Elongated stars - I'm sick to death of them!

[swag72]

I am utterly sick to death of throwing away hours of data because of elongated stars. In the example posted you can see them predominantly to the right of centre.

I'm happy it's not a guiding issue as although this is a 10 min sub, I get the same in 30s subs. I am using my SW120ED at it's native 900mm, so without a reducer / flattener as I want the reach. I am using an Atik 314L chip, so I am really not using the whole of the field at all. I have had this run through CCD Inspector, but just want confirmation that I can post these images. The curvature on CCD Inspector really doesn't seem that bad to me.

I would really welcome people looking at this and making some suggestions of how it can be sorted. I know that I can stick the reducer / flattener but really want a longer FL at the moment.

[johnrt]

This looks similar to the problem I had previously and what I'm trying to get rid of on my 6"RC. I found adding more weight to the counterweight end solved this - my guiding was good too but I still had this creep in every frame.

[swag72]

Was your scope balanced? So you added more weight and unbalanced it totally?

When guiding I adjust the weights accordingly, but as this is here on 30s subs that's aren't guided.

[lukebl]

You might have seen my thread on a similar subject. Although I'm imaging with a Newtonian, I have similar problems which I think I've finally put down to focuser sag. Stars were elongated whatever the exposure. The alignment of the cam (mine is an SXVF-H9 - same sensor as your Atik) has to be really precise, and I noticed that by putting the slimmest bit of card into the focuser to reduce the sag made a big difference. Still working on it, but I don't get many clear nights here in the UK to try it out!

[davew]

While working on the image Luke has beaten me to it !

Look at the lines I've drawn. They radiate from where I think the sweet spot is.

Could be focus sag, camera not square, lens cell not square etc. Start with focuser sag

Dave.

[swag72]

Excellent thank you - But ............. there's always a but. I don't believe I have focuser sag in my Moonlite?

Here's the CCD Inspector screens as well. To my eyes. it doesn't suggest a sweet spot as you have suggested? I'm probably wrong though!!

Regading the camera, it's all screw in attachements until I get to the extension tube and focuser, both 2" pushfit. But I have examined these with a fine tooth comb, they are sitting square.

[buzz]

you mention that you used CCD inspector - it should give a tilt figure as well as a curvature figure. That would indicate if the focus tube and couplings are sagging and making the image plane not orthogonal to the optical axis. You can rule out tracking issues if you do a very brief exposure on bright stars and confirm the issue is still there. hope that helps

Chris

[buzz]

ah - you just did it our mails crossed- it does show the same pattern as in the stars, note the dark blue top left and bright blue bottom right - I never found out what the curvature %age actually referred to - but assume it is the distortion of a star from a perfect circle - in which case it is indicating some of your stars will be 17% elongated.

[swag72]

So this is enough focuser sag or whatever to cause the elongation I am seeing?

Personally I don't think this is focuser sag - I have had issues with the flange of the SW not being absolutely flat so that it pushes the Moonlite up by a bit when it's screwed on.

[davew]

Some of what CCD Inspector shows is strength of light fall on each pixel I think. Once the light has come down the focus tube and filter it should be centred.

If the camera is dead square and there's no sag then we may be looking at CCD or lens cell.

Dave.

Will there ever be a no but day ? Not round here anyway

[Starlight 1]

Take one photo for 30s with the 314l and the same again with the canon 1000d and overlay the two photos if thay are still going the same way its got to be in the mount ,

I say this as the 314 and the 1000d are not the same weight and the focus tube will not move as much.

[swag72]

I am convinced that it's the focuser not sitting properly on the flange. To that end I will tonight try bits of plastic etc under one side of the flange to see if that helps.

I'll keep you posted.

@ Starlight 1 What would you suggest would be the problem in the mount if the cameras are going the same way?

[Starlight 1]

I still be looking at counterweights or backlash .

When tracking from left to right move weights down , if going from right to left move weights up a bit when doing the photos as in my last post.

if it is a clear tonight here I have a play as I had the same thing some time ago.

[themos]

Perhaps you need to throw that Evostar as far away as you can and never look back.

[davew]

One thing that worries me about it being a mount problem is why would you get round stars at the top and elongated stars, getting worse as you go out from those round stars, at the bottom ?

I've had similar problems too. Camera lens I'll admit but the same general idea. The answer was to mount a guide scope ring at the front of the lens and adjust until the stars were even across the frame.

I my case the stars were good on the left and gradually worse to the right.

