Stupid question : Does attaching a DSLR directly to a scope reduce the magnification

[Catanonia]

As I said a stupid question.

Let me explain.

With scopes you can get Barlow lenses etc that increase the magnification of the viewing image.

You can also attach the dslr directly to the scope to effectively make it a lense.

I am making an educated guess that the image you get on the camera is not the image you would see on a scope through a barlow lense.

Am I correct ?

Therefore with scopes of say 1200 and under, what advantage would that have over a decent dslr with high quality lenses up to that size other than being able to view the image in higher magnification by eye but not on film.

I understand that you can also mount a standard camera over the eyepiece to get that magnification view. Is this correct ?


And one lst question.... I am guessing here that the higher the pixel count of your DSLR the better the images are going to be. True or false ?

Thank you for being patient with a quickly learning newbie

[chris.bailey]

WOW!

A telescope is a lens of fixed focal length. It creates an in focus image at a point in space behind the back of the scope. You can verify where this is by putting a piece of white card behind it and seeing where the image of a terrestial object comes into focus.

An eyepiece magnifies a part of this image depending on its size and gives a field of view depending on its construction. I have a couple of Nagler UWAN eyepieces that are quite high magnification and yet have an 80 degree field of view. amazing to lok through but at a cost! The combination of eyepiece and scope gives the magnification factor of the combination (FL/EP). Your 1200mm scope with a 12mm eyepiece would have a magnification of 100x. Depending on the optics of the scope there is a theoretical maximum magnification but atmospeheric conditions will kick in before this limit in most cases. A Barlow increases this magnification by a factor (2x or 3x) but the optics of a Barlow are rarely as good as going up a size with a decent eyepiece.

DSLR lenses are not really designed for astro use but some are quite good. Field flatness and colour correction are the major issues. Scopes are a lot cheaper than the equivalent lens as they are dumb optics designed for use at infinity and beyond . Even my TAK FSQ is a lot cheaper than an equivalent 600mm lens and much better for astro use having a very flat field with brilliant edge to edge colour correction, albeit at some cost of even illumination (you cant have perfection with all three).

A DSLR "samples" a part of the infocus image (the so called image circle) and it is important that the scope creates an image circle that wholly encompasses the cameras CCD otherwise the image will be vignetted. Few scopes will evenly illuminate a full frame 35mm CCCD.

The image on a camera will be very different to that seen through an eyepiece as the above hopefully explains. I normally plan targets in advance and choose a scope/CCD combination that allows coverage of the targets angular size (Stellarium gives many objects angular size) and programme like New Astronomy CCD will tell you the image size of a scope/CCD combination.

You can use eyepiece projection and it can be useful for high magnification images of the moon and planets but the magnification is too high for deep sky imaging.

Lastly yes and no. The aim is to image at an image scale of less than 2 arcseconds per pixel. Having said that most of my imaging is closer to 3 arseconds per pixel so that is not a hard and fast rule. If the atmospheric seeing is limiting to around 1.5 arcseconds per pixel, you can throw as many pixels at it as you like and not improve what can be resolved.

[Catanonia]

mmmm, ok this is going to take some digesting, understand most of it, but need to work through it.

I understand the vinegrating and the magnification calculations as well as you get what you pay for.

Not sure what flatness is, can you explain in simple terms please ?

Were my basic assumptions correct in the questions I asked ?

The reason I ask is because I shot wildlife and glamour and so have some expensive lenses at hand. £600 and upwards and I know I will want a scope soon, just need to figure it all out in my head 1st.

I am currently looking at the Meade ETX 120 PE (Profession) scope as this will help me out with finding targets and allow me to do some basic photographs. It also has tracking, ok sideline tracking which should surfice for 30 second subs. Can pick up a new Meade 120 PE with loads of extra's for about £690 delivered. I have looked at the 70, 80 and 90's, but for the extra 200 notes, might as well go large.

I know this isn't what is recommended, but not going pro here and the skies in the UK do not justify full out reflectors / german mounts etc when it is a sub hobby for me. I like the idea of. take out the Goto, setup, press a button and take some stacks. Hey, I could even mount my DSLR directly on top of the scope and use it's guidance considering the price of a AstroTrac is nearly 500 notes new.




Many thanks again, looks like skys are clearing tonight.

