field rotation?

[Galaxyfaraway]

Thanks very much Paul.

The link I provided to download the fits files from is just my google drive/doc; it shouldn't be 'dangerous'.

The HA flats were about 7 seconds I think and the clear flats were about 0.04 seconds or so.

In any case if it works with Jpegs then that's fine. Thanks for the testing of the flats. I guess using white laptop screen is not a good way of taking flats (with a tshirt over the scope). I should try pointing at the sky or get a dedicated flatbox I guess would be the best option. I am not quite certain whether I am supposed to be looking at achieving higher percentage (of what?) across the images as per CCD inspector or whether that percentage distribution should be more even? I can certainly do the stretching of the flats to work out where the corners end up being uneven. If it's tilt, how do I correct it? I worry it's been brought about or made worse after installing the lakeside focuser. I notice that it is harder to tighten the screw under the focuser as it now also supports the lakeside bracket as well and I can't get to it with pliers or similar after fitting the refractor onto the mount. I will send a photo once I am back home. A couple of clear nights this week and it's frustrating to have to be away from the scope and not being able to image with it!

[Thalestris24]

57 minutes ago, Galaxyfaraway said:

Thanks very much Paul.

The link I provided to download the fits files from is just my google drive/doc; it shouldn't be 'dangerous'.

The HA flats were about 7 seconds I think and the clear flats were about 0.04 seconds or so.

In any case if it works with Jpegs then that's fine. Thanks for the testing of the flats. I guess using white laptop screen is not a good way of taking flats (with a tshirt over the scope). I should try pointing at the sky or get a dedicated flatbox I guess would be the best option. I am not quite certain whether I am supposed to be looking at achieving higher percentage (of what?) across the images as per CCD inspector or whether that percentage distribution should be more even? I can certainly do the stretching of the flats to work out where the corners end up being uneven. If it's tilt, how do I correct it? I worry it's been brought about or made worse after installing the lakeside focuser. I notice that it is harder to tighten the screw under the focuser as it now also supports the lakeside bracket as well and I can't get to it with pliers or similar after fitting the refractor onto the mount. I will send a photo once I am back home. A couple of clear nights this week and it's frustrating to have to be away from the scope and not being able to image with it!

Hi

Re Flats - I think to some extent it depends on your camera. I used to use a laptop screen and it worked fine with a dslr and with a qhy8l colour ccd. But you have to tune things so they the camera doesn't pick up the screen refresh signal and with the qhy8l it needs an exposure of several seconds. I bought a usb-powered flat panel a while back but it's not quite so easy to use though it's much more portable and easier to position. It has a variable touch setting but still needs a sheet of white A4 in front of it. I do my flats with the screen close to the end of the scope and after dark to make sure the light only comes from the screen/panel. The right exposure is when the histogram peak is 20-30% from the left.

Hth

Louise

[ollypenrice]

I find that the dimmer control on our USB panel doesn't give a consistent brightness. I use it at full power and dim it with paper. It may just be the model we have here, of course.

Olly

 

 

[ultranova]

13 hours ago, Galaxyfaraway said:

Thanks very much Paul.

The link I provided to download the fits files from is just my google drive/doc; it shouldn't be 'dangerous'.

The HA flats were about 7 seconds I think and the clear flats were about 0.04 seconds or so.

In any case if it works with Jpegs then that's fine. Thanks for the testing of the flats. I guess using white laptop screen is not a good way of taking flats (with a tshirt over the scope). I should try pointing at the sky or get a dedicated flatbox I guess would be the best option. I am not quite certain whether I am supposed to be looking at achieving higher percentage (of what?) across the images as per CCD inspector or whether that percentage distribution should be more even? I can certainly do the stretching of the flats to work out where the corners end up being uneven. If it's tilt, how do I correct it? I worry it's been brought about or made worse after installing the lakeside focuser. I notice that it is harder to tighten the screw under the focuser as it now also supports the lakeside bracket as well and I can't get to it with pliers or similar after fitting the refractor onto the mount. I will send a photo once I am back home. A couple of clear nights this week and it's frustrating to have to be away from the scope and not being able to image with it!