Also, why would the stars be the same after 30 seconds and 10 minutes ?

I see this as an othogonal problem. Not necessarily the focuser though.

Dave.

[Starlight 1]

ok start with Sweg72 post , Did not say if this is a new problem or not.

So I take that as a new problem, so if it is a new problem its going to be down to something that moving or seting up.

ok the ccd do look a bit out of aline. and more to the right hand side of the photo.

So you got to look at have I move any of the seting from when it was working fine.

you just have to look at every thing .

[Ibbo!]

How many thumbscrews does the 2" extension tube have?

[ollypenrice]

This certainly is not a guiding issue. I'd independently identified Davew's lines of elongation though he has done the job far better than I. Guiding errors follow the same direction across the chip.

Sara, there is no sag in your Moonlite. The whole Moonlite itself is sagging in the tube. The SW method of attaching the focuser is, if I'm not mistaken, to have three radial screws holding it in. Is this so? They do it like that on the cheapo guidescopes I have and the idea is a very unfunny joke. Pure tat. Fine on a cheapo ST80 but on an apochromatic refractor no, no and treble no!

You will have to get that Moonlite orthogonal to the light path or ditch the whole thing. You've already had a good go, I know. This isn't good news but I'm sure its the truth. The focuser is misaligned.

Olly

[swag72]

Cheers all - I agree with Olly here, this is an issue that I thought had been sorted, but clearly not. Yes the flange is held into the OTA on one end and the focuser on the other end with 3 screws - I am going to have a go tonight with packing one side of the flange to see if I can get it square that way. I really do not want to fiddle with the collimation of the focuser itself.

Do you think that a new flange would help?

The 2" extension did have just one screw holding everything in - Totally woeful, so we tapped in another 2 screws, now held much better with three!!

[Ibbo!]

You say its a cheap 2" ext are you sure that the end is square

I've got a cheap threaded ext here that is not square (it is now as it had a quick spin in the lathe)

[swag72]

I don't know about that - It's a TS one - I've got the latest CCD inspector results in another thread - Doesn't look good! Goodness knows what it is.

[George]

Easiest way I have found to make sure the focuser is central to the focal plane is as follows.

Make a paper mask (tracing paper is ideal)the exact size of your lens and mark the centre with a small cross, tape this over your lens.

You then need a laser collimator fitted in your focuser and see where the red dot lands, if its anywhere but on your centre mark slacken off the 3 screws and centre the laser dot on your mark. Mine was quite a way off and I had to elongate one of the screw holes with a needle file to get enough adjustment.

Tighten the 3 screws until nice and tight making sure the centre dot does not wander off your mark, when finished your focuser should be perfectly centred to the focal plane.

The steps above are not a cure all for your problem but it does take out a non central focuser out of the possible causes

This is worth a read and is applicable to all refractors http://f1.grp.yahoofs.com/v1/kBtQTz4hqsriBVBlrI1clzWC4F3RjnIuLpPwDEj3_uVAjTjjITZal59AXQH3_uU9lg3xHWNwMbu4ujN_daAaBkX8lGuT0MPQ1Q/Synta%20Refractor%20Collimation/SyntaRefractorCollimatability.pdf

[ollypenrice]

I'm pretty sure Sara's tried this method. The trouble is that it cannot identify all possible forms of misalignment. Imagine the laser-focuser being off to one side but tilted to point at the centre of the lens. The dot will appear in the right place but the focuser is still badly misaligned. The test is only useful if you are sure that the axis of the laser is parallel with the optical axis of the scope - but how can you know that? When you enlarge your holes are you not just tipping the laser to defeat, inadvertently, the whole point of the test?

The axis of the fouser needs to be the same axis as that that of the objective's light cone.

Tilting the foucuser does not acheive this. It just means that the two axes meet at the objective but then diverge.

What I would look into is finding someone to make a new tube with a proper threaded fit for the focuser because three radial screws are neither use nor ornament.

Olly

[swag72]

Ah, yes I did try the method mentioned by George. Tried to collimate my laser yesterday in order to make sure I had a fighting chance, broke it and it's now in the bin!! Put me off ever owning anything other than a refractor!!!!

I hope on the next clear night to get out with my larger sensor DSLR on the scope. I know that he field will not be flat as I'll not be using a reducer and just try with packing of the flange and screw tightening to get the focuser somewhere square. Then when I put my Atik back on, it should be in the sweet spot and my problems will evaporate (Yeh right!!)