F4 Mirror

[scarp15]

Hi, I would like to ask some advice for using a telescope fitted with an F4 mirror.

I am looking into various options for increasing aperture to a 12" scope, portable enough for transporting and comfortable enough to enjoy seated observation. One possible solution would be a 12" truss, 1200 FL dobsonian fitted with an F4 mirror.

Firstly I expect that such a fast scope reaches the margins in which some form of coma corrector is a necessity to clean-up the view, namely a parracor? Or can I get away with just using premium eyepieces?

Also is a good level of collimating experience required, plus good quality (more expensive type) collimating tools? As far as collimating is concerned, I only have some rudimentary experience of infrequent and easy star test collimating with my C8.

Thanks

[angusb1]

I've used my F4 8" with and without an MPCC with plossls, BST Explorers and Explore Scientific 82 degree EPs and I would say that I would want to use a coma corrector with a scope this fast or there will be noticeable coma at the edges of the field. The Paracorr would be the best solution from a convenience standpoint. You ought probably to get a decent collimation device as well if you don't already have one. I just bought a Hotech laser collimator because although I can get the secondary collimated in daylight with my cheshire, I usually need to tweak the primary before each session to get the best out of the scope especially if it's been in the car to get to my observing site. I have had a revelation laser collimator and its good enough to get the scope close - especially with a barlow, but the hotech is better.

HTH

[Moonshane]

hi Iain

I have a 16" f4 scope (solid tube) see http://stargazerslounge.com/1914604-post320.html and this is truly a delight to use. With top quality eyepieces you could get away with no coma corrector I think but with a Paracorr this is like an f8 scope with virtually no coma at all even in 100 degree eyepieces.

Collimation is required for all newtonians before each session as far as I am concerned and occasionally during a session. It's not hard to learn and quick to do, even in the dark. It's the same process for a f11 as it is for a f4, there's no difference other than needing to be closer to spot on before you notice the results of poor collimation with a fast scope - personally I'd prefer to observe with a scope that's properly collimated no matter what speed it is.

You don't have to buy Ethoi or Naglers either. I have BGOs, Radians and TV Plossls too and they all perform well with the scope. Especially with a Paracorr, I suspect that you could get away with other options.

Collimation and a fear of having to buy expensive eyepieces (not true but if you can......) need not put anyone off a faster newt in my experience and opinion.

[andrew s]

Iain - You will need top quality eyepieces to cope with the steep angles the F4 light cone makes at the eyepiece. They do not however, prevent or reduce the coma.

For this you will need a coma corrector but again check it is suitable for use at F4.

If you need coma correction or not depends on how sensitive you are to it. Some people find it more annoying than others.

Andrew

[Moonshane]

True Andrew, in fact good eyepieces often reveal the coma (where before it was hidden in bad eyepieces by e.g. astigmatism).

[scarp15]

Thanks for all the input, My personal preference is for seated observation on an adjustable chair. On reflection, I think that an F4 mirror at larger apertures, such as Shane's 16" solid tube might be more of an advantage in this aspect, than at 12" in a truss or flex design, when up to F5 may be a preferred option (for myself) and with a bit less coma to contend with.

As for collimation, being the owner of an F10 SCT, I do not possess a collimation tool, applying instead the occasional 'donut' test., and only really get a sense of how in or out the collimation is on rare nights of steady seeing when it is possible to increase the magnification some what.

I do look forward to the eventual step up to a 12" mirror and the hands on approach to collimation though!

[Moonshane]

I completely agree with all you say Iain. although if making your own truss (or solid tube) system though one advantage is you can put the scope at pretty much any height you want if you have no major size restrictions. I had a f5 12" and it was great. no CC needed for that.

[scarp15]

Hi Shane, I agree that custom builds, and looking at all options will accomodate height and posture better. I am slightly above average height (not quite 6') so F5 and perhaps up to f5.3 viewing whilst seated will be fine, especially if design is taken into account. One core issue is the position of the focuser. For example on the 12" f5.3 OOUK you referred to, if fitted to their dob base I believe the tube can be realigned a bit to permit the focuser to be placed at a more relaxed angle.

I have also been in email conversation with David Lukehurst. Interestingly he can modify the focuser - on a 12" f5.3 truss, from the standard 45 degree - from horizontal angle, to a reduced position for seated observation, and can attach two sets of studs for positioning both (standing and seated ) options. I am also still considering the flextube, which I will modify with a new focuser, and at f4.9 should be fine concerning height.

I have to say that unlike yourself, I have so far not ventured into looking at my own build though (yet).