Your welcome, I would not concern your self to much over the flats at the moment

A computer screen should be ok, for testing  your on axis, maybe you could set it up indoors

in the warm,

what we are using the flats for is to try and determine how your camera is sitting with

your optical axis, if your set up is  like my little WO  71 zenith star your R/F is held in place with two screws

which push against a brass type band that holds the R/F in place there's quite a bit of weight hanging off the r/f

when you put the filter wheel and camera on, but hopefully you have a screw on the under side which you can

loosen or tighten by taking flats and using the combination of the three screws you hopefully should get

your camera orthogonal with your light path.

with your camera set up pointing at your computer screen take a picture stretch it in a art package

so you get a good definition of the corners of your camera, they should be darker in the corners

but even a bit like the last pic I posted, if its not something is out of kilter, try and work out what corner

is out then make an adjustment with the hopefully two finger screws and screw driver screw underneath,

make small adjustment take photo stretch keep redoing until you get it as best you can , make sure you tighten things up tight when happy

just don't over do the screw driver one as you can crank this to tight, just tighten the finger ones tight. 

Triple check you back distance is as good as it can be, as your optical path is slightly lengthened by your filters from

your 55mm standard for your type of R/F you could probably set it for 56-57mm, definitely not less than 55mm

I am assuming that it is 55mm,

once you have it all set up as good as it can be keep the kit on the back of the scope rather than breaking it down

every time,.

Make sure your P A is ok, , try shorter subs  and see if the problem still persist,

worst case disengage the auto focuser, and once you have focus nip it up,

if after nipping it up it has moved then you know it is a problem with your focuser,

after focus if I nip mine up it does move a little but mine is not a top notch focuser,

do one thing at a time , take your time and systematically work your way through it,

and hopefully all will be revealed,

hope you sort your problem out

Paul

 

 

 

[Galaxyfaraway]

Hi again, I did some more tests with flats last night and it seems that it's very easy to make it look as if the corners are not even (after stretching the flat) if the laptop screen is not shining 100% at the correct angle (i.e. if it's slightly to the rights, then the middle or the brightest part shifts a little to one side). I don't think it is possible to get a 100% evenly illuminated field without a special flatbox and hence determine whether something is off axis with any degree of certainty.

Is there any other way to determine whether the axis is off? (by rotating on an axis while taking images for example?)

I have tightened everything up and will try again tonight.

[ollypenrice]

Well, you could insert a high quality laser collimator into the EP holder an put a piece of translucent paper or plastic over the objective with the centre point marked. You would need to check the orthogonality of the laser by rotating it in the EP holder. If it described a small circle on the card during this test then I dare say you could take the mid point of that circle as the true axis of the focuser. (Thanks to Steve FLO for laser-and-card idea.)

It is also possible, though, that a lens cell might be miscollimated. This might be testable by putting a screen in the back of the EP holder and focusing (briefly!) on a light source like the sun - though this suggestion is a nervous one. Maybe the full moon would be a better idea. Likewise have a care with any laser, especially with children around. (I know you know this but I say it anyway!)

Olly

[Galaxyfaraway]

I think I might send it back again and let WO test it; it's still under warranty. I am a bit concerned working with sun & lasers.

[Galaxyfaraway]

This image (from last night) has pretty obvious (horizontal) oval shapes in the top right and bottom right corners in the stars while the left hand side, the oval shape is more vertical. Definitely not spacing issue or field rotation?

I will get in touch with William Optics once again  I wonder if this scope is not optimised for photography as well as some of the other scopes?

[ultranova]

On ‎29‎/‎02‎/‎2016 at 08:33, ollypenrice said:

Well, you could insert a high quality laser collimator into the EP holder an put a piece of translucent paper or plastic over the objective with the centre point marked. You would need to check the orthogonality of the laser by rotating it in the EP holder. If it described a small circle on the card during this test then I dare say you could take the mid point of that circle as the true axis of the focuser. (Thanks to Steve FLO for laser-and-card idea.)

It is also possible, though, that a lens cell might be miscollimated. This might be testable by putting a screen in the back of the EP holder and focusing (briefly!) on a light source like the sun - though this suggestion is a nervous one. Maybe the full moon would be a better idea. Likewise have a care with any laser, especially with children around. (I know you know this but I say it anyway!)

Olly

Hi This is good advice by Olly, at least you will get a visual clue as to what is going on

Your top right hand corner is very bad, which still leaves me to believe that some thing

is not in line with your light cone.

your scope is more than up to the job of producing good results.

