Scope info - SW 8" & 10" Newts

[Carl M]

I heard that the GSO f/4 newts didn't have good made mirror cells in comparison to Skywatcher's. Collimating an f/4 is generally no different to collimating an f/5 the f/4 just needs to be kept more accurately collimated. The best thing about f/4 is that you can keep exposure times short but still get good data without having so much noise on a single exposure, the coma corrector is a must though and I found out the annoying way that it's meant to go in the 2" adapter and not straight in the focuser DOH!

[John]

.... Collimating an f/4 is generally no different to collimating an f/5 the f/4 just needs to be kept more accurately collimated......

My understanding is that the "sweet spot" for accurate collimation gets pretty small as the focal ratio reduces, ie: an F/5 newtonian has a "sweet spot" of 2.8mm but with an F/4 it's just 1.4mm. In contrast an F/8 allows a relatively generous 11mm.

It figures that the mechanical accuracy of mirror cells gets critical with really fast scopes too - from some of the examples I've tried, I wonder if some of the "standard" mirror cells supplied with normal production scopes could hit and maintain a really small collimation "sweet spot"

[Gina]

Coma corrector adds nearly a ton. Changes the cheap look of Newts

I had a good look at my obsy roof clearance yesterday afternoon and it's not that much. In one way this was a result of a design change part way through the build. I still think the design is good though. Giving more headroom for the scopes would have either meant a higher roof or lower walls and pier. In either case it would have meant a heavier roll off section. I'll have to look into the possibility of parking differently - I might just be able to accommodate a bigger scope than would seem possible ATM. I think some cardboard mock-ups might be in order

[htj]

So if going for a long focal length scope in order to get up and close to galaxies and planets in more detail is off the table due to what the seeing will allow, is Gina better of concentrating on light grasp and fast optics therefore something like an f/4 Newt?

Only for DSO imaging. For planetary one does a lot of short explores, say +30 frames/second, which allow you to get images when the seeing is stable.

For DSO one needs longer exposures as the objects are very faint (planets are very strong in light, and we can get away with short exposures).

Faster optics will improve the signal-to-noise ratio (which is what DSO imaging is all about). So yes, a faster focal ratio would improve that. The simplest way to that for Gina right now is probably the reducer flatner. Another way to improve the signal-to-noise ratio is get a more sensitive or less noisy camera, i.e., a CCD, or somehow cooling the DSLR (which I think Gina have already done). A third way to improve the signal-to-noise ration is go to a place with less light pollution. Longer exposures or more exposures also improve the signal-to-noise ratio, though light pollution can drown the signal with too long exposures, and galaxy cores can burn out, etc., so there are some balancing issues there.

I've never collimated a newton, but I've rage quitted while collimating my intes rumak a couple of times . I would think that collimating an f/4 system is a lot easier after one have picked up some experience with a slower one.

/Henrik

[Chris]

Thanks Henrik, I'm not that used to Newts myself, I've tried collumnating a 200p f/5 by using an out of focus star to get a donut effect, then lining up the circles so they are concentric, the resulting views looked good so I'm thinking that as long as you use a Cheshire or laser columnator and get a good coma corrector the f/4 might be ok as long as you check collumnation before each imagine session, well also as long as you can hit the 1.4mm sweet spot as stated above:)

[MikeD]

I know the 250 newt is out because of size issues for Gina but thought I might add that I only collimate my 250px (permanently set up in an obs) about once a year and then only because I feel I ought to or I have cleaned the mirror.

Mike.

[Gina]

Thank you folks And that's very interesting and encouraging, Mike I already have a Cheshire that I bought for collimating my Celestron 130 Newt but never used it because I bought my ED80 just afterwards and abandoned the Newt.

I'm going to have another look at whether I can park my scopes at a different attitude and make room for a bigger one. I know the requirements for DSO and planetary and different and that ideally you want two separate scopes.

[JamesF]

May be worth keeping an eye on http://stargazerslounge.com/discussions-scopes-whole-setups/187044-gso-6-imaging-newtonian-f-4-looking-opinions.html if you aren't already, Gina.

James

[Gina]

May be worth keeping an eye on http://stargazerslounge.com/discussions-scopes-whole-setups/187044-gso-6-imaging-newtonian-f-4-looking-opinions.html if you aren't already, Gina.

James

Yes, I've been watching that thread.

Just sitting here waiting for our neighbour to come and help lift my observatory roof off the fence!!

[Gina]

Roof now off fence and upside down on lawn - more info in my obsy build thread. Damage doesn't seem too bad