My Camera Won't Come to Focus - What to do?

Discussion in 'Tips Tricks and Tutorials' started by Doug, Oct 3, 2014.

  1. Doug

    Doug Staff Member

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    This is a common problem especially for fast telescopes with short focal length. There isn't enough "back focus" after the eyepiece holder.

    You can find the focal plane of your telescope quite easily. Remove the camera or eyepiece and accessories from the focuser, wind the focuser all the way in, then point at the moon. Hold a piece of paper behind the focuser and move it until the moon appears in focus. This is your focal plane.

    If you measure the distance to the piece of paper from the focuser, that is your "back focus".

    The sensor is a certain distance behind the front surface of your camera. You can find this information either in the manual or at http://www.sbig.com/support/application-notes-archive/. This has to be less smaller than your back focus distance or you will not reach focus.

    You also have to account for the thickness of your accessories, such as filter wheel, off-axis guider, and adapters. If the distance is too large, then you may have make some adjustments or compromises. Perhaps you can eliminate the off-axis guider by going to a piggyback guide scope or optimizing your polar alignment and PEC so you don't have to guide.

    If you have a field flattener, try removing it. Focal reducers tend to greatly reduce back focus. On the other hand, barlow lenses tend to increase it.

    You should also talk to the manufacturer or distributor of your telescope; they may have some suggestions for you. In some cases they may have accessories or suggestions for simple modifications that may help.
     
    artem and jerryyyyy like this.
  2. jerryyyyy

    jerryyyyy Cyanogen Customer

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    Hi,

    My first post to get things rolling. I have an SBIG STT 8300M attached to a Takahashi 180 ED. The critical focus thickness for this scope is 17 micros.. formula is F*F*2.2... so with a fast scope F/2.8 focus is critical.
     
  3. Doug

    Doug Staff Member

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    I have a Takahashi FSQ... fairly fast, but it's really temperature sensitive. I have to periodically refocus when I'm using it, sometimes every 30 minutes. I use the autofocus feature; seems to work for me. How do you focus your 180 ED?
     
  4. THouse

    THouse Cyanogen Customer

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    This should be CFZ 'Critical focus zone' just so we are all on the same page.

    Tom
     
  5. Mauro NArduzzi

    Mauro NArduzzi Standard User

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    I think it could be nice to make a list of the various optical backfocus of each SBIG CCD camera with every combination possibile (for instance SBIG STT + self-guiding FW withor without threaded adapter plate). This will save much times for every (new) SBIG customer!
     
  6. GlennS

    GlennS Cyanogen Customer

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    That information is already in the product manuals.
     
  7. Mauro NArduzzi

    Mauro NArduzzi Standard User

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    Unfortunately there are mechanical distances but not the optical backfocus that takes into account the cover glass (0.76 mm for KAF-8300) and the sapphire window (1 mm thick for STT). Moreover there is no word about the thickness of the threaded adapter plate (3.76 mm when using the 2.156" adapter plate of the STT ). In any case I meant to make a list such as the one of the Takahashi instruments (e.g. http://www.astropixel.it/takahashi-backfocus-distances/). This info could be very useful when a customer thinks to buy a SBIG product but also for customers that needs to make an adapter. Just a suggestion :)
     
  8. Doug

    Doug Staff Member

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    We actually do plan to put together better documentation on that front, but in the meantime just drop Tim Puckett a line. He should be able to help you with that.
     
  9. jerryyyyy

    jerryyyyy Cyanogen Customer

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    I wish I could autofocus. When I attached my Tak 180 ED to the SBIG STT 8300 there was not enough room to attach my Robofocus equipment... so I get to do this every night manually with a mask etc. It is always off then next day.

    I have to get a third party focuser to do this correctly.
     
  10. jerryyyyy

    jerryyyyy Cyanogen Customer

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    I had to get a custom adapter from Precise Parts.... circa $150. I had 1mm of space to play with. They did a great job.
     
  11. jerryyyyy

    jerryyyyy Cyanogen Customer

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    For the record I use Bahtinov grabber software to quantify the focus for my mask. The mask is laser cut....
     
    Last edited: Oct 15, 2014
  12. Tim Puckett

    Tim Puckett Staff Member

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    I owned one of the older TAK-180's. The newer 180's have a more robust focuser.
    Not sure what year model you have.