Did you try what I suggested, taking flats and using the screw underneath to try and

get every thing centred, that's if you do have the screw.  

[Galaxyfaraway]

Yes, I tightened everything and cannot detect tilt anywhere. The focuser doesn't move, maybe it's something in the train (spacer or the clamp, that's holding the reducer, using two thumb screws). I should perhaps try taking some photos without the reducer and try and attach the camera directly to the scope, if I have the right adapters. (to reduce and relief the imaging train of weight and see if it makes a difference. This might also tell me if the problem is focuser or the rest of the imaging train). 

William Optics sent me a message, indicating also that it might be tilt. I notice that the star shapes seem fine when scope is pointed at zenith.

Thy included a drawing (see below): the two screws pressing on the focuser are already quite tight (any tighter, and the focuser won't move in or out anymore). I don't see any other way how to manipulate or change tilt of the focuser.

Shame missing out on clear nights doing troubleshooting... Perhaps I should look at the sun option suggested by Olly, though quite terrified of this. Do I use some sort of filter? Also won't it burn something inside the scope?

[Galaxyfaraway]

Had a closer look at my imaging train this morning. It seems that the focuser seems to be pulled out by more than 3/4 (when in focus), so it might be possible that this would induce the tilt because the focuser is probably not going to be held by the second screw (see picture; the top two silver screws are supposed to apply pressure on focuser to hold it in place). I am going to try to use an extender instead and push the focuser in (like in picture 2) and see if it makes a difference. The trouble is that it introduces another set of thumb screws to hold the extender which might re-introduce tilt 

Are there no telescopes that don't have these thumb screws/clamping at the back, but instead threads to screw the components in? These clamps seem completely unsuitable for imaging as they can also rotate the camera accidentally during tracking.

But then the trouble with threads is that sometimes these things are impossible to unscrew again (especially if you forget to apply WD40 or similar). I am having real trouble unscrewing one of the adapters on the Atik EFW2.

[Davey-T]

Who said astro imaging was easy  so many things that all have to work in harmony.
The W0 Star 71 has screw on camera etc, I lubricate all my threads with silicone grease, I've found that all "T" threads are not equal some adapters are really tight, I've got variuos ones that won't actually screw together.

Dave

[zicklurky]

I had to tighten everything up on my Star 71, when it arrived my focuser was drooping with the weight of the CCD. The tube also screws apart, and this bit needed tightening as well. At first I was getting spiky and odd shape stars, but once everything was tightened they were corrected.

[Galaxyfaraway]

How far does your focuser come out, to be in focus? On the GT81, mine has to come out by about 3/4 (or 68.5mm) and I suspect it will have more tilt if it's out too much.

Incidentally, in my quest of tightening everything, this is what happened when I tried tightening the thumbscrews on the extender with pliers 

The screw top broke off (bottom of the picture) and now I can't get the screw out and reducer is now stuck there forever... Any suggestions how to get the broken screw out? The guys on various youtube tutorials don't look anything like me and have a lot of tools I have never heard of...But I need to get the screw out! Help!!

[swag72]

If your stars are all round when pointing at zenith, then there is indeed something sagging in the focuser somewhere.......

[Davey-T]

If you're lucky it may be aluminium, if you're unlucky it may be stainless steel.
Take out the unbroken one and get a drill that fits loosely in the thread.
If I was doing it I'd make a jig to hold the drill central though if it's aluminium you may be able to do it freehand,measure the depth of the other hole and mark the drill with tape and drill away and pray
Hopefully the reminding thread can be wiggled out with a small bradawl.

Good luck

Dave

 

[Galaxyfaraway]

I don't think it's steel to be honest, otherwise it wouldn't have broken so easily. I am going to try and drill into the broken screw and then use one of these to get it out: http://www.amazon.co.uk/gp/product/B003JTDJSY?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s01 No idea if this will work.

I will probably break the reducer as well now...

[Davey-T]

Don't think there's a small enough extractor in that set.

Try and centre punch it to get started in the right place, probably a good idea to start with a smaller drill in case you're off centre.

Dave

[SilverAstro]

9 hours ago, Galaxyfaraway said:

The screw top broke off (bottom of the picture) and now I can't get the screw out and reducer is now stuck there forever... Any suggestions how to get the broken screw out?

