RH Field edge out of focus stars - MN190

[Kbramley]

All,

Having recently got back into imaging after house moves, babies, weather and obsy construction I have found to my woe that my recent images appear to show out of focus stars just on the RH side. I have done basic collimation with a laser collimator, and done a very simple "look down the barrel) to see how well its colimated and it looks reasonable (as well as it did before anyhow). Could this be a symptom of bad collimation, or is it more likely that my imaging rig (Atik 383L - Filter wheel - 5mm spacer - OAG - Focusser) is not sat perfectly square on the focal path due to it sagging slightly due to the weight. Oh for info Its the stock MN190 single speed focuser.

I am debating whether its worth getting a Cheshire to try and get the collimation better.

Thanks for any advice in advance

I will try and post a typical sub later when I get home

Keith

[Peter Drew]

I would suspect imaging rig sag in the first instance.    

[Kbramley]

Agreed, I am leaning to this myself, I guess a good test would be remove OAG and filter wheel, then try with just 383L, I would still have wind out the focusser some way but the weight would be nearly halved, if problem reduces significantly then I'm possibly into new focusser territory :0

Keith

[opticalpath]

A couple of thoughts, Keith ...

Good idea to remove the OAG and filter wheel to lighten the load. If you have a suitable length threaded extension tube, I would fit that in front of the camera so you can keep the focuser as near to fully racked in as possible.

My dual-speed focuser has a pull-out extension section that locks with a single thumbscrew - is yours like that? It's a horribly loose fit and very prone to tilting even when the thumbscrew is well tightened. I found a way of improving that; let me know if you are interested.

Last, I know from painful experience how difficult it can be to collimate this scope. The symptoms you described *could* be a collimation problem, particularly if you have adjusted the primary mirror tilt by much. The thing with the Mak-Newt is that, as well as the usual optical alignment of primary mirror and focuser axis that you do with a Newtonian collimation, in the Mak-Newt the primary mirror axis also has to pass right up the centre line of the OTA and through the centre of the corrector. It's quite possible to do an apparently perfect 'Newtonian' collimation, but end up with the primary axis a little tilted. The Newt. will tolerate that happily but the M-N will not.

As a first step I would certainly see if you can correct the problem by actions on the focuser as above, before touching the collimation. If you have to re-collimate, there's a long thread elsewhere on the forums describing several MN190 owners experiences of doing this. Check there also for comments about some looseness in the primary mirror cell; some owners found that the primary mirror cell was shifting slightly in the OTA causing centring errors that affect the M-N collimation and/or coma correction.

My other advice would be don't rush to change the focuser. I swapped mine for a Moonlite and discovered that the absence of slotted mounting holes on the focuser base meant that adjustments and recollimation were required .... OK now but was not at all an easy process!

Adrian

[Kbramley]

Thanks for advice, some good info there soon as I get chance I will try a few out. The focusser has two screws, one larger than the other, I found I had to slacken them slightly otherwise the electronic focusser didn't have the power to move anymore

keith

[opticalpath]

Thanks for advice, some good info there soon as I get chance I will try a few out. The focusser has two screws, one larger than the other, I found I had to slacken them slightly otherwise the electronic focusser didn't have the power to move anymore

 

keith

I guess that one of these screws is a locking screw that prevents any in-out movement at all of the draw tube; this one needs to be loosened off so it never prevents the focuser motor from turning.

The other screw will be a tension adjuster that presses the focuser spindle against the flat on the drawtube. It does need to be tightened a little. Loosen it off until the drawtube slips with the weight of the camera vertically underneath when you run the motor to lift it. Then gradually tighten it just enough so the focuser will reliably lift the full weight of the camera without slipping. A heavier camera/ filter wheel etc may require a little more tension but don't tighten more than necessary.

Adrian

[Kbramley]

Quick bump on this with an update, I narrowed the problem down to the OAG. The OAG has a bespoke adaptor to take it to a T-Thread and three thumbscrews to hold it tight (this allows one to rotated the OAG a little to get a guidestar). As it turns out I'm not man enough to provide sufficient tightness to prevent the whole thing flexing (albeit by a very small amout) when the Filter wheel and 383L are on. with the help of some pliers I added a 1/2 turn extra to the thumbscrews whilst ensuring the adaptor was flush and true and after some 10min OIII subs I took last night the focus is now much flatter across the whole field . There is still some residual off centerness going on which is now most probably linked to the focusser itself, but this is far more bearable.

Keith

[ollypenrice]

Good to know but have a care with the pliers. Those thumbscrews shear off easily. Could you not add a little packing to the male part? That shiny metallic sellotape can be quite good.

Olly

[Kbramley]

Yeah was only a tweak, but I know where you are coming from, the metal is quite soft. If only I had a 3D printer, I could knock out a solid spacer, the gap between the filter wheel is about 5mm.  I do have some  acrylic sheet which I could cut to  size , I made some spacers like this before.

[opticalpath]

Not sure what you mean about the 5mm gap. Is that a threaded component that tightens up before it meets a shoulder that would hold it square on?  In which case a spacer might help .... so you're not relying solely on the accuracy of the threads to hold the components squarely together.

If there's looseness between a tube and the socket it fits into, as Olly says, try a sliver of something to make it a more snug fit. I found that Teflon baking parchment is good - cuts with scissors and doesn't tear so easily as tape.  One thing to try is to run two parallel strips of tape to the male (or female) component - not too far apart, say 45-60 degrees around the circumference - with just one clamping screw opposite them that pushes the inner tube on to the two 'rails' of tape and helps to keep the tubes parallel. That's copying the idea of the Howie Glatter Parallizer; he points out that with clamped push-fit components, getting the tubes parallel is more important than getting them precisely concentric.

Adrian

[Kbramley]

Sorry for the confusion, here is a quick piccy of the set up, the 5mm extension screws onto an insert, the insert is held in the OAG main shell via three thumbscrews, this allows rotation if needed. I needed the 5mm extension to get it co-focal and allow enough clearance to get some purchase on the thumbscrews and fit myguide cam on. The pic is not great as I knocked it up in Word

[opticalpath]

It looks like the body of the OAG and the section of tube that it clamps on to are very short.  Unfortunately that will make it more susceptible to tilting than if there was a longer overlap of the tubes.  Is there any option to make a direct threaded connection to the OAG body, or do you have to use the push-fit and clamp screw arrangement? 

If you have to use the clamp screws, the best setup would be to use a short adapter/ nosepiece with a bevel cut so that, as you tighten the clamp screws (with any internal clamp ring removed), the nosepiece is pulled in and the body of the filter wheel is pulled tightly against the body of the OAG with no gap.  That isn't possible since you need a 5mm gap but an oversize 5mm spacer ring might work if you make it as big as possible to maximise the surface areas in contact.  With a bevel cut nosepiece, the thumbscrews don't have to be so tight; you're relying on them to pull the components square against one another, rather than to clamp down on the inner tube.

Adrian

[Kbramley]

Thanks for the advice Adrian, as you said there is no option to use a threaded attachment to the OAG, this particular choice of connection  is most likely to allow the OAG to be rotated and then locked more easily. My picture doesn't show it  but the bit the screws tighten down on is beveled slightly and not flat as in the diagram. I guess it's probably not intended to be used with such a heavy CCD and FW. I shall try an oversized spacer when I get chance and see if this makes the set up more rigid.

Keith