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What's Wrong With My Gobo?

Discussion in 'Lighting and Electrics' started by zackw250, Jun 10, 2005.

  1. zackw250

    zackw250 Active Member

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    Location:
    Bellingham, WA, USA
    Ok, I ordered a custom gobo, and gobo holder. I put it in my Colortran 5/50 Ellipsoidal... focus is OK, but I get this wierd light bleed around the middle. Can you help?

    [​IMG]

    Click HERE to see the full size image

    I read somewhere I need a donut, but what is it, where does it go, and what does it do? Thanks - Zack
     
  2. SuperCow

    SuperCow Active Member

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    Location:
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    Check the bulb alignment and position. I'm not sure how to do that on your fixtures, but if it's not centered, then that might explain the bleeding.
     
  3. zackw250

    zackw250 Active Member

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    I know how to adjust the alignment - but what is aligned. Even distributed light, hot spot in the middle, dark spot in the middle, what?

    I assume aligned means even light through the beam, but I dont know for sure.
     
  4. propmonkey

    propmonkey Well-Known Member

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    after awhile gobos start to burn up and spill/bleed light. how old is that gobo?
     
  5. zackw250

    zackw250 Active Member

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    brand new - just put it in.
     
  6. propmonkey

    propmonkey Well-Known Member

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    then you'll need a tophat/dounut. its is a tube with a lense that you put on the front of the instrument. not sure how it works but thats what its for.
     
  7. SuperCow

    SuperCow Active Member

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    Not necessarily a top hat (or snoot). That prevents spill light out beyond the beam. A donut is what you;re looking for It goes into the front gel frame slot, and it's basically a piece of metal the same size, with a smaller hole in the center. You need to be careful to buy one that is the right size for the degree of your fixture, so a 50? fixture obviously needs a bigger hole than a 19? one. That should clean up the image.
     
  8. propmonkey

    propmonkey Well-Known Member

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    whoops i ment snoot not dounut. but tophats help with gobo images
     
  9. ship

    ship Senior Team Emeritus Premium Member

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    Top hats and dounts have use and are often good - even on a Leko if seeming odd to clean up the stray light on the ediges. But correct me if I'm wrong but both more catch the stray light around the edges of the beam and in general sharpen the image.


    What is shown here while no doubt a dount would help, is more a question of bench focus or alignment of lamp within the reflector - them funky knobs behind the light fixture that screw around with the seat height and position of the lamp within the reflector.

    In addition to this, what lamp are you using? Given a Colortran fixture is not known to be using the most efficient of reflector or optical sources of light, are you using the at least proper and adjusted lamp in it, or can you upgrade to a better one?

    Still the hot spot should easily go away with some adjustment of lamp to it's reflector by way of playing with them little knobs at the rear of the fixture. Hot spot in the center is what it is and should if right lamp be easy to correct.

    Add the donut but funk with the lamp base seat height in getting the right bench focus for the image.
     
  10. zackw250

    zackw250 Active Member

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    So Ship - I should have a hot spot in the center of my beam if my lamp is properly aligned? I am using the HX754 lamp - is this good? It is what this fixture is speced for.
     
  11. ship

    ship Senior Team Emeritus Premium Member

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    The GLD should be a very good lamp. Should be but I have not used one yet. More condenced filament than that of a FEL it might be emulating in output.

    So given a decent lamp and a fixture that normally uses a G 9.5 lamp with a LCL of 2.3/8", any lamp matching this will function in the fixture assuming the wattage is within rating which I don't doubt.

    Given this, yes you would want a "flat field" across the beam of light. The intent is to have a beam of light that's constant with little to no drop off or bright spots across the beam. Otherwise for hot spot center with drop off you would be wanting a PC spotlight. Different intent and fixture, though you can focus a Leko to have a more hot spot to it's beam center. Is your fixture focused for a hard edge to it's beam of light?

    Flat fields are what they are for the equipment in use - not all are perfect. On a 360Q for instance, you can for the most part get a flat field but due to reflector in-efficiency, the center will often be slightly darker than the main part and edges of the beam of light. That's what the internal lamp reflector on a HPR lamp corrects for - the loss of light at the center of the beam where the lamp pops thru a hole in the center of the reflector. On the other hand, dependant upon how good that reflector is, on more later fixtures you should not notice as much this hole due to the concept of the ellipsoidal reflector, baffle and lens train.

    Pre-bench focus for this even field before installing the gobo. One detail not mentioned is that sometimes even of a S-4, you just can't get a perfectly even flat field of light. Sorry but it happens not all that infrequently that your beam of light just can't be cleaned up sufficiently. It's much based upon fixture efficiency of light by way of design and individual fixtures or problems with them. Make sure that your lamp is fully seated and centered in the reflector before bench focusing.

    Could also have some individual fixture problems. On a Lycian 1290 follow spot for instance, if that spot gets a good bump there is a chance that the ellipse of the reflector will warp and no longer provide a true ellipse. Much less in getting that bump, the fixture itself can get out of square. Once something like this happens, there is no amount of bench focus that will correct for this. Normally this out of square or ovaling of the reflector will more show up more as a double image or say instead of a hard sharp blue "hilation" all the way around the edge of the beam, you will instead have almost a double ring if not half blue half amber when you otherwise have the most optimized beam possible for bench focus. I thus doubt that this is your case but at times you might find say dots of bright spot across the flat field that are also lens/reflector problems. One good way of bench focusing I have found is to first do the bench focus but while looking at the beam of light, move it up and down on the wall. This movement of the beam will allow you to notice even slight problems with the beam more so than looking at a non-moving image that your eyes will balance out for you so you can't otherwise notice them.

    Ensure the curve of your lenses are facing each other, in their proper order of lenses given they are of different sizes and in their correct mounting position. If at some point your wash of light is not sufficient for a even beam of light after bench focus, try another fixture or two. Perhaps you have pulled initially a fixture that just is not as good as another one. If even after that you can't get a perfect beam - and this should only be a bench focus question, perhaps a different brand or type of lighting fixture - one more optically refined will be necessary.

    Anyway just some thoughts. Most of the Leko companies will have supplemental information and advice on how to best bench focus their gear. While what knobs are used to do this will be different, the same intent of what you are looking for will be in place.

    Still all and all, given the proper hard edge focus of the fixture is not producting the hot spot - which I think would also effect the rest of the image quality, it's probably a bench focus of the lamp issue.
     

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