dumb question?

[ebdons]

Hi, sorry if this a an obvious question, can I use a 2"meade 5000 type tele-extender with an 2" coma corrector on an F4 newt? with a QHY10, or just use the meade and crop the coma out of the image, I know meade TE  are more suited to PI and SCT than newt and deep sky, but not having tried as yet, as I'm trying to reduce the size of image scale to a suitable size and gain more definition of the object in the process, with an increase of exp time that will be needed on something faint, I used the meade ok with moon imaging but no yet tried with DS objects. Tony

[Gerry Casa Christiana]

I don’t think that you can use a SCT coma corrector on a Newtonian I doubt they would be compatible. Someone will give you the technical answer I’m sure. 

Kind regards

Gerry

[RT65CB-SWL]

As above. An interesting discussion here --->https://www.cloudynights.com/topic/449767-would-a-coma-corrector-work-with-an-sct/

[ebdons]

Ok thanks chaps, that link to the article was a wealth of info. my meade is more use on a frac and sct than anything else but always want to get more out of my equipment. this is the tele-extender I have. maybe a f6 newt would give me the right view?

[Gerry Casa Christiana]

I'm pretty sure it won't work with a Newtonian  Unfortunately you will need one if you want to do imaging. 

[ebdons]

It did work on my f4 newt, but only on the moon and then I had focus issues on the sides of the FOV. thanks Ton

[Nigella Bryant]

My understanding is that you don't need a coma correcter if working above f5 so if you're imaging with an f4 Newtonian and adding a 2x Barlow that'll give you imaging at f8. So all you need is a 2x barlow lens. I may be wrong, I'm new to imaging and haven't tried my f4 Newtonian at f8 yet for those tiny planetary nebula, but I'd love to have a go one day. 

[John]

Despite increasing the effective focal ratio, I don't think a barlow lens actually corrects coma because that is already "in the system" from the fast primary mirror.

I might be wrong but thats my understanding.

[BinocularSky]

2 hours ago, John said:

Despite increasing the effective focal ratio, I don't think a barlow lens actually corrects coma because that is already "in the system" from the fast primary mirror.

I might be wrong but thats my understanding.

You are correct. A Barlow increases the effective focal length of the system, not the primary.

@Nigella Bryant It might help to visualise coma as a sort of lop-sided chromatic aberration.

In the case of a Newtonian mirror, the surface is parabolised, but it only brings parallel rays to a point focus if they are parallel to the optical axis (the axis of the paraboloid). The further something is off-axis, the less well it is corrected. Similarly, the faster the mirror, the deeper is its curve, so coma is increased relative to a slower (shallower) mirror. A Barlow merely narrows the cone-angle of the cone of rays reflected by the mirror; it cannot correct for spherical aberration or coma.

In the case of the Schmidt-Cassegrain, there is a fast, spheroidal mirror (approx f/3) and the correction is carried out by the corrector plate. In an f/10 SCT, the secondary mirror acts optically like a x3.33(-ish) Barlow, and makes the light cone narrower. Despite being f/10, the coma is about equivalent to that from a f/5 Newtonian of the same aperture.

The job a coma corrector has to do in each instance (Newt and SCT) is slightly different, so a corrector designed for one is unlikely to do a good job on the other.

If anyone really wants to get to grips with this, Rutten and van Venrooij's "Telescope Optics - Evaluation and Design" is an excellent resource.

HTH

[ebdons]

Ok thanks chaps, the meade is/has been described as a variable barlow and then not a barlow at all, more like a pseudo parracorr that has elements of both, I will revisit this idea of imaging the smaller PN, and the meade can be placed anywhere along the optical path and still be effective at x2 so the bumf says?  so it has to go behind any corrector optics like a CC? just need a clear night to test maybe?

[Nigella Bryant]

55 minutes ago, BinocularSky said:

You are correct. A Barlow increases the effective focal length of the system, not the primary.

@Nigella Bryant It might help to visualise coma as a sort of lop-sided chromatic aberration.

In the case of a Newtonian mirror, the surface is parabolised, but it only brings parallel rays to a point focus if they are parallel to the optical axis (the axis of the paraboloid). The further something is off-axis, the less well it is corrected. Similarly, the faster the mirror, the deeper is its curve, so coma is increased relative to a slower (shallower) mirror. A Barlow merely narrows the cone-angle of the cone of rays reflected by the mirror; it cannot correct for spherical aberration or coma.

In the case of the Schmidt-Cassegrain, there is a fast, spheroidal mirror (approx f/3) and the correction is carried out by the corrector plate. In an f/10 SCT, the secondary mirror acts optically like a x3.33(-ish) Barlow, and makes the light cone narrower. Despite being f/10, the coma is about equivalent to that from a f/5 Newtonian of the same aperture.

The job a coma corrector has to do in each instance (Newt and SCT) is slightly different, so a corrector designed for one is unlikely to do a good job on the other.

If anyone really wants to get to grips with this, Rutten and van Venrooij's "Telescope Optics - Evaluation and Design" is an excellent resource.

HTH

Okey dokey, I got it wrong. I thought as it was above f5 it was okay,  what a numpty. Glad I said it though cus I was going to have a go sometime at f8 with my 2X Barlow so I'll have to use my cc as well with my f4 newt. Thanks, you learn something each day in this hobby. Brilliant. 

[Nigella Bryant]

Sorry to high jack the thread, so if I was imaging planet's at say f20 with a 5X power mate I'd still have to put my cc in the train too? 

[ebdons]

Hi, that's sort of what I was thinking with regard to the CC, think the meade can be placed along the imaging train? but maybe behind the CC will be difficult for the ccd spacing, unless the barlow is very short? if you put the meade in front would that negate the CC properties then. might test tonite and see.

[david_taurus83]

On 08/05/2019 at 12:57, Nigella Bryant said:

Sorry to high jack the thread, so if I was imaging planet's at say f20 with a 5X power mate I'd still have to put my cc in the train too? 

No, you shouldn't need a cc if imaging planets or the moon.

[Nigella Bryant]

1 hour ago, david_taurus83 said:

No, you shouldn't need a cc if imaging planets or the moon.

Thanks