The saga of the F4 8" Imaging Newtonian

[Rocket Pooch]

Hi,

I thought I'd give you an update on the imaging newtonian I have, as you may remember the 1st one I got fell to pieces, this has since been replace, still in its box.  I will be unpacking this today and having a look at it.

I have done quite a bit of reading about the scope and in communication with some people who know what they are doing and there seems to be two key issues to be resolved.

Firstly the tube around the focusor needs to be reinforced and secondly depending on what camera your using you need to be careful what coma corrector you use.

I have an MPCC and QSI, the QSI is 1.1kg, this is quite a heavy camera, so I have to get the focusor to extend as little as possible.

Also, the collimation is obviously an issue with this type of scope, apparently some of these scopes have their centre spots out by 2mm and the secondary out of alignment a little, I'm going to get a cats eye kit to help me get critical primary and secondary collimation.

So, I'm going to have another go, not just on the word of the forums but also because of some of the images which are now being produced using DSLR's images, such as this 30x3 minute image with a DSLR, I guess with the QSI I should be able to go deeper.



Horsehead Nebula by Phil Hosey, on Flickr


One small note, a 6" astrograph F4 with a reinforced tube and focuser is about to come out as well, this could be a better scope for budget newts.

[mickw]

the QSI is 1.1kg, this is quite a heavy camera, so I have to get the focusor to extend as little as possible

If the tube is strengthened with thicker material than necessary, this would also move the focuser further from the secondary and reduce the amount of travel needed.
You could even add spacers behind the focuser.

[RobertM]

Almost sounds like it's worth persevering.  That's a very nice horsey for a DSLR but as you say it's a fairly light camera...

[Rocket Pooch]

Hi,

Mick, the trick seems to be to put another two scope rings each side of the focuser, this seems to have worked for D70 and QHY9 users, these cameras are going to be about 400g lighter than mine, but one has a filter wheel.

Robert,

Another interesting thing that is happening is when this batch is gone I guess they will redesign it, the reason why I say this is because the new 6", 10" and 12" F4 scope all have rail based focusers the larger scope has a 3.3" focuser.  So it looks like the F4 new newts from Taiwan are going to up the anti a little, esspecially with the carbon tube ones coming along.

I love newts for the value, if I can get an 8" and 6" working as F4 astrographs it will be a result.

Chris
 

[Fay]

I will have to get my 8" out again, I suppose, but I seem to think I have it collimated & it isn't.

I know it takes great images, it is that I do not have the knowhow of the reasons for certain problems. Perhaps it is collimated & the problem is the spacing & if the camera is sitting correctly in the tube.

[mickw]

the trick seems to be to put another two scope rings each side of the focuser

I can understand that reducing the flex in the tube, but not eliminating it on the downhill side of the focuser where any stress would distort the tube inwards, although a quick fix would be a bolt or pop rivet fixing the ring to the tube.

[MarkS]

I've just noticed this thread.

Chris,

You have a choice really - you either persevere with this Newt or you pay an awful lot more money for a dedicated, fully corrected, heavier duty astrograph.

You've correctly identified the 2 main issues:
1) A fast F-ratio, F4, is not very forgiving when it comes to collimation issues or sag from around the focuser
2) You need to make sure the corrector is well matched to the scope (as you discovered with the ED80)

A Cheshire (including the sight tube) is the only way to accurately collimate these things - at least you'll be able to follow it up with a star test.  Star testing won't work on the Banana because the secondary is designed to be off centre and this give non-concentric circles in the de-focused star.  

It's a heavy camera you are using - good luck!

Mark

[RobertM]

If I were you I'd wait till you see what the SW Carbon Fibre tubed newts are like.  They may not be cheap but there should be far less tube flex; dependant on how well they've been designed that is.

The idea of the extra two tube rings is a good one and something I seriously considered for the SW190 before I got the Moonlite.  I've even seen them fitted to ASA Astrographs - even carbon fibre tubes may need some support!

One option is to use a wide focuser adaptor machined to the tube profile with a reinforcer underneath.  The Moonlite adaptor I use is wide and machined to the tube profile; there's no reinforcer but it does a really good job even with my heavy camera.   I'm not sure how much that will help you but it may be worth a try.  Certainly bolting an extra tube ring directly to either side of the focuser base will remove one element of the equation but that's no good if the focuser flexes (it will but is it noticable !).

Once you've got that sorted then a Wynne Corrector is the way to go... f2.8 ... hey that's banana scope territory ... maybe that's a bit too scary ... 3" focuser and the like... could be a banana scope is not so expensive after all.

Robert

[Rocket Pooch]

Hi,

Yes I could add a carbon tube, £600, then a 3" Focuser £500 and also four rings £300, oh crap thats getting expensive.

I already have my name down on one of the carbon tube F4's, but I'm going to play with this one, oh a side note, got out of the car tonight and although there are some clouds the sky is stunning.

[RobertM]

You don't need expensive tube rings, just use a few more of the cheap SW cast rings.  It's surprising how stable thay are if you fit them to a decent dovetail plate.  I was going to use 4 for the SW190 but two fitted to a Losmady plate worked out stable enough, which did surprise me.

got out of the car tonight and although there are some clouds the sky is stunning

That's not what I'm calling the clouds at the moment