Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Light leak or something else?


Recommended Posts

I've seen an interesting issue with my first few captures; the dark frames and light frames I took with the camera mounted on the scope featured this odd glow towards the middle/left of the frame. Conversely if I take a dark frame with a cap after the EFW with the camera on my desk, I get a perfect dark frame with the ASI183's usual amp glow. Is this likely to be a light leak, or something else altogether?

Skywatcher 200PDS, ASI183MM-PRO, Mini ZWO EFW, all clamped up in the standard Skywatcher focuser.

rgb-denoised.jpg

M33_300sec_1x1_Dark_frame3.jpg

M33_120sec_1x1_Lum_frame8_c_r.jpg

dark-300s-nobias.jpg

Edited by discardedastro
Link to comment
Share on other sites

It looks like an internal reflection coming from the bright star that's on the very right-hand edge of the image.

Tracking down where the light is reflecting from could be tricky....it could be the inside of the OTA, the focuser draw-tube or any extension tubes. It could even be reflections between the camera sensor, cover glass or filters.

 

images where you have a bright star in the frame can be tricky.

 

Edited by Zakalwe
Link to comment
Share on other sites

I concur. Filters edges are also a consideration - have been instances of unmounted filters creating reflections if the edges aren't darkened, not sure if it is possible for mounted filters, suppose it depends on the mounting material.

Link to comment
Share on other sites

I should clarify though that the dark frame (second image in the post) was taken _with the telescope cap on_ - the really bright star on the right, do you mean in the last frame? If so, that's a dark frame taken with the lens cap on the EFW, rather than the telescope cap - I repeated dark frame capture with the camera cooled on the bench after I saw the first dark frames and the lighting issue, and the bright star in that frame is just the amp glow the ASI183 tends to suffer from. So I don't think it's a bright star causing issues - my thinking has mostly been local pollution, lights from near the scope etc.

The Baader filters have a black mounting disc and I can't immediately see anyone reporting similar issues with them. I've got some flocking material I haven't yet fitted to the tube, so I might give that a shot - slightly terrified of removing the primary mirror cell to get at it all but hopefully it'll all go back together without too much issue! My suspicion is mostly that there might be light leaking in around the focuser's drawtube (on the outside) that's then reflecting off the secondary.

  • Like 1
Link to comment
Share on other sites

The second image is a dark frame with the lens cap on the scope, so it can’t be anything to do with what is being imaged in the light frame eg bright star at the edge of the FOV.

I’ll start the betting and say it’s light coming in from behind the primary, maybe your computer screen or a light. 

  • Like 1
Link to comment
Share on other sites

You have the usual ASI183 "starburst" amp-glow on the right hand side.  This will calibrate out.  The big oval shaped bright patch in the upper centre is probably some kind of light leak.  Does it happen if you take darks with the lens cap on in a totally darkened room?  If not then it's definitely a light leak. You will just have to experiment putting a black cloth over possible entry points until you narrow it down.

Mark

 

  • Like 1
Link to comment
Share on other sites

Can't really add much to what has been said, except you say you did the dark away from the scope with a cap on the EFW, so it's not light leak from the secondary.

I must admit I do tend to do my darks when it is dark to avoid any chance of light leak.   I used to get light leak with a manual filterwheel, but yours is EFW so should be enclosed and leak proof.   But my primary leaks light so I do them in the dark. 

Carole 

Edited by carastro
Link to comment
Share on other sites

Right - thanks for all the tips. The scope is set up in my garden but still close enough to the house there are some potential sources of light.

I've got the scope off the mount while I fit a focusing motor to the focuser (which has turned out to be Exciting, the Skywatcher focus motor mount does fit, but blocks up the parallelising adjustment screws, so had to expand some holes in that plate with some files and a drill), so I'll go around with a torch on the bench and see where there's potential for leakage.

I'm guessing something as simple as a blackout cloth over the back of the primary would be enough to stop any leakage there, though I might be brave enough to pop the mirror cell off and flock around the interior near the primary.

The focuser's the other potential entry point - I've now applied flocking around that area to try and minimise the impact of any leakage, but might also paint the (non-contact) external surfaces of the drawtube with some matte black enamel I've got lying around to mitigate bounced light there. Everything upstream of the focuser is sealed and I can take perfect dark frames with the lens cap on the EFW in a brightly lit room, so that lot's OK!

Link to comment
Share on other sites

I've flocked the inside of my tube now - took out the primary and very carefully cleaned it while I was at it. Once I had it all back together on the bench I assembled the camera and took some 60 second and 5 minute exposures.

With the scope in a bright room there were similar-looking halos in the frames. Covering the focuser made little difference, but covering the rear of the scope by the primary with a dark coat resulted in perfect frames showing very no sign of any light. So it looks to me like I need to find a way to properly cover the rear of the tube to minimise the risk of light entering the tube and causing these problems again - either I can find an appropriate plate pre-made or get a friend to CNC some sheet aluminium. Thanks all for the help!

