Resolved Aluma Questions

Discussion in 'Aluma Series' started by Paul Haese, Aug 6, 2019.

  1. Paul Haese

    Paul Haese Cyanogen Customer

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    Dec 26, 2014
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    I recently took possession of an Aluma 694 with integrated guiding filter wheel. I have a few questions as these cannot be answered by looking in the manual.
    1. First up is when using the 36mm to 1.25" inserts, where does the O ring go when using Astrodon filters? Does the O ring go at the bottom of the filter wheel or the top?
    2. What is the overall back focus of the camera and guiding filter wheel integrated in millimetres? The back focus table does not have the Aluma camera listed or the filter wheel. Odd give you sell the product.
    3. What is the back focus of the adapter plate 2.156" x 24? I have this on the front of the filter wheel. Is this extra to the above back focus or not?

    I think that is about it for now.
    Paul
     
  2. Colin Haig

    Colin Haig Staff Member

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    Hi Paul,
    1. O-ring and Inserts:
    Normally O-ring first, then the insert.

    You may have to try it both ways, I'll explain why.

    There seem to be different "vintages" of AstroDon filters, with different filter cell thread lengths and inserts.

    Bottom Clearance:
    Make sure the filter threads are clear of the drive belt and the mounting screw in the small green PCB (printed circuit board).
    Test this with one filter and insert before you install everything.
    When you thread the filter into the insert, if the filter cell protrudes past the bottom of the insert, then there is a small risk that it could hit something.
    Disconnect the connections between the filter wheel and camera.
    Check this isn't happening by gently moving the filter carousel around.

    Top Clearance:
    Make sure the top of the AstroDon filter is not going to hit something above when it rotates. This can be tricky to check.
    If it does, then put the o-ring above the insert, and recheck the bottom clearance.

    If you have a micrometer, I'd appreciate it if you could give us these dimensions of the AstroDon filters you have (because I'm building a list of filters from various manufacturers to know which ones are problematic):
    Total height of the filter from the bottom of the threaded part to the highest point on the top.


    2. Back Focus:
    The 694 + FW8G-Aluma has a backfocus of 54mm, depending on the refractive index of the filters (I've assumed the filters shorten by about -0.6mm).

    3. Back focus of 2.156 x 24:
    If your draw tube or focal reducer threads inside this adapter plate, then you don't count the thickness.
    If however there is a stop flange, for example with a nose piece, then you may need to add this in plus the thickness of the nosepiece flange at its base.

    If I get a few minutes, I'll put a couple drawings together for you.
     
  3. Paul Haese

    Paul Haese Cyanogen Customer

    Joined:
    Dec 26, 2014
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    Hi Colin, sorry I have not had a chance to test things until now.

    Ok for filter thickness. From top of the filter to the bottom of the thread is 9.88mm. The actual filter holder is 7.54mm

    Clearance looks ok if I have the rubber O ring at the bottom. Everything rotates ok top and bottom. Nothing seems to hit. No contact with the belt or the screw head. Nothing appears to contact on the lid either.

    However I did find something concerning which is separate from this issue. I found the mirror on the pick off prism was detached from the piece at the back of the mirror. The adhesive circle appears not to have any adhesion. What should I do about this? The mirror was leaning forward. Do I simple re-glue this down with some loctite? Or have I got this wrong. I have a photo to show you if you need.

    Drawings of the Backfocus of the 2.156 x 24 would be good. I will need to use a thread in adapter off a corrector. One is about 1mm and the other corrector gives me about 23mm.
     
  4. Doug

    Doug Staff Member

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    That's a concern that the mirror came loose. We typically use a high quality RTV to attach optics. Photo would be appreciated.
     
  5. Paul Haese

    Paul Haese Cyanogen Customer

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    Here is a photo. In the photo the pinkish horizontal line is the edge of the mirror tilted up. This is how I found the mirror upon opening the case today. It has been sitting in my shed for 4 weeks in the carry case. It's the middle of winter so no heat. What do I need to do to solve the problem? Glue etc?
     

    Attached Files:

  6. Colin Haig

    Colin Haig Staff Member

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    If you wish to Do-it-Yourself, the adhesive used is Room Temperature Vulcanizing (RTV) Silicone. e.g. the kind that can be used to make aquariums out of sheets of glass.
    (GE Silicone II is an example that is often available at hardware stores).

    @Doug may have an alternate recommendation.

    There should be a small blob of it on the back of the mirror and there should be evidence of it on the metal tab it attaches to.
    You'll want to use a razor blade or plastic tool to remove any existing RTV Silicone from the back of the small mirror, and the the tab.
    Be careful not to damage the mirror (don't scratch it).
    When you apply the replacement silicone, use a tiny amount, as it will spread out, and you don't want it to spill over onto the mirror front surface.
    You might use nitrile gloves or a piece of paper to put a bit of finger pressure on the mirror to get it stuck down, but not leave fingerprints.

    If you're not the DIY type, we can arrange a repair.
     
  7. Paul Haese

    Paul Haese Cyanogen Customer

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    Hi Colin; there is a small flattened blob on the rear of the mirror. It is not on the metal tab though. I have just the sort of silicone here and will sort it out. Thanks for the information regarding stripping and installation.
     
  8. Colin Haig

    Colin Haig Staff Member

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    Ok, be careful. Thanks for the update. Let us know how goes.
     
  9. Paul Haese

    Paul Haese Cyanogen Customer

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    Hi Colin, I have done the repair and I think it went ok. It is a fairly simple procedure, but the tips were helpful. Mirror appears to be seated correctly and without any finger prints or silicone on the surface.
     

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