Argh! When you remove(ed) the other 2 thumb-screws from the other side did it not relieve the tension on the inner tube such that the broken one becomes manageable - with a little jeweler/watchmaker screwdriver as a drift, inserted tangentially and a bit of an incline to give it purchase,,,  little taps (with a glass hammer !) ?

However, it may be that the offending screw did not bottom on the inner tube but may have become bound in the (imperfect?) tapped hole (which would tie in with its inability to rectify the orig problem ?) in which case relieving of inner objects is not sufficient. Then we are down to differential materials and their expansion coefficients. Try warming and then the above or cooling and then the above ?

Sorry, just a few random thoughts in case ! (PS I've recovered a few situations that way, including a cylinder head stud in a 1947 side-valve engine, but substitute large screwdriver ) Good luck thoughts etc. ,,

 

 

 

[Galaxyfaraway]

There are only two screws that hold the inner tube and for some reason, when I release the other screw, the reducer still won't come out

Anyway, I will try all I can before smashing the whole thing with a hammer. I am waiting for a bench vice and some protective glasses first (mainly, just to give out the impression like I know what I am doing. As well as a first aid kit.

 

[Galaxyfaraway]

Update:

Have been taking some images and didn't do enough testing. I wanted to ask: is there a way to definitely exclude field rotation as an issue? It seems when I align 3 channels (either RGB or HSO etc), the stars on either sides of the images don't really align properly and I think someone mentioned that if this is the case, it is likely to be field rotation...Since I am using an offaxis guider, it seems plausible that the rotation wouldn't need to be symmetrical in all the corners, if I am guiding on a star that is quite a bit off-center??

I have also taken images of 5 secs exposures only (pointing east, west and zenith) and in all 3 instances, the stars appear to be fine. I am mystified by all this. However there is definitely something still wrong with the stars in the longer exposures...William Optics said there might be too much weight arising from camera, filterwheel, reducer etc. But aren't these scopes supposed to be designed to withstand this? I have always been worried about mount not being able to take all the weight and it never occurred to me that the focuser may also be  a source of weight problems.

Some recent images I took. Not enough integration time yet and processing is terrible but I wanted to get something to keep me going...

Update:

Have been taking some images and didn't do enough testing. I wanted to ask: is there a way to exclude field rotation? It seems when I align 3 channels (either RGB or HSO etc), the stars on either sides of the image don't really align properly and I think someone mentioned that if this is the case, it is likely to be field rotation...

I have also taken images of 5 secs exposures only (pointing east, west and zenith) and in all 3 instances, the stars appear to be fine... So I am still mystified by all this. However there is definitely something still wrong with the stars in the long exposures...William Optics said there might be too much weight arising from camera, filterwheel, reducer etc, pushing on the tube. But aren't these scopes supposed to be designed to withstand this? I have always been worried about mount not being able to take all the weight and it never occurred to me that the focuser may also be a source of weight problems.

Some recent images I took. Not enough integration time yet and made a pig's breakfast out of processing but I wanted to get something to keep me going...

[Galaxyfaraway]

A couple more

[Galaxyfaraway]

[Galaxyfaraway]

Hi again,

I took a few 5 secs exposures (slightly defocused) of stars (pointing east, west and zenith, in that orderwest 5 secs.tifwest 5 secs.tif). It seems all three images are not great in terms of star shapes however I don't see such bad elongations as on longer exposure images. Any clue as to it is spacing, guiding/polar alignment or tilt?

[ultranova]

HI, after looking at your pictures of your set up ( please tell me if I am wrong)

as I am only guessing some of the dimensions here and what make the filter wheel is

If your  R/F is the same as mine then it will have the standard 55 - 56mm back focus

As I am looking at the photos going by what I can see is a coupling ring to connect your F/R to your

OAG then OAG to you filter wheel then your camera.

Guess work so could be wrong, coupling ring 7 mm / OAG 25 mm / filter wheel Atik efw2 21mm

then your 13mm back focus for the camera = 66 mm  where as you should have 56/57 mm maybe

58 at a stretch and that's allowing 25 mm for the OAG,

I could be looking at this wrong, please let me know if that is the case

The 56-57mm is from the back of the  R/F

hope this helps Paul

Also that would explain the strange star shapes, you might still have a bit of tilt

But I am thinking this could be your main  culprit if my sizes are some where near