Which SCT OTA and mount combination?

[michaelmorris]

My present set up is a venerable Meade LX200 8" EMC in the observatory with an Orion 80ED piggy backed on it. For star parties I use a Skywatcher HEQ5 Pro with the Orion 80ED with a 66mm Apo as a guide/second scope.  At the moment I use the LX200 mainly for visual and planetary/lunar and the 80ED for imaging DSOs. 

I've decided to start saving to upgrade my set up so that I can have the following facilities.

In the observatory

• Better tracking on the SCT so that I can use it for deep sky imaging as well as planetary/lunar imaging.
• Better tracking on the 80ED

In the field

• Facility to use an SCT for visual observing and for lunar/planetary imaging as my views at home are getting increasingly restricted by neighbours trees

To achieve this goal (before this decade is out ... ), I'll need a new mount in the observatory and a new SCT OTA.

At the moment the plan is to get a Skywatcher AZ  EQ6 GT for the observatory and keep the HEQ5Pro for start parties and other off site observing/planetary imaging.  (I don't want to have top remove the observatory mount for off-site observing).  The plan would be to swap between the SCT and a dual scope wide field rig on the observatory mount.  The same principle would apply for the HEQ5Pro.

But which OTA to go for?

I've narrowed down to a choice of three within my budget (no more than £1700 - preferably less).

• Celestron 9.25 XLT - great for planetary/lunar and should be well within the carrying capacity of both mounts for imaging (HEQ5Pro for planetary and  AZ EQ6 GT for DSOs),
• Celestron 11 XLT - great for planetary/lunar but pushing the carrying capacity of both mounts for imaging.
• Meade 10" LX200 ACF - Should be better for DSOs? and within the carrying capacity of both mounts for imaging 

Your thoughts would be greatly appreciated.

[Mr Spock]

If you are looking at portability the C9.25 is a doddle to pick up and will be ok on an HEQ5. Realistically a C11 is too big for an HEQ5 and a 10" LX200 is probably pushing it too. Both would be fine on an EQ6.

From what I've seen from planetary imagers the next step up from a C9.25 is the C14. With the C11 not offering enough of a difference and a C14 being an observatory instrument, this is why I moved from a 10" newt to the C9.25.

My EQ6 can carry the C9.25 and a piggy back 80ED using an ADM guide ring system, but it is a close thing. I had to upgrade from a Vixen dovetail to a Losmandy as the Vixen was just too flimsy. If I were ever to use this for DSO AP I'd want something much more substantial than an EQ6.

[ollypenrice]

You're talking mainly about payload here. For DS imaging at long focal lengths this is only half the story, or less than half. The other concern is tracking accuracy, which becomes vastly more exacting as you image fewer arcseconds per pixel. You don't say what camera you are using but its pixel size is hugely relevant here. A small pixel camera in a long focal length scope will not resolve to the capacity of the telescope if the seeing or the autoguiding won't support it. This means that you are gaining nothing from using a scope of this size. With a mono camera you can increase your effective pixel size by binning 2x2 but if you are using OSC you cannot do this. You probably know this calculator but it will give you a good idea of what to expect. http://www.12dstring.me.uk/fov.htm

Once you get down to 1 arcsec per pixel or so, guiding does really start to become a tricky business. It can be done but it may not come straight from the box on the sub-premium mounts. Seeing is site dependent but I often hear UK imagers say that 2 arcsecs per pixel is the sweet spot, though many go below that. Here I've been down to 0.6 but I wasn't convinced it was doing anything that 1.0 wouldn't have done.

The Edge and ACF variants are far better DS options than standard SCTs but watch the focal reducer situation. The Edge reducer is costly. I don't know what works on the Meade. Check the image circles for your camera, too.

Olly

[michaelmorris]

You're talking mainly about payload here. For DS imaging at long focal lengths this is only half the story, or less than half. The other concern is tracking accuracy, which becomes vastly more exacting as you image fewer arcseconds per pixel. You don't say what camera you are using but its pixel size is hugely relevant here. ...

Olly

For probably at least the next 5 years, DSO imaging will be with a DSLR camera. Planetary imaging with Imaging Source cameras.

