More SLR advice

[Trevor-Austin]

While waiting for my 300D to arrive I want to understand exactly how this works.

I am going to use a T-Adapter in my 2" diagonal and then the T-ring to connect to my camera? I think that's right

However I can't make my mind up on reading the adverts for the various adapters how they work. I thought the camera used the telescope as a lens in the same way that my webcam does, however reading the Sky's the Limit adapters then it appears you put an eyepiece in the adapter, which sounds better as I can then change the FOV and the magnification unlike the webcam.

So what bits have I understood and what bits have I got wrong?

[russ]

You can use both methods:

Camera with no eyepiece, just using the scope as a lens is called 'Prime Focus'

Connecting the camera to an adapter with an eyepiece is called 'eyepiece projection'

There's a third option, mainly applies to the digicam owners, where the camera, with lens attached, is put upto the eyepiece. This is called 'Afocal'.

Most DSLR owners will go with Prime Focus. Perhaps adding a barlow for increased focal length/image scale if needed.

[Trevor-Austin]

Ah OK,

So what equivalent lens would my 300D act as on my scope and what is the advantage of prime focus over eyepiece projection?

The one I'm looking at is;

the deluxe one here, Camera adapters

Thanks

[bus_ter]

See diagram below for all the different ways to connect your dslr to the scope, and how to work out the focal length.

Not all of these modes work equally well, for several reasons. First, DSLRs excel at deep-sky work, not lunar and planetary imaging. Accordingly,we want a bright, wide-field image. That means we normally leave the focal length and

f -ratio of the telescope unchanged (with direct coupling) or reduce them (with compression). The modes that magnify the image and make it dimmer – positive and negative projection and, usually, afocal coupling – are of less interest.

Second, if you do want to increase the focal length, positive projection (eyepiece projection) is seldom the best way to do it. Positive projection increases the field curvature that is already our primary optical problem. Negative projection, with a Barlow lens in the telescope or a teleconverter on the camera, works much better. The appeal of positive projection is that, like afocal coupling, it works with any telescope that will take an eyepiece; you don’t have to worry about the position of the focal plane. Indeed, positive projection with a 32- or 40-mm eyepiece can give a projection magnification near or below 1.0, equivalent to direct coupling or compression.

Regarding negative projection, note two things. First, a teleconverter on the camera has a convenient, known, magnification, and the optical quality can be superb, but the DSLR may refuse to open the shutter if the electrical system in the teleconverter isn’t connected to a camera lens. The cure is to use thin tape to cover the contacts connecting the teleconverter to the camera body. Second, Barlow lenses make excellent negative projection lenses, but the magnification is not the same as with an eyepiece; it is greater, and the best way to measure it is to experiment. The reason is the depth of the camera body, putting the sensor appreciably farther from the Barlow lens than an eyepiece would be. Compression is the opposite of negative projection. Meade and Celestron make focal reducers (compressors) designed to work with their Schmidt–Cassegrain telescopes. As well as making the image smaller and brighter, they help to flatten the field.

[russ]

Ah OK,

So what equivalent lens would my 300D act as on my scope and what is the advantage of prime focus over eyepiece projection?

The one I'm looking at is;

the deluxe one here, Camera adapters

Thanks

To work out what lens your scope will work as on the 300D, you need to know the focal length of the scope and then times that by 1.6 (crop factor of the 300D chip).

A Skywatcher ED80 for example, which has a focal length of 600mm, will work as a 960mm F7.5 telephoto lens on the 300D.

[Trevor-Austin]

Right, great reply thanks.

So its starting to make sense, as far as my camera is concerned it is going to have an F10 3200mm lens, awesome!

Need to look at those calcs to see if I can get any idea of what I will be able to see in the resulting pictures. i.e. what can I image in one go. Looking forward to lots of messing about

Now I guess I need the Meade wedge and find a way of guiding it, this is going to get expensive as I just know I'm going to want to keep making it better!

[Trevor-Austin]

Not all of these modes work equally well, for several reasons. First, DSLRs excel at deep-sky work, not lunar and planetary imaging. Accordingly,we want a bright, wide-field image. That means we normally leave the focal length and

OK, so negative projection looks like the way to go. is there any disadvantage to using the 2"diagonal to do the prime focus coupling with over connecting directly to the back of the SCT? Just means I can check and find my views without having to keep connecting and disconnecting the Diagonal?

Damn, I was pretty much decided on eyepiece projection

[russ]

Right, great reply thanks.

So its starting to make sense, as far as my camera is concerned it is going to have an F10 3200mm lens, awesome!

Just a word of warning. You'll have a mare getting long exposures with a focal length of that size. Even with the worlds most accurate EQ mount, it will still be a big ask. Plus the slower F10 focal ratio will be working against you, making things doubly difficult.

I had the same problem with a 10" LX200. Connected up my 300D expecting great things but was mightily disappointed to see trailing after only 20secs. I soon changed tact and bought a small refractor on a small EQ to go with the LX200. Keeping things small and wide made all the difference.

[Trevor-Austin]

Sadly I'm already expecting that

[bus_ter]

OK, so negative projection looks like the way to go. is there any disadvantage to using the 2"diagonal to do the prime focus coupling with over connecting directly to the back of the SCT? Just means I can check and find my views without having to keep connecting and disconnecting the Diagonal?

Adding a diagonal between the telescope and camera can affect two things. Firstly you've effectively added an 'extension tube', so you will be focussing the image at a different point further back. You will need to significantly adjust the telescoopes focus (which may or may not allow this much adjustment) and when you do focus you'll have a bigger image, so you've increased magnification and decreased f-stop.

Secondly you've added more glass (prism) or mirrors which will only cause to deteriorate the image a little more. This could be an insignificant amount, or very noticeable depending on various factors, particularly the quality and cleanliness of the diagonal.

I don't know what you mean about connecting and disconnecting the diagonal. When the camera is in place you'll be looking through the camera (or using the LCD viewfinder) to locate and focus the subject. Swapping the camera for an eyepiece will produce an entirely different view and focus.

[Trevor-Austin]

Certainly don't want to slow it down anymore or make the image bigger, so connected to the back of the sct then.

Thanks

[Merlin66]

A favoured option is to use a flip mirror infront on the camera.

You can then focus and frame the target object...flip the mirror and start your exposures.....

The good back focus of a SCT makes this a nice option.