Hello! + Autoguider/guide scope question

[johnno07]

Hi all!

My name's Chris and I have just brought home my first telescope. For as long as I can remember I've been interested in the universe, astrophysics and astronomy. In fact, one of the earliest books I can remember reading as a little fella was the "Solar System" instalment of the Time Life "Planet Earth" series (does anyone else remember these fantastic books?).

So today, after many weeks of researching, my first telescope arrived: a SkyWatcher Black Diamond ED100. I decided on a smaller (semi?)APO refractor as opposed to a larger reflector as my main interest is astrophotography and I dig high quality optics (plenty of experience with regular photography). Am I correct in saying that a doublet refractor is only "semi" APO and has to be a triplet to be "full" APO?

I picked up a CG5 GoTo mount secondhand through the classifieds here and am extremely happy with it. Though obviously, after weeks of mostly lovely clear skies in Brisbane, today is stormy and overcast haha.

As I mentioned above, I'm looking at doing astrophotography with my regular camera (a NEX-7). I've bought a suitable T-adapter and am all set to go. I do have a question about autoguiders/guidescopes though. Basically my question is... what's good? Is an autoguider the way to go? Or is there merit in buying a cheap, secondhand DSLR (Canon or the like), sticking that on the back of the guidescope, hooking it to my laptop and using some form of software to do the guiding? Is that even possible? Also, what sort of thing should I be looking at as a guide scope (focal length, aperture etc)?

So I just want to say hi, thanks (for all the help I've got from perusing IIS already!) and sorry for the terrible weather, Brisbanites!

[tlgerdes]

Hi Chris,

You can use just about any sort of optics for the guide scope, my first was a 70-300mm zoom camera lens. As long as the camera attached can achieve focus and you can mount it steadily on you scope.

Your camera though is probably more important, most guiding software will not talk to DSLRs. So you will probably want to look at potential guide cameras and see what software is supported by it.

One of the most popular is the QHY5/Orion SSAG (essentially the same beasts). Relatively cheap, reliable and supported by every man and his dog from a software perspective. It also has the benefit of inbuilt guide port, so it will talk straight to your CG5s guide port.

[Wavytone]

Chris,

There is no such thing as 'semi APO'. Either a lens is an APO, or its not.

There are achromats - which bring 2 wavelengths to the same focus, or apochromats, which bring 3 wavelengths to the same focus. For some lenses designed for astronomy, one of the 3 wavelengths may outside the visible spectrum.

Until about 10 years ago, to make an APO meant either the lens had to be a triplet, or for a doublet, one element was fluorite - which is very fragile, easily damaged by moisture, as well as expensive.

In recent years the use of ED glass has allowed doublet designs to achieve apochromatism, and are frequently diffraction limited at f/7. ED doublets that are apochromatic are a huge improvement over achromatic doublets. However a well designed triplet or four-element lens should be even better especially when the size of the field f view and field curvature are taken not consideration.

The other issues to consider are
- whether the lens is truly diffraction-limited,
- the amount of'secondary spectrum' and
- lateral chromatic aberration off-axis,
- the amount of field-curvature present, and whether to use a field flattener to correct it.

With three of four elements a lens designer can do a great deal to control all these aberrations over a pretty decent field of view For example http://www.vixen.co.jp/en/index.html
...

In larger apertures however, the best of all however are all-reflecting optics where all wavelengths are focused at exactly the same point from UV to infrared, or those with thin correctors such as the maksutov-newtonian or Riccardi-Honders.

[johnno07]

Thanks for this, Trevor. So I'm assuming the longer the focal length of the guidescope, the better the guiding and subsequent correction - as it can correct smaller movements? Though there will obviously be a limit to how much a motor can be corrected. I will look at the QHY5 and Orion SSAG :)



Thanks for clearing that up, Wavytone :)

[2stroke]

Could have just said its classed as an apo not semi lol.

+1 on Trevors advice, unless your really tight for cash they are the best options for ease of use and also offer a 1/2" sensor so theres plenty of stars to choose from. For your config its the easiest route though there are cheaper ways and better using less expensive and more expensive camera's.

[brian nordstrom]

:thumbsup: Chris , don't worry mate your 100Ed at f9 is in the APO territory easily , nice scopes these. Not everybody can afford a 4-6K Astro Physics , Takakashi , TEC . etc , etc , your scope will produce very good results ,,, once you learn how its done , good luck .
On the guiding have you looked at the 'Orion' 50mm guider packages for sale at Bintel , these work very well from what I have seen .
Brian.

[Jon]

Focal length is not all that important. The autoguider software picks up mere brightening of adjacent pixels and interprets that as movement, and corrects for it - the software itself is VERY accurate. In my experience, what limits the accuracy of guiding is flexure between the guidescope and main OTA, weight imbalance, wind, and polar alignment error. So you want a guidescope setup that is lightweight and rigid.

As the others have said, the Orion mini AG package from Bintel fits the bill perfectly.

[johnno07]

If flexure is the biggest issue, would that mean an OAG is just a better option?

[Lee]

Generally, yes...
no flexure, lighter, easier balance, cheaper....

the only real cost is trouble finding good guide stars, depending on your scopes F/L and sensitivity of the guide cam.... (just get a lodestar and be done with it!)