FW8G-Aluma adapter and back-focus question

Discussion in 'Aluma Series' started by Kalpesh Jasapara, May 13, 2020.

  1. Kalpesh Jasapara

    Kalpesh Jasapara Standard User

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    Hello,

    Thank you so much for all your help.

    Can you please confirm the M48x075 is the the correct adapter to connect the FW8G-Aluma.
    https://diffractionlimited.com/product/adapter-plate-m48-thread/

    I am trying to connect The FW8G-ALUMA with Celestron Edgehd 9.25 (with / without Focal Reducer).
    I have the Celestron SCT camera adapter (both M48 and T2), with the T2 adapter, however when I connect the FW seems I cannot make the connection.

    Here is my back focus calculation,
    Celestron edgehd 925 with or without focal reducer (0.7x) requires 146.05 for focus.

    Celestron 48mm adapter (Wide -T?) - 91mm --> M48x075 adapter plate --> FW8G ALUMA --> SBIG ALUMA
    I am using Astrodon LRGB E Series filters.

    Q1. Is the above connection appropriate, or did I miss something ?
    Q2. The back focus seems to be 91 + 14 + 42.26 - 1.66 (for 3mm filters) = 0.45
    Q3. Does the M48x075 add any additional back focus?

    [ I found the above numbers 14 for SBIG ALUMA and 42.66 (FW8G) from one of the links in this forum ].

    Regards,
    -Kalpesh
     
  2. mike m benjamin

    mike m benjamin Cyanogen Customer

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    Hello, I have the same setup as you, Aluma/FW8G/48mm adapter plate. All together your looking at about 57.5mm, about 2.5mm more than a standard DSLR so you need to shortened your adapter from reducer to filter wheel by this amount.
    This statement is confusing, it reads that you physically cannot thread the filter wheel with adapter plate to your Celestron T thread adapter. I am sure you can as long as its a 48mm T thread.
     
  3. Kalpesh Jasapara

    Kalpesh Jasapara Standard User

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    Hello Mike,

    Thank you for your reply.
    Sorry about the confusing message, you are right I am unable to physically connect without the adapter plate.

    Can you please share the details of the "shortened adapter" you are using?
    Is there a connector that can be used to connect the OTA (or reducer) directly to the FW8G?

    Looking for a back focus table for the Aluma.

    Regards,
    -Kalpesh
     
  4. mike m benjamin

    mike m benjamin Cyanogen Customer

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    Kalpesh,

    You must use an adapter on the filter wheel for a physical connection, otherwise it's just a big open hole. The adapters are 3.75mm thick, the Aluma and self guided filtwheel is 54mm, so 57.75mm. Your using 3mm filters, subtract a third of the filter thickness, so -1mm. I get 56.75mm used backfocus. I said I have the same camera combination as you, I don't have a Edge HD, I have a Meade SCT with a Moonlight, you simply attach your camera and focus, no specific back focus number needed. You may need a special adapter made, precise parts maybe? It would be cheaper to get it done locally though. Sbig hasn't come out with a official backfocus chart with the aluma models added. Someone else created a older thread on the matter.
     

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  5. Colin Haig

    Colin Haig Staff Member

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    From a backfocus perspective:
    The Aluma 8300 back focus from sensor to top of blue metal frame is about 14.3mm.
    FW8G-Aluma is about 42.3mm
    The refractive index of the filters is usually closer to -1mm for a 3mm filter; your number seems a bit high, but I don't have data on those particular filters here.
    The T-thread adapter plate is about 3.72mm thick, and screws on to the focuser-side of the FW8G-Aluma. Since your stuff threads in to this, your actual backfocus will vary. It depends how far it screws in.
    So I get a number closer to Mike's, estimating about 55.6 to 59.3 if your hardware went to the outside of the adapter plate instead of threading in.
     
  6. mike m benjamin

    mike m benjamin Cyanogen Customer

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    Colin, I have four of the adapter plates kicking around here, they have different coatings, some dull finish, some shiney. They range from 3.7 to 3.85mm. If he wants to be super precise, he'll have to vernier his.
     
  7. Colin Haig

    Colin Haig Staff Member

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    Mike, thanks for chiming in, much appreciated.
    We've been working with some of our suppliers on tolerances and coatings - for example, some parts are anodized, others are powder-coated and have broader tolerances.
    The tolerances add up. It's best to bolt it all together and measure to a common reference point. I tend to use the top of the blue metal case nearest the filterwheel or faceplate.
    Also, no 2 Celestron scopes seem to have the exact same image plane.
    There have also been a few refinements to the Aluma cameras over the past 2 years, such as small changes to the thermal cooling that affects the position of the sensor relative to the chamber window.
    My advice to amateurs is to actually point the scope at the moon, figure out the focal plane distance, and then test-fit the camera based on that, prior to making a final adapter or locking down a drawtube position.
     
  8. mike m benjamin

    mike m benjamin Cyanogen Customer

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    I am glad that I didn't have to get into that with the Meade ACF optics.
     
  9. mike m benjamin

    mike m benjamin Cyanogen Customer

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  10. Kalpesh Jasapara

    Kalpesh Jasapara Standard User

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    Thanks @Colin Haig that is very helpful.

    @mike m benjamin -- Thank you also for all the details, I agree with you the Variable Locking Extension will help buffer for any variations in any of the adapters, before locking down to a final adapter. I believe I have all the information I need give it a try.
     
    mike m benjamin likes this.

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