Whats the differance

[scagman (John)]

Hello all,

I am woundering what the differance in setup and results of the following OTA's would be using the same mount, autoguiding scope and a meade DSI II.

GSO 10" f/8 RC
10" f/10 SCT
ED127mm f/7.5 APO

e.g. would you need a focal reducer for 1 setup but not the others,
Different exposure times to get the same detail
Spend twice as long to setup and focus as the others etc.
(I am a total beginner so dont know what other accessories/adj would be needed to make things work)

Reading the marketing hype for the various scopes all are great at imaging and will produce stuning images etc.etc.. (for their price range).

I know things like quicker f/r mean shorter exposure time, larger diameter - more light.

Would there be any significate diferance between them the results.

Any comments would be helpfull.

Thanks
John.

[ArcaneMagik (Craig)]

I am in no way an educated guesser even about this topic, but I didn't think your thread should get dropped to the bottom without a reply.

Well field of view would be different, the refractor wouldn't produce diffraction lines, refractor would need a field flatterner, and I think similar would be needed for the SCT and RC. Operation and maintenance would be different for each of the scopes. Weight and length of the OTA would come into play as well as they would affect your mounting choice.

As far as beginner equipment I personally would suggest against it. The shiny pretty bits are wonderful, but difficult equipment that drives you from using it will just make you abandon the hobby. A good refractor can always be used as a guide scope when you upgrade down the track, which makes it a great choice for beginner astro scope.
.

[scagman (John)]

Thanks Craig for the reply. I appreciate your thoughts on the matter.
363x 173x 181x

Could you elaborate a bit on the differance in opperation please. (I probably have a very nieve view, but I would have thought that once the OTA was mounted to the mount via the appropriate method, and the focuser and camera in place, then the operation would only be the focusing(which I gather is ONE of the hard parts).
Other than collimating, what kind of maintance is involved and how often does it need performing. I havn't found much on maintance so far other than collimating.

I have read and learnt alot about FOV with regards to visual viewing since I started this thread and how same EP would give smaller FOV and more mag. in larger apertuer ATO's . Playing with the FOV feature in starrynight I can see this in practice.

I am having difficulty understanding the FOV and CCD camera.

In Starrynight I can setup what equipment I want to use and it will show the various FOV's of the OTA and EP by drawing a circle, theoretically what I would see thru the selected setup. and workout the various magnifications.

When I select the CCD camera (Meade DSI II) I get a rectanle on the screen representing the ccd's chip. This is the same size (left to right) FOV as the 5.5mm EP at X magnification would give.

One thing I dont understand is when I use the 127mm (F/L 952mm) with the CCD the FOV is 3 times bigger than the FOV would be if the 10"(F/L 2000mm) with the same CCD is used.

I was expecting it to be the other way around.
Any words of wisdom would help putting this matter straight would help greatly in understanding.

I hope my understanding/use of the FOV feature in Starry Night is correct and I really hope my discription/question isnt to confusing.

Thanks John.

[ballaratdragons (Ken)]

Hi John.

If you are going to be DSO imaging with the DSI II, then a Focal Reducer is one of the best tools you can use.
It will give you are much wider (and nicer) Feild of View, and will also increase the brightness of the objects in the FOV.

None of the scopes you suggest are very fast. That's why I would suggest a Focal Reducer.

Your confusion over the FOV in the different scopes is a common puzzle.
Yes, they have different focal ratio's (f7.5 for the Refractor and f10 for the SCT) but they are overriden by Focal Length.
The 127mm is only 952mm long, whereas the SCT is 2000mm! The 2000mm will magnify a lot more.
It sounds a bit confusing, but that's how it works.

Shorter Focal Length equals wider FOV
plus a Focal Reducer, nice and wide. A brighter image in a shorter time.

[Rigel003 (Graeme)]

Yes, the SCT has nearly 3 times the effective focal length of the refractor so the field will be 1/3 the size. Given that the DSI II has a fairly small chip, you really want to pair it with a short focal length scope if you want to encompass a reasonable field. So of the three, the ED127 is definitely the easiest (ED 80 better still). If you're beginning in astrophotography you definitely don't want to start with long focal lengths. They make extreme demands on the solidity of your mount, length of exposures, guiding tolerances etc.

[scagman (John)]

Thanks guys for the replies.

It is all slowly taking shape and I'm starting to make sense of it all.

I have pretty much settled on the 127mm APO.
I have read that the ED 127mm APO's from Northgroup preform well so I have sent an inquiry to them and just waiting on a reply. I have sent them 3 inquires but havnt had any respone from them yet so I will wait a little longer as I do believe they can be slow to respond.

I have been looking at focal reducers for the 127mm APO, but I'm having trouble finding one that is designed for an APO OTA, plenty for the SCT's. I have read that some people have used these with success.
Any suggestions on a reducer for the APO would be good. I found this one, NextGen .7x ultrawide.
http://www.optcorp.com/product.aspx?...-327-345-10161
This would give me a f/5.25 or should I be looking for a faster ratio eg .6x or .5x (f/4.5,f/3.75 respectivly).

Would I also need a flatener when using the reducer?

Are any of the reducer/flatener combos available for APOs?

Thanks again for the help
Regards
John

[ballaratdragons (Ken)]

Yes John, they make Flattener/Reducers.
But all you will need is just a Reducer. The chip in the LPI is small and won't be bothered by the Field curvature outside of its FOV.

And yes, 0.6x or 0.5x will give you better FOV for your images.

The ED127 is not a true Apo. Don't let the advertising fool you.
ED's are basically Achro's but have 1 lens made of Extra Dispersion Glass (hence the name 'ED').

They are very very good scopes

Try this Reducer from Bintel: https://www.bintelshop.com.au/Product.aspx?ID=7458

[scagman (John)]

Thanks Ken,

Will check them out.