/FR is a myth. It does reduce the FL of the scope by whatever factor it is designed to do but it will not bring in more light through the aperture of the scope so your FOV will be wider but whether this translates into shorter exposures I wouldn't bet on. Yes it is worth the money if you are into imaging.

A.G

A.G you are right it can't increase the amount of light through the aperture stop but it reduces the image scale and so the number of photons per pixel goes up (with a corresponding opportunity to reduce exposure and assuming typical UK seeing). Assuming the design of the scope can fully illuminate the chip at the shorter focal length/ wider field of view then more photons will hit the CCD rather than missing it at the longer focal length.

Regards Andrew

[lensman57]

A.G you are right it can't increase the amount of light through the aperture stop but it reduces the image scale and so the number of photons per pixel goes up (with a corresponding opportunity to reduce exposure and assuming typical UK seeing). Assuming the design of the scope can fully illuminate the chip at the shorter focal length/ wider field of view then more photons will hit the CCD rather than missing it at the longer focal length.

Regards Andrew

Hi Andrew,

Thanks for the clear explanation, but if the image scale is reduced does this not affect the resolution so when the image is scaled up we are back to square one?

A.G

[andrew s]

Hi Andrew,

Thanks for the clear explanation, but if the image scale is reduced does this not affect the resolution so when the image is scaled up we are back to square one?

A.G

Hi A.G, Generally speaking once you have reduced the resolution you can't get it back. The FR will reduce the resolution and attempting to get it back by scaling it up will just increase the size but not the resolution. Eventually, you will just see the pixels which can be smoothed by re-sampling but the resolution will stay the same or get a bit worse!

There are techniques to get around this. If you have a set of dithered images then you can use an image processing technique called Drizzle to reconstruct an image with higher resolution than the individual frames. This method exploits the fact that different parts of the image fall across different pixel boundaries and by varying amounts and this holds the additional information needed to reconstruct the higher resolution image.

Regards Andrew