Targets are M3 and Andromeda at 500mm and 1000mm with my lenses. Prob will use 5 second subs over an hour if the weather holds.

[Nighthound]

Originally Posted by Catanonia

mmmm, ok this is going to take some digesting, understand most of it, but need to work through it.

I understand the vinegrating and the magnification calculations as well as you get what you pay for.

Not sure what flatness is, can you explain in simple terms please ?

Were my basic assumptions correct in the questions I asked ?

The reason I ask is because I shot wildlife and glamour and so have some expensive lenses at hand. £600 and upwards and I know I will want a scope soon, just need to figure it all out in my head 1st.

I am currently looking at the Meade ETX 120 PE (Profession) scope as this will help me out with finding targets and allow me to do some basic photographs. It also has tracking, ok sideline tracking which should surfice for 30 second subs. Can pick up a new Meade 120 PE with loads of extra's for about £690 delivered. I have looked at the 70, 80 and 90's, but for the extra 200 notes, might as well go large.

I know this isn't what is recommended, but not going pro here and the skies in the UK do not justify full out reflectors / german mounts etc when it is a sub hobby for me. I like the idea of. take out the Goto, setup, press a button and take some stacks. Hey, I could even mount my DSLR directly on top of the scope and use it's guidance considering the price of a AstroTrac is nearly 500 notes new.




Many thanks again, looks like skys are clearing tonight.

Targets are M3 and Andromeda at 500mm and 1000mm with my lenses. Prob will use 5 second subs over an hour if the weather holds.

I've been experimenting with my 100-400L for long exposure work and it does well at around 200mm. Like Chris mentioned, field flatness is one issue and chromatic aberration (false color around bright objects like stars) is another.

Field curvature(lack of field flatness) is an effect caused by the optics of refractors and lenses. In reflectors the distortion is referred to as coma. As you look out toward the edges of an image the stars will become elongated or stretched. Field flatteners/focal reducers are made to place between the camera and the scope to correct this effect while also lowering the f ratio you're shooting at. Which means faster optics and faster is better, especially when shooting relatively short exposures.

This leads me to something else you mentioned and that's the Meade 125 telescope. The fork mounted 125 is great for planetary and lunar work but at f/15 I'm afraid you'll be disappointed on the light gathered in short exposures. You'd get far better results between f/4 to f/6ish for deep sky.

Good luck tonight, post your captures.

[Catanonia]

Originally Posted by Nighthound

I've been experimenting with my 100-400L for long exposure work and it does well at around 200mm. Like Chris mentioned, field flatness is one issue and chromatic aberration (false color around bright objects like stars) is another.

Field curvature(lack of field flatness) is an effect caused by the optics of refractors and lenses. In reflectors the distortion is referred to as coma. As you look out toward the edges of an image the stars will become elongated or stretched. Field flatteners/focal reducers are made to place between the camera and the scope to correct this effect while also lowering the f ratio you're shooting at. Which means faster optics and faster is better, especially when shooting relatively short exposures.

This leads me to something else you mentioned and that's the Meade 125 telescope. The fork mounted 125 is great for planetary and lunar work but at f/15 I'm afraid you'll be disappointed on the light gathered in short exposures. You'd get far better results between f/4 to f/6ish for deep sky.

Good luck tonight, post your captures.

oh F15, that is a bit pants really and will struggle to collect light. mmm, will have to research a bit more.

Ok here is what I am after.

Sub £700 mark and I want

1. Large scope, 100mm or bigger with focal length over 1200mm
2. Goto technology in it
3. Decent motorised mount so can put my DSLR and large lens on top. Think EOS 50D and a 1000mm lense. Not fussed if Alt or EQ, would prefer EQ
4. Polar alignment if i need it
5. Tracking capabilities for exposures of upto 30 secs. The more the better.

oh and

6. Low F Stop


What makes (would prefer a package so don't have to mess as very new to this) would you recommend.


Chris : Dude, that is awesome kit and well OTT (ok jealous)

[Adrena1in]

Originally Posted by Catanonia

Sub £700 mark and I want

1. Large scope, 100mm or bigger with focal length over 1200mm
2. Goto technology in it
3. Decent motorised mount so can put my DSLR and large lens on top. Think EOS 50D and a 1000mm lense. Not fussed if Alt or EQ, would prefer EQ
4. Polar alignment if i need it
5. Tracking capabilities for exposures of upto 30 secs. The more the better.

oh and

6. Low F Stop

Hi, and welcome from a fellow Brit!

