#1  
Old 01-04-2008, 12:33 PM
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Moonman (Michael)
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Imaging with Refractor or Reflector?

Hello to all!

I'm currently setting up for autoguiding and direct PC control of my EQ6 mount. I am tossing over whether to stick with my 8" Newtonian as an imaging scope or invest in a good 80mm refractor (limited budget). I have my eye on a skywatcher ED80.

What are the pro's and cons of each or is it just personal preference?

Does the smaller aperture of the refractor require more exposure time?

thanks

mike
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Old 01-04-2008, 12:42 PM
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iceman (Mike)
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Hi Mike

The exposure time is dictated by the focal ratio, not the aperture directly. Your 8" newt is probably an f/5, whereas an ED80 is an f/7.5.

Some factors to consider:
- Focal length
. 8" newt focal length around 1000mm
. ED80 focal length 600mm
What this means is that the ED80 is good for wide field views, whereas the 8" will be better for closer in views of galaxies etc.

- Focal ratio
. 8" newt f/5
. ED80 f/7.5. With a focal reducer/flattener (almost a requirement, rather than optional), it brings it to around 480mm f/6.
With the ED80 you're letting less light in (aperture) and the focal ratio is slower meaning you need a longer exposure.

. The 8" may need its mirror moved up the tube if you want to hook a DSLR up to it.
. The 8" will need a MPCC (coma corrector) to get a flat field.
. The ED80 needs a focal reducer/flattener to get a flat field.

I'm currently using an ED80 with WO 0.8x reducer/flattener, with my Canon 350D. It's great for widefield, and the shorter focal length has less demands on it for accurate guiding and less impacted by bad seeing.

However for close-up views of galaxies, it just can't cut it (check out my M104 or M83). My next DSO imaging scope will me an 8" f/4 or f/5 newt.

Whichever way you go, you'll also need a guidescope, guide camera, and GPUSB or similar.
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Old 01-04-2008, 01:23 PM
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Moonman (Michael)
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Hi Iceman,
Great info! thanks for that.

This is an intuitive question but if I mounted an ED80 on the Newtonian can I switch between the two i.e. When I want to image widefield could I guide with the Newtonian and image with the refractor or would I need another smaller refractor to guide the ED80?

The focal reducers look complicated and there seems there is an element of choice. I've noted some of the threads here dealing with the ED80 so I'll investigate some more.

cheers

mike
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Old 01-04-2008, 01:48 PM
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Hi Mike.

Certainly, a good idea to get the ED80 and they can be interchangeable. Image through one and guide through the other, and vice versa.

You'll still need to potentially move the mirror up the tube for the 8".

Will you be using a DSLR? which one?
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Old 01-04-2008, 02:22 PM
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No DSLR for now. I hope to get a Canon of some description down the tracks maybe a 350D or modified version depending on budgetary factors.

For now I'm just setting up using a very basic DSI and LPI as a guiderscope. I'm mainly focused on developing techniques in guiding and allignment etc. I think a DSLR at this stage will be too much too soon.

Eventually the DSI will become the guiderscope when I can take the next step up. I also have been considering using one of the Imaging Source Cameras as an alternative to a DSLR.

cheers

mike
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  #6  
Old 01-04-2008, 02:43 PM
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Also I've already have the GPUSB on hand as of yesterday.

mike
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Old 01-04-2008, 03:56 PM
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ah great, well if you're going to use a DSI then you won't need to move the mirror up the tube. It should come to focus just fine.
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Old 01-04-2008, 04:08 PM
gbeal
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Both scopes are perfect for what you want, and will provide all the challenges you need.
The idea of 80mm & 8" dual mounted is a good move as well, and the ability to interchange for guiding/imaging another great idea. Weight distribution will vary however, and this could cause you some grief.
Also there is no mention of the mount, but bear in mind an 8", plus ED80 will give most small mounts a hard time, so consider that as well.
Best bet is to try it, and also sidle up to someone in your area who is already imaging and see what they are doing.
Gary
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Old 01-04-2008, 04:19 PM
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Thanks Gary,

I have an EQ6 mount so wieght shouldn't be a problem. I will have to include an extra counterweght for the extra weight of the refractor.