[ollypenrice]

In that case, Michael, the focal lengths are very much on the overkill side. The C11 and Canon 1100D would give you just 0.38 arcseconds per pixel. That is totally intenable. You would never succeed in realizing that resolution. Yes, you'd get a picture but with no more detail than you'd get from a much smaller instrument.  Focally reduced to to F7 you would still be on about 0.55 arcsecs per pixel and even that is well below where I'd want to be for DS. With reducer and my Atik 11000 with 9 micron pixels I'd be on 0.95 arcseconds per pixel which is a lot more likely to deliver, and I'd still be on a  respectable 1.89 arcsecs per pixel in Bin2, so binning the colour would be a good option.

Deep sky, small pixels, long focal lengths... don't add up. The scopes have other strengths, though. I'm only talking about DS (and mount accuracy.)

Olly

[michaelmorris]

In that case, Michael, the focal lengths are very much on the overkill side. The C11 and Canon 1100D would give you just 0.38 arcseconds per pixel. That is totally intenable. You would never succeed in realizing that resolution. Yes, you'd get a picture but with no more detail than you'd get from a much smaller instrument.  Focally reduced to to F7 you would still be on about 0.55 arcsecs per pixel and even that is well below where I'd want to be for DS. With reducer and my Atik 11000 with 9 micron pixels I'd be on 0.95 arcseconds per pixel which is a lot more likely to deliver, and I'd still be on a  respectable 1.89 arcsecs per pixel in Bin2, so binning the colour would be a good option.

Deep sky, small pixels, long focal lengths... don't add up. The scopes have other strengths, though. I'm only talking about DS (and mount accuracy.)

Olly

Thanks Olly

The consideration of SCTs with a larger aperture than my present 8" is mainly for improvements for visual observing and planetary imaging.

[ollypenrice]

Thanks Olly

The consideration of SCTs with a larger aperture than my present 8" is mainly for improvements for visual observing and planetary imaging.

In which case it will deliver. Have you checked out Damian Peach's website where he compares the Celestrons? Worth a look.

Olly

[Xplode]

If you want to image DSO's with a larger scope and get good results with a DSLR a newtonian is your only choice at a reasonable price.

A 10" f/5 sounds like it would work, but is pushing the limit on your HEQ5,....i think it can be done with a carbon version if you keep the guide scope light (guiderfiner). To get best of both imaging and visual you could get 2 secondarys for it. Changing the secondary is a fast process with the correct tools.

A newtonian would not be as good for planetary as an SCT, but i have no doubt you could still make great images with it.

One thing to think about before getting a large newtonian is it's size. It probably takes at least double the space of a 10" SCT.

Here's the scope i would recommend

For secondary centering i found this works great and should allow changing of secondary mirror in less than 5 min and probably 2 min with more experience.

[michaelmorris]

In which case it will deliver. Have you checked out Damian Peach's website where he compares the Celestrons? Worth a look.

Olly

No I haven't seen this. thanks for the heads up.

[michaelmorris]

I was thinking about a fast Newtonian as an alternative to an SCT.  However I assume that for planetary imaging I would need a 5x barlow as a minimum,

[gnomus]

... Have you checked out Damian Peach's website where he compares the Celestrons? Worth a look.

Olly

Just visited his site.  Could see a review of the C9.25 but not a comparison of the different sizes.  Have I missed something? 

[Xplode]

I was thinking about a fast Newtonian as an alternative to an SCT.  However I assume that for planetary imaging I would need a 5x barlow as a minimum,

A 5x would work perfectly, but you could also use 2 barlows. With 2 you could go with 2 different ones so you have more choices for visual.

[ollypenrice]

Just visited his site.  Could see a review of the C9.25 but not a comparison of the different sizes.  Have I missed something? 

When I last looked I remember something about there being little to choose between the 9.25 and the 11 under UK skies for planetary imaging.

Will I think I read that!

Olly

[Mr Spock]

It says in the C9.25 review there's little to chose between the C9.25 and C11. If you look at some of the Saturn images taken with the C9.25 and C11 it's hard to tell which is taken with which.

[michaelmorris]

It says in the C9.25 review there's little to chose between the C9.25 and C11. If you look at some of the Saturn images taken with the C9.25 and C11 it's hard to tell which is taken with which.

My other requirement for an SCT is visual observing. Lunar, planetary, DSOs and doubles.