What is the difference between an astrophotography OT and a normal OTA?

[Gem]

Hi all. Can someone please explain the difference between an "astrophotography optimised OTA" and a regular OTA? Does it make a big difference?
Thanks!

[tlgerdes]

Field flatness and illuminated imaging area would be the two key points.

Field flatness can often be achieved through external adapters but illuniated imaging area is something you cant correct without changing your telescope parts.

Take a standard visual 8" Newt, most have a small secondary of around 50mm, whereas an imaging Newt will have a secondary of around 70mm or greater, creating a larger illumunated area for your CCD to minimise vignetting.

[mswhin63]

I am not sure but I also think chromatic aberration may also be a feature, for refractor scopes.

[tlgerdes]

CA is something that will effect visual users as well as astrophoto. It has more/different issues for AP around focusing. Different light waves lengths (ie R,G,B) focus at slightly different points and you can be focused on Red but be out of focus on Blue and Green if you do not have a good Apochromatic OTA.

[Gem]

Thanks!
It was on reflector OTA I was mainly interested in, but all info helpful! Learn something new every day...

[ballaratdragons]

Grant,

A Telescope designed and optimised for Astrophotography is called an Astrograph.

Often you can modify a scope to be an Astrograph which involves shortening the length of the optical tube, which gives the ability to bring an imaging chip much closer to the focal plane where the image is flatter.
On a true Astrograph you can do visual but you need to use an extension tube to move the eyepiece back out to the original Focal plane.

Many people have moved their mirror up the tube of their reflectors, or cut off an inch or 3 from their refractors so a camera can reach focus and take advantage of a flat plane zone. Those scopes are now Astrographs.

Other modifications are the use larger secondary mirrors and the use of Feild Flatteners and Focal Reducers (which often determines how much a focal tube must be shortened).

[Gem]

Wow! I didn't know that!
So, to be clear, if I buy an OTA that is an "astrograph", will it be able to be used for visual observing as it is? Or do I need to mod it?
*mind is spinning a bit*

Maybe a better way to go would be to have a standard 200mm reflector for observing and when I want to image to use a piggybacked scope that is purely for imaging... The only problem then is weight: both for the mount and also for portability/useability.

Thanks for all this! Everyone is most helpful!!!!:)

[ballaratdragons]

Grant, you don't need to buy an Astrograph to do Astrophotography.

Almost any scope will do, as long as it will reach focus on the camera chip.
But to answer your question, no you won't need to mod an Astrograph to do visual.

All you will need is an extension tube (an Eyepiece will do with the lens taken out).
Extension tubes are cheap.
All it is is a tube that you put in the focuser, then you put your eyepiece into the tube. It just moves the Eyepiece away from the scope :thumbsup:

[Gem]

Aahh! Now I am clear.
Still need to make a choice though! Too many factors to consider!

[alistairsam]

Hi,
if there is no vignetting with a visual ota and the 50mm secondary and the illuminated area is correct, by using a custom focuser (lower than a low profile) to achieve focus and without moving the primary, will a bigger secondary still be required?
in theory, this would not cause vignetting as the secondary would still cover the light cone?

[ballaratdragons]

Which one are you going to be doing the most?

Visual or Imaging?

If you are going to be doing more imaging, and can afford a true Astrograph, go for the Astrograph.

Or just buy the scope you want and modify it (if needed).
Not all scopes need modding for imaging.

Here is an example of the extension tube you need for doing visual through an Astrograph: http://www.myastroshop.com.au/products/details.asp?id=MAS-028D

GS has a nice range of nicely priced RC Astrographs at Andrews Communications: http://www.andrewscom.com.au/site-content-section-10-guansheng.htm

[Gem]

Initially, mainly visual. Later, a bit of both.

[ballaratdragons]

Bert (Avandonk) gave a fantastic description, with some of his images, on how to beat CA when imaging with an Achromat.

In his 6" Achromat he used a B&W camera, with coloured filters (as many do in imaging) but he refocused on each colour filter.
With this method his image out of a $900 Achro was as colour corrected as an image out of a $12,000 Apo.
:)

[ballaratdragons]

You probably don't need an Astrograph.

My suggestion would be to start with something like an 8" Dob for your visual work, then later on it can be mounted on an EQ mount for imaging.
Best of both worlds :)