Link to comment
Share on other sites

I had the same problem with my 150pds and like you traced it to light leak around the primary.  My solution was to use a bungy loop to hold a piece of black polythene over the back of the scope, this can easily be taken on/off for collimation and star tests etc.

Martin

  • Like 2
Link to comment
Share on other sites

I find I can't do darks on the scope at all and that's that. I have to do them with the camera out of the system and the metal screw-on chip cover fitted. And this is on refractors with fully sealed electric filterwheels. How does the light get in? Not a clue - but it does. I compared 30 minute darks done on the scope and off. They were certainly very different from each other.

There do seem to be a lot of threads discussing reflections in CMOS cameras.

Olly

Link to comment
Share on other sites

I haven't yet put the camera on and taken any dark frames, but thought I'd share my mod to the scope - I had RazorLabs laser cut a circle of 5mm acrylic with slightly oversized holes (not quite oversized enough if anything) for the collimation knobs, and tight holes for the locking screws. This lets the locking screws (which don't do much if anything anyway for the mirror stability as far as I can tell) hold the plate in place securely while adding nearly no length to the tube. Total cost was about £25. I drew it up in Fusion360, exported to DXF, cleaned up in Inkscape and shipped it off. I've attached the SVG if anyone wants it - it's to RazorLabs' guidelines for cutting and fits a 200mm scope, other sizes will need adjustment.

The plate completely covers the whole end of the scope with no gaps, so should work pretty well at keeping the light out. It won't help tube cool-down particularly, but I'm mostly keeping the tube outside at the moment under a Telegizmos cover so that's less a concern. I did wonder about putting some slots and mounting holes for a fan in, but figured I'd want a slightly more complex mounting arrangement to stop light leaking in through the fan itself (some indirect paths and offsets, which would involve a couple of bits of acrylic and some antireflective paint) so left it for this iteration.

Once I get a bit more time I'll get it back indoors or drag the camera out in the daytime and see if I can take dark frames that are as good as the ones with the cap on the camera!

signal-attachment-2019-01-21-161405.jpeg

signal-attachment-2019-01-21-161322.jpeg

tubecap.svg

Link to comment
Share on other sites

8 hours ago, sharkmelley said:

Nice mod!  But I'm worried you keep the tube outside in the UK.  Don't you worry about the effects of humidity on the optics?

Mark

So far it's been OK - I've kept it out for a full year now more or less. I cap it all off at the end of the night and use FLO's desiccant 2" cap to help reduce the moisture levels in the tube. I've noticed no issues thus far. I had the mirror out as part of modifying the tube to add flocking and it looked pretty good - gave it a fingertip/water soak and distilled water rinse and it looks good as new. I'm more worried about the mount than the tube, really, but that also seems OK so far. The Telegizmos cover is pretty decent. I've got space for a RORO so that's planned when I've saved up enough for it!

Link to comment
Share on other sites

Well, thanks for all the advice - this dark frame I took on the scope! Not a shred of light leakage visible to my eyes, so I think that's done the trick (by and large). All my lights from the night appear to be free of any light leak style artefacts, too, so the flocking appears to have helped too. I ran without the dew shield tonight to give myself the worst odds, too.

integration.thumb.jpg.fd33315e2e022cce670d9a3c7950c0e4.jpg

One 60s luminance exposure, M33:

M33_60sec_1x1_Lum_frame9.thumb.jpg.6960a9c1833b5d255fca8208c1c118ab.jpg

Link to comment
Share on other sites

  • 1 year later...
5 hours ago, ashortfallofgravitas said:

Sorry for bumping this - I've got a 200PDS and just tracked down an identical issue.  @discardedastro how easy was it to fit the blocking disk?  I'm looking at cutting your design (thanks!) myself, just a little worried about removing the spring loaded adjustment screws

The little screws are totally safe to remove - they don't really do anything but allow you to put tension against the big screws, and in practice they're irrelevant; I've seen no movement between sessions of the mirror other than the usual tiny amount of mirror flop/sag you'd expect in a 3-point mirror mount. The disk is trivial to fit using those little screws - all of 30 seconds to drop it into place and do the screws up.

It's continued to work flawlessly as a mod for the last year, by way of a progress report.

Link to comment
Share on other sites

4 minutes ago, discardedastro said:

The little screws are totally safe to remove - they don't really do anything but allow you to put tension against the big screws, and in practice they're irrelevant; I've seen no movement between sessions of the mirror other than the usual tiny amount of mirror flop/sag you'd expect in a 3-point mirror mount. The disk is trivial to fit using those little screws - all of 30 seconds to drop it into place and do the screws up.

It's continued to work flawlessly as a mod for the last year, by way of a progress report.

Thanks! I need to have a closer look at the CAD file - I take it the plate drops over the large adjustment bolts, instead of having to remove them?