    For reference. If you need support I take calls days and evenings by phone for SBIG customers.
    You can reach me via Skype at: puckettccd

    Tim Puckett
    Marketing Director
    SBIG Astronomical Instruments
    59 Grenfell Crescent, Unit B
    Ottawa, Ontario,
    Canada, K2G 0G3
    Phone: 404-281-5686
    Fax: 613-225-9688
    SKYPE: puckettccd
     
  13. jerryyyyy

    jerryyyyy Cyanogen Customer

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    Thanks for the contact Tim. I appreciate the conversation over SKYPE. You certainly known a lot about the angles on SBIG products.
     
  14. Ethan Broadaway

    Ethan Broadaway Standard User

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    Not sure if this is suited for Doug or Tim but I am suffering the same back focus issue with an f/4 10" SN since adding the OAG to my STF-8300M. I was made aware of the backfocus table yesterday and and while my heart initially sank as I appear to need about .85" when accounting for the image train including the OAG a call to JMI has a replacement focuser enroute that should provide a lower profile on the order of ~.75" or so. That said I am still shy of the remaining ~.10 of an inch. From the back focus table the difference in width between the FW+OAG and the self-guiding FW option for the STT is about 5mm. I am reading that the SGFW is only STT--any way around that? The AO option reads that is can reduce back focus requirements as well but does not work with the STF either. Is there a way to convert my STF to an STT? Are there any other means of shaving a few mm off of the imaging train I am not considering? Any focal reducers I should consider? Any guidance would be greatly appreciated.

    Much Thanks,
    Ethan
     
  15. Doug

    Doug Staff Member

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    Newtonian reflectors, especially fast ones, are often a problem for back focus. If there's any way to move the primary mirror forward in the tube, you will gain some back focus. In some telescopes you can simply drill three holes and move the mounting screws for the primary cell.

    Converting the camera is not financially practical. You would be better off selling your current camera and putting the proceeds towards a new one.

    Focal reducers will make matters worse, not better. A Barlow lens would extend the focus position outwards.
     
  16. Ethan Broadaway

    Ethan Broadaway Standard User

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    Much thanks for the follow up! Liking the primary mirror shift option the best. I'm tapping the tube already to accommodate the new focuser and a few new holes should be no problem. That said I've got to pull out my thinking cap on this one--the trick being to determine how much forward the secondary will permit before the edges of the return cone are spilling over the sides (if that at all made sense). Additionally, is it as simple as a .5" mirror shift forward equates to a .5"focal plane extension or does the schmidt-corrector and/or secondary throw some sort of curve in there?
     
  17. Doug

    Doug Staff Member

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    Yes it will be 1:1.
     
  18. Ethan Broadaway

    Ethan Broadaway Standard User

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    Awesome news and good to know! I have decided to trim some weight off of the OTA along the way as the mirror cell fits over the lip of the tube and will require the tube to be trimmed rather than just adding new holes--so since that must happen I'm going to reduce that rolled steel tube to a custom truss assembly and to make sure I get those lengths correct, I will play around with the set-up on a rail first. If that sounded like a long project I thought the same to myself so I will be acquiring a 10" f/8 RC to image with in the interim--as there should be adequate back-focus on the slower system to accommodate the 8300 w/ OAG. The trick will be how much distance the temperature compensating focuser eats up...but when in astro-imaging does it ever work out as planned? : )

    That said I will keep you posted as my efforts progress--I cannot thank you enough for the insight! Viva SBIG!
     
  19. Thabet Al Qaissieh

    Thabet Al Qaissieh Standard User

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    Hi!
    I have the Meade LX850 16" and the SBIG8300 with filter wheel. I always have problems capturing images of objects, whether it is the moon, or deep sky objects. I have tried different functions, filters, even smaller telescopes, with no avail! Any tips?
     
  20. Tony Lazar

    Tony Lazar Standard User

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    Can anyone point me to where I can get a short SCT 2" to STL 2.156" adjustable adapter? I have the T-thread version, which works great with my ST-4000XCM and AO8, but now I have to attach a FW8G-STT to an F/6.3 focal reducer behind a C-11. PreciseParts? ScopeStuff? This should be simple, but I can't find it.
     

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