Mirror Quality for Imaging?

[glend (Glen)]

Well Bird, in the case of the imaging newt, yes, my primary interest for it is in DSOs. That said, I do have a Istar 127mm refractor, f12, at a 1500mm fl, which I use mostly for planetary work; with it's long fl I can push it to fairly high power, and yes I can image with it. My choice of these two budget scopes gives me the reach I want, in a light weight package (important for us retirees), and at a price point that is affordable for me (I built the Istar around the Istar provided objective). They both ride my NEQ6 comfortably.

For people about to point out the Istar is an Achro, don't bother, with it's long focal length and the appropriate filter and post capture processing there is no CA in my images.

[alpal]

Originally Posted by bird

I presume most of the imaging being discussed here is low power / wide field - ie I can't see anyone above talking about the optics needed for high-res planetary work...

I have a 16" f/4 newtonian and optical quality matters a lot to me :-) See this image for what is possible in good seeing, you won't get that with low quality optics...

http://www.acquerra.com.au/astro/gal...-174959utc.png

Jupiter was about 50 arcsec in diameter here, you're welcome to measure the image to see what resolving power it has. At the time this was taken I had a 14.5" f/5 mirror in my newtonian, and central obstruction about 28%.

cheers, Bird

Hi Anthony,
it's nice to see someone appreciates quality.

cheers
Allan

[sharpiel]

I have an 18" f5.5 Galaxy Optics Newtonian mirror reputed to be 1/25 wave. I haven't used it for some time now and (almost criminally) it sits in its mirror box under my bed. It always delivered stunning views on all objects. I really should find someone else who would love and appreciate it as I have. These days I am moving towards more compact folded optical designs for easier transport.

[alpal]

Originally Posted by sharpiel

I have an 18" f5.5 Galaxy Optics Newtonian mirror reputed to be 1/25 wave. I haven't used it for some time now and (almost criminally) it sits in its mirror box under my bed. It always delivered stunning views on all objects. I really should find someone else who would love and appreciate it as I have. These days I am moving towards more compact folded optical designs for easier transport.

A very nice mirror but it would be hard
to image with without a giant mount & a good mechanical design.

[Logieberra (Logan)]

This 10" RCOS, as already seen by many of you, sure is a performer for a scope of that aperture: http://www.iceinspace.com.au/forum/s...d.php?t=124993

[sharpiel]

Split ring or horse shoe

Originally Posted by alpal

A very nice mirror but it would be hard
to image with without a giant mount & a good mechanical design.

[alpal]

Originally Posted by sharpiel

Split ring or horse shoe

A horse shoe mount would need a rotator.

[alocky (Andrew lockwood)]

Originally Posted by alpal

A horse shoe mount would need a rotator.

Why? The AAT doesn't.
A beautiful design and one I always aspired to build. Jmi offered one in the late'90s.
Cheers,
Andrew.

[alpal]

Originally Posted by alocky

Why? The AAT doesn't.
A beautiful design and one I always aspired to build. Jmi offered one in the late'90s.
Cheers,
Andrew.

OK - do you have any photos of the setup for such a large scope?

[alocky (Andrew lockwood)]

Originally Posted by alpal

OK - do you have any photos of the setup for such a large scope?

Actually, I do, but they're not digital. There are plenty of pictures online of the prime focus cage with one David Malin therein, and it does not need to rotate. The horseshoe or split-ring is an equatorial design, so there is no field rotation if you have a good polar alignment.
Here's one...
http://ftp.aao.gov.au/images/captions/aat004c.html

[John K]

Originally Posted by alpal

OK - do you have any photos of the setup for such a large scope?

funnily enough I have a JMI scope - it is still being offered by JMI - mine is an NGT12 - a 12.5" f/5 scope - it does not need a rotator as a horseshoe mount is an equatorial mount.

https://www.flickr.com/photos/johnka...57594183210993

The scope has 1/16 wave optics from OMI and is used for both planetary and deep sky imaging. The quality of the optics means that focusing is a snap and considering the f/5 ratio is has low coma, although I do use a Baader MKIII corrector for deep sky imaging.

Some photos taken with the scope here:

https://www.flickr.com/photos/johnka...57640517313374

https://www.flickr.com/photos/johnka...in/set-1162379

https://www.flickr.com/photos/johnka...9369/lightbox/

https://www.flickr.com/photos/johnka...3154/lightbox/

In my mind, deep sky imaging may in effect mean that as we are imaging at a smaller focal length we can get away with lower quality optical quality - a flat field is more important - that's why the low cost RC's in the market now are getting so popular. But, as the focal length increases like that needed for planetary, then so does the need for better optical quality.

The Mars and Saturn images (links above) were taken at an affective focal length of nearly 9 meters - so you do need good optics imaging at the focal length.

John K.

[alpal]

Originally Posted by John K

funnily enough I have a JMI scope - it is still being offered by JMI - mine is an NGT12 - a 12.5" f/5 scope - it does not need a rotator as a horseshoe mount is an equatorial mount.

https://www.flickr.com/photos/johnka...57594183210993

The scope has 1/16 wave optics from OMI and is used for both planetary and deep sky imaging. The quality of the optics means that focusing is a snap and considering the f/5 ratio is has low coma, although I do use a Baader MKIII corrector for deep sky imaging.

Some photos taken with the scope here:

https://www.flickr.com/photos/johnka...57640517313374

https://www.flickr.com/photos/johnka...in/set-1162379

https://www.flickr.com/photos/johnka...9369/lightbox/

https://www.flickr.com/photos/johnka...3154/lightbox/

In my mind, deep sky imaging may in effect mean that as we are imaging at a smaller focal length we can get away with lower quality optical quality - a flat field is more important - that's why the low cost RC's in the market now are getting so popular. But, as the focal length increases like that needed for planetary, then so does the need for better optical quality.

The Mars and Saturn images (links above) were taken at an affective focal length of nearly 9 meters - so you do need good optics imaging at the focal length.

John K.

Some nice pics there - thanks for sharing.

cheers
Allan