apo vs semi apo

[Neo_uk]

Hi,

whilst only taking this hobby seriously over the last couple of months after a life long interest it has cost me a small fourtune (as a few members might vouch for ) to say my initial budget was £500 thats long since gone.I have taken to the photographic side of the hobby and I'm quickly learning i need more and more equipment which will have to come over a period of months .

Having just bought a canon d300 from a sgl member Ive taken some pictures on my explorer 200mm which turned out better than i thought they would .But i keep looking at the small refractors and see the excellent reviews they get but a few things still confuse me.

wo zenithstar 80 II semi APO vs WO Zenithstar 66 SD APO

allthough one has a larger aperture (Zs80II) and is semiapo and one is of a smaller aperture and APO what is the major difference ???.I was always led to believe aperture is king but i have seen images produced by both these little gemson various web sites that rival the larger telescopes so is it quality of the optics that make up for the decreased size in aperture ?.

Also are there any know factors regarding focusing i.e extra adaptors needed etc ?

so after waffling on for a while i think what I'm trying to say is ........

do i sacrifice a little of the aperture size for a full APO or go bigger aperture and settle for semi APO.....???

hope someone can follow this because Ive really confused myself now :scratch:

regards Chris

[Beyond_Vision]

The WO66SD is actually a semi-APO the WO Zenithstar 80ED II is classed an APO although some may class it somewhere between an APO and a semi-APO.

The new WO Megrez 72FD is a flourite doublet lens that is near as dam it fully corrected. There is also a Flourite version of the 80mm available which has better performance than the 80ED II either available as an 80FD or as an upgrade lens kit called an APOGRADE kit which includes a WO/TMB designed lens and a dual speed focuser.

As you can see it not a clear cut thing. The level of colour correction is kind of proportional to the money you pay with higher end APO's having 3 or maybe 4 elements to the lens. As per the HiFi world its a law of diminishing returns in that you dont get double the performance for double the money.

The main reason people use these scopes for astrophotography is that they have a widefield of view and that they dont suffer from the need to re-collimate like Newtonians and SCT's. They also have good contrast due to hardly any internal reflections and no central obstruction. They are not ideally suited to planetary work as they are a short focal length but they can be barlowed to get a reasonable level of magnification if needed. At the end of the day apeture is needed to get an increase in resolution when greater magnification is required.

Regards

Kevin

[Neo_uk]

Thanks again kevin for the advice the reason i ask is ive seen this http://www.astrobuysell.com/uk/propview.php?view=16369

is this a good pice good scope ?

regards chris

[Beyond_Vision]

I would rather go for the Megrez 72FD which is £299 new compared to the Megrez 80II semi-apo.

[Neo_uk]

OK Kevin one last question wo magretz 72 or skywatcher equinox 80

Ive seen them both on Flo .

do these scopes need extension tubes to achieve focus for dslr's

(OK maybe that was two questions lol)

i really do appreciate the time people give to new people to the hobby on this forum and as you mustget a lot of repetitive questions.

regards Chris

[radioactive]

I rather dislike these marketing terms of semi APO. Look its either an APO or not!

if its not such as my Antares Sentinal is not! and its not what they say it is a semi APO either!

it is a finely adjusted acromat in fact imho

Grrrrrrrrrrrr

rant over

[Beyond_Vision]

Neo I would go for the Megrez 72FD in preference to the 80EDII.

Regards

Kevin

[iancandler]

Its no contest chris, go for the megrez 72, at the current price its unbeatable.

If I was buying now its what I would go for in that price bracket.

[MartinB]

Chris, aperture certainly isn't king when imaging. These are the important things to consider

Exposure time - this is determined by the F ratio of the scope. For a given exposure time an F5 will give the same signal to noise ratio regardless of aperture. Obviously the sensitivity of the of the camera will also be a major factor in how much time you give your targets

Image scale - this is determined by the focal length. The longer the focal length the greater the magnification.

Field of view - this is determined by the focal length of the scope and the size of the ccd chip. DSLRs have a large chip so a big field of view. Small focal length refractors are great with cameras with small chips such as a DSI but with DSLRs small targets can be ... small! One way to check how well framed your images will be is to down load Ron Wodaskis CCD calculator http://www.newastro.com/newastro/book_new/camera_app.asp This is an invaluable tool and it's free!

Resolution - this is determined by focal length and pixel size of the camera. It can also be limited by the quality of the seeing conditions. In poor seeing resolution may not improve with some cameras when going above 600mm f/l on the other hand a 2000mm scope will produce great detail in outstanding seeing. Using focal length and pixel size it is possible to determine the plate scale/sampling rate expressed in arc seconds per pixel. If you download the ccd calculator you can see how this works. A sweet spot is between 2-3 arc seconds per pixel.

Hope that is some use.