Link to comment
Share on other sites

5 hours ago, ashortfallofgravitas said:

Thanks! I need to have a closer look at the CAD file - I take it the plate drops over the large adjustment bolts, instead of having to remove them?

That's right, so collimation is unaffected. I might make them a little wider around the large bolts - they can be rotated but it's a bit tricky to get a grip on them. Another 1mm around the edge would make a big difference.

Link to comment
Share on other sites

  • 1 month later...
On 11/01/2019 at 08:47, discardedastro said:

I've seen an interesting issue with my first few captures; the dark frames and light frames I took with the camera mounted on the scope featured this odd glow towards the middle/left of the frame. Conversely if I take a dark frame with a cap after the EFW with the camera on my desk, I get a perfect dark frame with the ASI183's usual amp glow. Is this likely to be a light leak, or something else altogether?

Skywatcher 200PDS, ASI183MM-PRO, Mini ZWO EFW, all clamped up in the standard Skywatcher focuser.

rgb-denoised.jpg

M33_300sec_1x1_Dark_frame3.jpg

M33_120sec_1x1_Lum_frame8_c_r.jpg

dark-300s-nobias.jpg

I saw something on you tube recently which has a similar patter on darks, and was being attributed to amp glow on a ZWO ASI model.

I can't remember which one tho'

Link to comment
Share on other sites

23 minutes ago, iapa said:

I saw something on you tube recently which has a similar patter on darks, and was being attributed to amp glow on a ZWO ASI model.

The glow on the right is a very well characterised pattern on the ASI183 models.

Link to comment
Share on other sites

  • 2 months later...
  • 10 months later...

Hello, just to reawaken this, I spent 4 hours today really trying to make the inside of my scope dark in the daytime.  My observing location is quite light bright  With an f4 8 inch Newt Astrograph my darks taken no the scope have some reflected light gradients and spikes and my 240s subs mostly have weird spectral rings and gradients that just cannot come from sky conditions. 

OK, the cover doesn't totally block the light, but I'm not so worried about that, as I want light frames to be all light from the sky and I can do darks with that covered in material while parked or on the bench. 

But on the bench in the daytime, with the front covered, it's like a theme park in there, when the fireworks are going on! 

So I started tracing the issues at the mirror end, adding self adhesive velcro strip between the mirror frame and the mirror holder frame (a 1.5cm wide strip does the trick).  But the areas around the adjustment bolts let in light so they needed more velcro. gaffer tape, nothing would stop the little holes, and I wasn't willing to disassemble my nicely collimated F4!  I also used a smartphone camera light to shine at the mirror. 

Then I remembered the black bag that a motorbike helmet came in, and I found with it a smaller bag for the visor.  Putting both on the end, with them already elasticated and black, bingo! 240 second dark frame. no light.....from that end...l

I moved the covers from around the camera and focuser......and Oh No - more light!  I read up on focussers and how not to worry about the small amount of light they let in through the gap to allow the tube to go up and down.  But, the dark shots (now that he mirror end was sorted) showed me the culprit for the weird circular refractions...yep focuser leakage onto the secondary and onto the primary and back to the camera.....What to do?....Baffles Baffles...hmmm....oh flock it!? a special adhesive skirt around the focuser tube that could ride up and down with the focus? 

Er - queue brainwave!!!.  I found an old black sock, cut the foot part off, and yippee, the leg part of the sock covers the base of the focuser right up to the camera ring.  And this sock still had good elastic to hold it securely.   And another test....very very small amounts of light....but most acceptable for a budget solution! 

I really could see the gradients and rainbow patterns on my lights, and even after applying flats, darks etc....so I would suggest the loss of a sock and an old black bag, a good investment!  Astronomy is sooo romantic.

  • Like 1
Link to comment
Share on other sites

  • 1 month later...
On 29/05/2021 at 21:02, MaltaNewtF4 said:

I moved the covers from around the camera and focuser......and Oh No - more light!  I read up on focussers and how not to worry about the small amount of light they let in through the gap to allow the tube to go up and down.  But, the dark shots (now that he mirror end was sorted) showed me the culprit for the weird circular refractions...yep focuser leakage onto the secondary and onto the primary and back to the camera.....What to do?....Baffles Baffles...hmmm....oh flock it!? a special adhesive skirt around the focuser tube that could ride up and down with the focus? 

Er - queue brainwave!!!.  I found an old black sock, cut the foot part off, and yippee, the leg part of the sock covers the base of the focuser right up to the camera ring.  And this sock still had good elastic to hold it securely.   And another test....very very small amounts of light....but most acceptable for a budget solution! 

I really could see the gradients and rainbow patterns on my lights, and even after applying flats, darks etc....so I would suggest the loss of a sock and an old black bag, a good investment!  Astronomy is sooo romantic.

 

 

I did an overhaul of my focuser this week, added stepper motor etc.. and i did a similar trick with an old rug + a 3d printed part.

 

IMG_0779.thumb.jpg.012ef0c0c2a1823897946a2033594b62.jpg

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.