Are there any circumstances you know of where tube flexure is a problem more than others?

cheers

Mike
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  #10  
Old 01-04-2008, 04:24 PM
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Quote:
Originally Posted by gbeal View Post
Best bet is to try it, and also sidle up to someone in your area who is already imaging and see what they are doing.
Gary
I'm actually heading down the river tonight to call in on "Phil" Mr Acropolite.
He has an ED80 I can have a look at which will help make decisions easier.

cheers

mike
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Old 01-04-2008, 04:27 PM
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Does HRH Liz know he as an ED80? And has he told her about the EQ6 yet or is he still hiding it in the cupboard from last Astrofest?


BTW it would be nice if you could go side by side with your 8" and 80mm but I don't know if you would have the room. This would help to get around the flexure problem to some degree.
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Old 01-04-2008, 04:36 PM
gbeal
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You're stirring again Paul, and you make it look so easy too. LOL.
Side by side is great, but factor in the extra weight a SBS unit adds. I did, and man the in-built chip of the SBIG grew in appeal overnight. SBS units need to be over-engineered too, and this adds weight. believe me.
Gary
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Old 01-04-2008, 06:02 PM
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Moonman (Michael)
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Side by Side

I have heard that side by side mounting is hard to balance. As I don't have a permanent setup for my mount I have to setup and packup each time I go out. One less technical difficulty potentially dispensed at least untill tube flexure becomes a problem. Also the bars and plate are quite expensive items too. Mind you when you see pictures of that setup it looks impressive!

I don't know about Phil's wife but I did warn mine that for the next couple of weeks lots of packages would be turning up and not to get too stressed about it. I even covered myself by quickly showing her a picture of the ED80without drawing too much attention to the price. She made a comment to the effect that she didn't begrudge me my telescope gear and that's as good as a "Yes" as I'll ever get from her in the astronomy department.


cheers

Mike
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  #14  
Old 02-04-2008, 04:04 AM
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citivolus (Ric)
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Quote:
Originally Posted by Moonman View Post
I have heard that side by side mounting is hard to balance.
It take me all of about a minute to balance my setup, being a 90mm refractor and a C9.25. In my case it takes 2x 10Kg counterweights on my CGE, using a Losmandy DSBS plate. After you've done it one or two times you will remember the balance points for each scope and the process even simpler. I've marked the dovetails with some fluorescent masking tape to speed things up even more.

Regards,
Eric
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Old 02-04-2008, 08:28 AM
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Moonman (Michael)
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Hi Eric,

How do you get mount your cross bar so your RA axis isn't at right angles to where it should be. Is there a setting on your software. The pictures I see don't seem to make sense to me about this setup.

regards

mike
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Old 02-04-2008, 08:50 AM
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Moonman (Michael)
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MPCC for Newtonian

Quote:
Originally Posted by iceman View Post
The 8" will need a MPCC (coma corrector) to get a flat field.
Hi Mike,

Is this what you mean re MPCC

http://www.myastroshop.com.au/produc...asp?id=MAS-221

regards

mike
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Old 02-04-2008, 09:34 AM
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yep, that's the one. Though not sure if you'll need it with a DSI.
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Old 02-04-2008, 01:26 PM
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Moonman (Michael)
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Quote:
Originally Posted by iceman View Post
Though not sure if you'll need it with a DSI.
Hi again Mike,

Is that because the chip size is small and doesn't include the area on fringes of the mirror where coma occurs?

I imagine it doesn't apply to the refractor because the aperture is significantly less.

mike

Last edited by Moonman; 02-04-2008 at 01:29 PM. Reason: spelling and additional comment
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Old 02-04-2008, 04:02 PM
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Or is it because....?

Or is it because of that there's no point having a focal reducer in the Newtonian when I can do wider field shots through the Refractor. Which perhaps means that all I need is a focal reducer/image flattener for the refractor. The WO one seems like a good option.

thanks

Mike
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