As others have probably pointed out, you can't have a 1200mm focal length scope with a 100mm aperture *and* a low f/stop. Focal Length divided by aperture determines the f/ value. My "big" scope is 127mm aperture and 1200mm focal length, and is pretty good for lunar and planetary work. I got it from www.skysthelimit.org.uk. It's only an Achromat, but it was only something like £150. I also got a 600mm with 102mm aperture, but don't use that so much.

I also bought an EQ5 mount from the above, which is really pretty good, and last night I was taking 1-minute subs with little or no trailing, (and I've not properly aligned it yet. Was going to last night, but it was the best night in ages and the first time this year I've properly been out, and I just wanted to image anything I could!)

EQ5 with GOTO will set you back about £400 or so, but that and either a high FL scope, or a smaller Apochromat, should be within your £700 budget.

Let us know how you get on...looks like you won't be short of a few words and posts, which is great.

Tim.

[Catanonia]

All Set, have tripod all setup, Laptop already with Stellarium configured to my garden, even a panoramic view of the garden to navigate inside it.

DSLR connected to the lappy to store images straight to it, power supply and well ready for the stars. Even got a few chilled ones (beers) ready for a few hours. Wish me luck with the clouds.

[chris.bailey]

Yep decent skies tonight in the UK here too. Takes me an hour or so to get my rig set up and guiding

so with work tomorrow I'm going to leave it tonight.

Andromeda is a big target, a 300mm lens with full frame CCD would cover it (its twice the size of the full moon). The image below was taken with the rig above. Needs more subs to capture the very faint outer reaches as it should go beyond that frame.



Flatness is the ability of the scope to render stars as circular right across. Cheaper scopes tend to having egg shaped stars at the corners. Worse the centre of cvurvature may not be at the optical centre leading to eggs at two corners and not the others. Its not uncommon to put specialist field flatteners on scopes to achieve that. The TAK in the rig above has one built in.

Good luck with tonights imaging. Dont bother with darks, flats etc. Just shoot plenty of subs and stack them.

[A.S.I.G.N. Observatory]

ED80 or ED120 on an EQ5 or EQ6 PRO at the very least. Anything less than that and you will be left wanting....

Look into Orion/Skywatcher/Black Diamond range.

[Catanonia]

What about this

http://cgi.ebay.co.uk/SKYWATCHER-EXP...lenotsupported

[A.S.I.G.N. Observatory]

Originally Posted by Catanonia

What about this

http://cgi.ebay.co.uk/SKYWATCHER-EXP...lenotsupported

Looks good mate. The only thing I would check is how stable it would be to mount a camera to the focus tube on the side. A cheap newtonian is not my preferred scope, but others have made it work well.

[chris.bailey]

I personally would start at a much lower focal length than 1200 (900 abolute tops). Its a steep learning curve as it is without adding extra complications. 1200mm needs a very sturdy mount, accurate polar alignment and a level of tracking that is not within your budget!!!!! I would also start with a refractor so as to avoid collimation headaches. You would also have to swap out the focusser on that Newt to get focus with a DSLR! As Baz suggested an ED80/100/120 on a HEQ5 is really the minimum starter packages if you want to be in a position to image deep sky. It stretches your budget a little but is a proven platform

[chris.bailey]

http://ukastroimaging.co.uk/forums/i...32.0;topicseen not sure if yo can see it without joining but ED80Pro for £200. A fantastic first imaging scope!

[Catanonia]

Originally Posted by chris.bailey

http://ukastroimaging.co.uk/forums/i...32.0;topicseen not sure if yo can see it without joining but ED80Pro for £200. A fantastic first imaging scope!

mmmm, ok just bought the ED80 with the eyepieces and case for £200

Now looking at Motorised EQ5 Equatorial Head with COAA Computer WIN CTC GOTO set up. Looks nice.

Can also piccy back my DSLR on the ED80

[A.S.I.G.N. Observatory]

Yep, that's exactly the same as the one I am using, only my ED80 didn't come with the aluminium carry-case. The beauty is, that as a beginners imaging scope, it is fantastic, but if ever you upgrade to a larger scope, the ED80 will make a PERFECT guide scope.