Central Obstruction

[aaron308 (Aaron)]

Just wondering what difference there is likely to be in contrast between a 25% and 31% central obstruction. The Skywatcher 200mm reflector has a C.O of 25% while the GSO 200mm imaging reflector has a 31% C.O. Is there likely to be a noticeable difference between the two?

[Camelopardalis (Dunk)]

Given they're manufactured by different companies, there's probably too many other factors that come into play. Even then, I suspect very few - if any - would be able to tell the difference visually...

[barx1963 (Malcolm)]

An imaging Newt is optimised for imaging. this usually means a larger secondary as that ensure a larger part of the field is fully illuminated to maximise the amount of light falling near the edges of the sensor. Visual scopes ( and I am assuming that the SW one is that) do not need to be so careful about some drop off in brightness as it usually quite hard to detect by eye and most users only concentrate on the centre anyway.
In addition if one is an f4 and the other f5, all other things being equal, the f4 will need a larger secondary.

Albert Highe in his book (see http://www.bintel.com.au/Accessories...oductview.aspx ) gives quite a detailed analysis of this matter which is well worth a read, although it can get a little technical.

Cheers

Malcolm

[torana68 (Roger)]

difficult to pick for sure, have a look here:
http://www.damianpeach.com/simulation.htm

[aaron308 (Aaron)]

Thanks everyone; I'm getting the feeling that the larger obstruction in the GSO scope isn't likely to cause too many problems; certainly not enough to be be a deciding factor, especially since I'm likely to use it more for deep-sky observations (which I understand isn't affected too much by the size of the secondary?). Considering the GSO is cheaper, has a better focuser and a mirror fan it might be worth going with that.

[barx1963 (Malcolm)]

Aaron
The thing to understand is that the GSO f4 imaging scope is just that, an imaging optimised scope. As such it has several differences between it and a more visual oriented one.

1 Shorter focal length. At f4 rather than f5 it reduces the length of the tube which makes it easier to handle on an imaging mount. Also it makes it "faster" so exposure times are reduced. The speed of the scope is of itself not relevant for visual purposes, but can introduce complications. The main ones are more coma in the image, which for imaging can be dealt with via a coma corrector and collimation becomes more finicky.

2 Position of the focal plane. An imaging scope is designed so the image focuses where a camera sensor would be which is usually further away from the focuser and would be used for an eyepiece. This may entail using an extension tube to achieve focus with an eyepiece.

3 Larger secondary as already discussed.

Generally speaking, these scopes can be used for visual use as long the user is aware of these issues and is prepared to live with them.

May I ask is it intended to mount this scope on an EQ mount or as a dob?

Malcolm

[aaron308 (Aaron)]

Hi Malcolm,

The scope is actually F/5 instead of F/4, and includes an extension tube. Hopefully those lend it more to being useful for visual work? The plan is for it to be equatorially mounted.

Aaron

[barx1963 (Malcolm)]

Ah so it is the f5 imaging scope, sorry that wasn't clear from your initial inquiry.

Have you used a newtonian scope on an EQ mount for visual? The reason I ask is that I have and no way would I go back to that setup. For visual observation a dob is almost always a better option for the following reason

1 No need to polar align
2 No need to level
3 Eyepiece position almost always in convenient position
4 Easy to setup and pack up
5 No electronics to fail
6 Maximum aperture for the $

Cheers

Malcolm

[aaron308 (Aaron)]

Yeah, I have used an EQ mounted Newtonian, and I quite like it; even though it does have its drawbacks, I find the conveniance of tracking (especially with a motor drive) to outweigh them. I've already got the mount as well, so I might as well use it!

[philipheaven (Philip)]

Stick with an eq mount! Polar alignment takes all of 2 minutes and then tracking objects through the sky is a breeze. I ended up getting rid of the dob cause I found tracking so annoying. Always have to make multiple adjustments just to keep your object centered. Another plus with eq mounts are celestial co-ordinates. Can't bet right ascension and declination!

Now I'm going to cop it from all you dob users (which is basically everyone!)

[barx1963 (Malcolm)]

The dob vs EQ issue is a matter of personal preference. As this is a beginners forum area, and it is hard to know the level of knowledge of the OP, I though a little advice may be in order. I have used both EQ and dobs mounts and for visual use, in my opinion, dobs win. I know others that happily use an EQ.
Personally I don't find pushing a well made dob to difficult. Anyway most of the time once I have found an object, 3 or 4 minutes of observing is enough so not too much nudging is needed.
As long as the OP is aware of the mounts and the options, it is all good!!



Malcolm

[doppler (Rick)]

Go the gem. I recently got a 10" f 4.5 dob and the first thing I did was to put rings on the tube so I could mount it to my eq mount. I find it much easier to handle but I can still use the dob mount for a quick viewing session. I guess it depends on how good the eq mount is (some of the light weight ones can be unstable and a chore to use) As for eyepiece position I guess that I am spoilt, my old 8" eq newt has a fibreglass tube with a tube rotation system.
Rick

[aaron308 (Aaron)]

With all this talk of dobs, I've actually had a look at the GSO F6 8" Dob, with the idea that I could buy some tube rings and a dovetail giving me the ability to use it either EQ mounted or as a Dobsonian. Only issue is whether it would be too heavy/too unsteady on an EQ5 (which was fine with my old F5 8", but don't know about the heavier and longer F6).

Any recommendations as to whether I should go with the F5 OTA or F6 Dob?

[raymo]

Hi Aaron, I have an 8" f5 Newt on my HEQ5 Pro Goto, and when I add
an 80mm f5 guidescope and camera, the mount is definitely at it's
limit. Based on this I suggest that your EQ5 would be overloaded when
carrying an 8" f6 and anything else you might put on it. The sensible limit
for the EQ5 is a 6" scope with accessories. If you prefer an EQ mount, I would go with the f5 OTA, which your mount would handle a bit better, and would be a little better for imaging, if you decide to go down that road in the future.
raymo