7 replies to this topic

[Demon]

Hi All,

Sorry for what I think is probably a daft question.

I'm new to CATs and intend to image at the native focal length at F10.

Obviously there is a corrector lens on the front of the C8 and I assume the optional x6.3 reducer simply flattens at the shorter focal length - but is not actually required for flattening if using at native FL.

If someone could confirm this is correct or put me right I'd appreciate it please?

Also I'm assuming there will be a limit to the circle(sensor size) that is flat in both cases, would anyone also know what the C8 is good for without the x6.3 please?

I'm not on a massive sensor so likely good for now but would like to know in case I think of going bigger.

Thanks very much,

Nick

[JMP]

I don't think it's a daft question at all. Try it. I think you will see distortion in the far corners with your 183 sensor, but only when the seeing is really good. And I expect you'd hardly notice with a 533 sensor. I was just looking through a classic C8 the other night. Even with a 40mm plossl the stars were pretty good to the edge. I was surprised, frankly. Field curvature and coma are not as bad as I see in a C6 or C5. So why not give it a shot?

[thierry martin]

Hi All,

Sorry for what I think is probably a daft question.

I'm new to CATs and intend to image at the native focal length at F10.

Obviously there is a corrector lens on the front of the C8 and I assume the optional x6.3 reducer simply flattens at the shorter focal length - but is not actually required for flattening if using at native FL.

If someone could confirm this is correct or put me right I'd appreciate it please?

Also I'm assuming there will be a limit to the circle(sensor size) that is flat in both cases, would anyone also know what the C8 is good for without the x6.3 please?

I'm not on a massive sensor so likely good for now but would like to know in case I think of going bigger.

Thanks very much,

Nick

I'm afraid the reducer x0.63 is for canceling the coma reduction of the stigmatism , reducing the EFL  but at the prize to increase the curvature of field.. To flatten the focus field, you need negative lens because the Petzval sum is opposed to the stigmatism error with the SCT. . When you reach 1/4 error of the Petzval sum , with the stigmatism the curvature of field goes towards infinite. The field of illumination will reach 60% of the full illumination at 37 x 0. 63= 23 mm diametre. If  you want the best available reducer you have to buy a 0.63 reducer from Starizona.

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[Demon]

Thanks very much for both replies. I thought that as with most things astro stuff only works to a point and after that it degrades. I think I get the gist and appreciate knowing that these still are not astrographs, but also is definitely worth a try. I'd read the starizona gives better flat circle etc than the celestron however i don't want either as I'm looking forward to getting back to the long focal length in a less wind prone ota. As you say the wider 183 can get cropped and my primary targets will more often be centred. Everything is a compromise in astro so looking forward to trying this. Rumour has it that this version of c8 albeit 25years old had all the best bits and as its spent 24 years in its case it's pristine. Plus will use autofocusing so hoping not much mirror slop which may become largest issue. Fingers crossed and thanks again.

[RichA]

I'm afraid the reducer x0.63 is for canceling the coma reduction of the stigmatism , reducing the EFL  but at the prize to increase the curvature of field.. To flatten the focus field, you need negative lens because the Petzval sum is opposed to the stigmatism error with the SCT. . When you reach 1/4 error of the Petzval sum , with the stigmatism the curvature of field goes towards infinite. The field of illumination will reach 60% of the full illumination at 37 x 0. 63= 23 mm diametre. If  you want the best available reducer you have to buy a 0.63 reducer from Starizona.

They actually never designed the f6.3 reducer/corrector for the whole field.  The assumption then was, that the C8 would be used to image single objects the images of which would be cropped to highlight the object, rather than have a vast expanse of empty space with a tiny galaxy or nebular in the centre of the field, as we see with the diminutive refractor/camera lenses today.

[thierry martin]

They actually never designed the f6.3 reducer/corrector for the whole field.  The assumption then was, that the C8 would be used to image single objects the images of which would be cropped to highlight the object, rather than have a vast expanse of empty space with a tiny galaxy or nebular in the centre of the field, as we see with the diminutive refractor/camera lenses to

The reducer was done to compete with the Meade 6.3 SCT. https://www.cloudyni...ucer/?p=7091000

[Demon]

Thanks gents, I'll find out when it gets dark but I had a look through astrobin and that has shown me what realistically I should get and it looks great, if I can achieve what others are getting with and without the reducer I'll be very happy. I would call my images far from the perfection of many and I think many of the other factors including collimation and guiding have just as much chance of screwing up the stars. I read one guys review of a load of reducer and in the first image the stars were not perfect, but the elongation wasn't pointing to the centre and they weren't comets so I think many can get mistaken. I've got a £55 angeleyes reducer on its way which will allow me to get a little more field of view for some dsos when required, but I'm still looking forward to imaging at 2m with this lovely little scope. Thanks again.

[RichA]

The reducer was done to compete with the Meade 6.3 SCT. https://www.cloudyni...ucer/?p=7091000

It was the better idea.  I don't think the Meade sold too well, it didn't last.