Refractor or Reflector for Imaging?

[FrankieValley]

Hi,

I have a 1200mm focal length reflector (with a Baader MPCC field flattener) and I'm wondering if all else is equal i.e. ccd, seeing, target, etc would a 600mm refractor with a 2x barlow produce a better or worse image?

I understand that the CCD pixel sampling rate in the two scenarios above would be identical and the MPCC will compensate for the non-flat field of a reflector, but are there any advantages or disadvantages between the two set ups (apart from size and weight)?

The telescopes I'm thinking of are a SW 250PDS f/4.7 and a SW Evostar ED80 DS-Pro f/7. I realise that exposure length would be a factor and that the reflector at f/4.7 is much faster, but is there a trade off with contrast and image quality afforded by the smaller aperture refractor?

Best Regards

David

[lukebl]

I would have thought that there's no contest in favour of the reflector.

An f/7.1 scope with a 2x barlow will be operating at f/14.2. So you will have two scopes with identical focal length, one at f/4.7 and one at f/14.2. Any advantages of the refractor, and I'm not sure what they would be anyway particularly with a smaller aperture plus a barlow added to the optical train, would be outweighed by the much longer exposures required to get an equivalent signal. Longer and more exposures will leave you open to other issues, such as guiding accuracy, vagaries in the weather, satellite trails, and whatever.

If you're talking about planetary imaging, then you'd need an even longer focal length than 1200mm, so the reflector plus a barlow would win too.

[FrankieValley]

Thanks Luke.

[ronin]

The ED should give a sharpr and better contrast image, the reflector will be a little "softer". The secondary and the spiders that hold the secondary are the cause of this. Also the spiders can (will ?) give rise to small spikes on star images.

You can get spikes for a refractror if the optics are pinched.

I tend to look on the fact that prime camera lens are lumps of glass and not a mirror, and suspect there is a reason for this.

[lukebl]

I'm not convinced that an 80mm refractor, however good it is, plus a Barlow would really compete with a half-decent 250mm Newt. Better, possibly, for wide-field imaging without the Barlow than the Newt, but not if you intend using it at 1200mm FL.

[Mike Hawtin]

David, I think you will find that it's not reflector vs refractor which you need to consider as as an imaging scope but rather the quality of your mount first and foremost. Also whether your interest lies with deep sky or lunar/planetary imaging. A newtonian with a focal length of 1200mm on a low end mount will present real difficulties for deep sky imaging whereas lunar/planetary should be a straightforward proposition. Alternatively a refractor at 600mm should produce satisfying results of deep sky objects even on a mediocre mount. It's horses for courses really but if you are serious about deep sky imaging at 1200mm you will need a capable mount and therefore a considerable budget. In my experience it is pointless and ultimately much more expensive to try to wring decent performance from cheap kit than to bite the bullet and buy decent stuff from the outset. I don't mean to discourage you but simply to point out the pitfalls and save you from frustration and needless waste of drinking vouchers!

Lots to think about, don't be tempted to just bash on regardless.

Cheers

Mike H

[FrankieValley]

Hi,

Many thanks for the superb comments and advice. You have provided the answer to the question in my mind which is would a refractor provide much better images at the same focal length compared to a Newtonian? It sounds like the images would not be that much better to justify the increased exposure times required. Short focal lengths are best used for wide fields (no barlows).

Thanks again.

David

[JimStan]

I tend to look on the fact that prime camera lens are lumps of glass and not a mirror, and suspect there is a reason for this.

Don't discount the many mirror "lenses" that are used for telephoto work in terrestrial photography, Ronin ! The compact size and ease of use of a good Schmidt camera lens far outweighs any loss in quality of the final picture. And some of those good lenses can photograph the hair on a gnat's @$$, if you know what I mean !

I believe the prime reason that most lenses below around 500mm are "lumps of glass and not a mirror" is because it is much easier to correct for abberations caused by bending light to a focal point that close to the objective with lenses having different indexes of refraction, than it is to try to do the same thing with a parabolic mirror !

Jim S.

[Polaris B]

The best way to judge between the two is to look at images taken. I have used both a 10" f/4.7 newt and a 80mm ED f/7.5 refractor to image. They both have their advantages, but not at 1200mm. I can't imagine even trying to image at f/15.

The stars in the refractor are round, so if you take the rig down one night and want to go back to the same object the next night, you can, easily. In the newt, the stars have spikes. If you go back the next night and the camera isn't exactly in the same place, the stars have blurred spikes or two sets. I have to set up and take down each night. When imaging with the newt, I try to get the image all in one night.

I love the view at 1200mm. I bought the newt specifically to image at that focal length. It's the beginning of the focal length needed for galactic detail. But if you are going to image galaxies, you will want a faster scope. Imaging at f/4.7 is much easier than imaging at a slower speed.

Having said that, I agree with the post above about the mount. You will need a hefty mount to control the 10". Moreover, on my scope (an Orion), I found too much flexure between guidescope and main camera. I now use an off-axis guider. I know of people who guide a 10" newt with a guidescope, but my scope would not do it beyond about 120-second exposures.

Polaris B

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