How to magnify DSLR prime focus

[Paul.L]

Hi, I wondered if someone could clear this up for me, I don't quite understand what is done to magnify when using 'prime focus'.

I would love to know how to take shots of deep sky stuff.

I have a barlow lense that's x2

My telescope is 70/700mm

Does it depend on the telescopes focal length on how far you can see using just DSLR ?

Thanks

[shaunster]

The focal length does determine the field of view yes

The shorter the focal length the wider the FOV

You dont want to be using a barlow for DSO imaging as it will increase the exposure time needed I think

[jgs001]

The scope is f/10 based on the details, and that's not ideal to start with for deep sky... it's rather slow... If you add a 2x barlow, you multiply the time for an equivalent exposure by 4, which puts a lot more strain on the mount as it has to maintain accurate tracking for that time.

It depends on what you mean by how far you can see... If by that you mean how far into the universe, that's more a function of exposure time... although it's closely linked to the focal ratio of the scope... If you mean by see things more closeup then yes, the focal length defines the field of view...

[Catanonia]

Personally I would not try this with that scope or really any to be honest unless you are planet imaging / viewing as high fstops are desirable due to the bright planet surfaces.

As stated, adding a 2x barlows will do the following.

1. Add extra glass into the optics train and not being nasty, but the barlows you have is most probably a cheaper one and not that good.

2. It will increase your zoom / range / focal length by a factor of 2 or more depending where you place it in the optical train.

3. It will increase your F stop by 2 so make it much much harder to see and focus

4. It will increase exposure times by a squared factor of the barlows to compensate for point 3. Ie 4 times the amount of exposure to get what you would without the barlows.

5. Your guiding will have to be spot on and last for the time.

This is why most of us have many telescopes of differing specifications for differing targets.

How do I know all this...... Well I fell into the same logic trap and tried and field with a 80mm refractor.

Guess what.... I now have 3 scopes

[Paul.L]

Ok so I'm not missing anything silly like a zoom feature on more expensive DSLRs. Guess I will have to get a new telescope. I just wanted to try and test it before I upgraded.

Thanks

[Catanonia]

Ok so I'm not missing anything silly like a zoom feature on more expensive DSLRs. Guess I will have to get a new telescope. I just wanted to try and test it before I upgraded.

Thanks

Afraid not mate, you are using the scope as a prime FIXED length lens.

[Shibby]

Well... as far as the camera is concerned, this also will affect the field of view. You can't zoom at prime focus, but smaller sensors provide smaller fields of view, but to actually 'see' more detail you need higher resolution. However this typically means smaller pixels which has a similar effect as slowing down the optics, again requiring longer subs. There is no way round this whether at the optics or at the sensor. (again better tracking also is required). These problems lead to aperture fever

Ps: can some of the more knowledge members sanity check what I've just said as I'm far from an expert on this, im only repeating some things I think I've previously read on sgl!

[Catanonia]

These problems lead to aperture fever

LOL Well summed up

[jleach29]

I take the top part off of my barlow lens that the lens goes in, and screw the botton half of my barlow with the lens into my T adapter, but I only use it for Lunar shots.

[ollypenrice]

This has been well answered. However, there is nothing wrong with 700mm as a focal length. You will have an image scale ideal for all sorts of targets. Eg M33 would look great on a DSLR chip at that focal length. My own line up has FLs of 85 and 200mm (camera lenses), 328 and 450mm (Small Takahshi refractor with/without reducer) and 980mm (big refractor).

To see what fits onto what FL/chip combination try this:

The New CCD Astronomy Home Page

The trouble is that F10 is very slow, as stated. F5 would be 4 times as fast.

Small pixels are not a great idea in astro cameras. They tend not to have them too small because low light cameras use larger, more sensitive pixels.

Olly

[Paul.L]

Thank you all kindly for the very good answers and additional advice. I will be busy in the upcoming weeks now I know where I stand

[Catanonia]

Thank you all kindly for the very good answers and additional advice. I will be busy in the upcoming weeks now I know where I stand

as long as it gives you the proper expectations, all is good.

The hobby isn't cheap, but it can be if you are happy with what you have.

Unfortunately, all that have answered here are not