Summer Glamping Sale - 5% off Domes - Use Code DOME5

Adapter - Astrodon 1.25" Eyepiece Extender

Astrodon 1.25" eyepiece extender

  • Vendor
    Sale price
    Regular price
    $34.00 You save 18%
    Unit price



    MOAG means Manual Off-Axis Guider. It allows you to guide your telescope to obtain sharp, round stars by guiding in front of your filters using unfiltered light in the same optical beam as your imaging camera. The MonsterMOAG comes equipped with a dovetail to 2.7?-24 males threads. This works for most RC (e.g., RCOS) and Astro-Physics scopes with 2.7? extension tubes, and adapters that go from 2.7? to larger (3.5? and 4?) systems. This is where you make adjustments to bring the focal plane of the imaging camera at the specified distance from your scope, focal reducers or field correctors.

    There are dozens of different connection possibilities at the back of your scope, such as reduces, field correctors, focusers, rotators, etc. We cannot make adapters for all possibilities, so we just provide the male 2.7? adapter described above. In many cases you will need a custom adapter. We suggest you utilize They provide a “configurator” where you specify the threads on both sides of the part and the thickness of the part. It will provide a drawing and a quotation immediately. It has the MonsterMOAG dovetail specifications, so on one side you will specify the Astrodon MonsterMOAG.

    For Takahashi FSQ and TOA owners, you can add the CAA-APF (0.4?) adapter along with a Spacer27. This converts the back of the 100 mm camera-angle-rotator (CAA) from 92 mm to 2.7?-24 female threads. The MonsterMOAG then screws into this. You will also need our Spacer27, a 0.2? thick 2.7? washer/spacer.

    For those with Meade or Celestron SCTs with 3.25? visual backs, you can purchase a 3.25? female to 2.7? female adapter. This screws onto the visual back of your SCT and accepts the 2.7? male threads from the MonsterMOAG. In this way you can enjoy the benefits of off-axis guiding in the main beam without worrying about differential flexure with guide scopes or possible primary mirror movement as the scope tracks across the sky.
    You will need to balance the following distances for your optical system. These constraints are interconnected. PLEASE attempt to make these calculations prior to contacting Astrodon or any of our resellers. Of course we are available to help you finalize your choices, but making the calculations in advance will expedite the process and increase your confidence that the overall system will work for you.

    Balancing Distances
    Focal plane distance of the imaging CCD to the back of your scope, such as from field correctors or reducers. This can be adjusted with scope-side adapters or extension tubes.
    Distance from the MMOAG prism to the focal plane of the imaging CCD (distance B below)
    Distance from the center of the prism to the focal plane of the guider CCD (distance A below)
    A must equal B to reach focus for the guide camera


    Select MMOAG camera side adapter (backfocus/thickness = Ad)
    Determine B = 0.625? + CBF + Ad
    Match A to B by using the Helical Focuser, as supplied with the MMOAG or replacing it with the provided shorter 1.25? manual nose cup
    For the Helical Focuser, A = 3? + GBF, or
    For the short 1.25? cup, A = 2? + GBF

    Camera-Side Adapters from Astrodon to Determine Ad

    SBIG STL (dovetail to 2.156? male)

    MMOAG-STL-S (Ad = 0.1?), MMOAG-STL-L (0.75?)
    SBIG ST (dovetail to t-threads – 42 mm male)

    MMOAG-ST-S (0.1?), MMOAG-ST-L (0.75?)
    SBIG STX, Apogee U16M/9000, FLI Proline (dovetail to 3? male)

    MMOAG-CFW (included, 0.1?), MMOAG-CFW-A (0.2?), MMOAG-3-L (0.75?)

    Some Camera + Filter Wheel Backfocus CBF (please verify with your vendor)
    SBIG STL/FW5 1.6?
    SBIG STL/FW8 1.95?
    SBIG ST/FW 1.65?
    SBIG STXL/CFW7 1.76?
    SBIG STXL/CFW with Guider 2.44?
    STT/FWG8 2.16?
    Apogee U16M/FW-7S/9R 2.15?
    Apogee U16M/FW-10S 2.25?
    Apogee U16M/FW-7S/9R (deep cooling) 2.53?
    Apogee U16M/FW-10S (deep cooling) 2.63?
    FLI Proline/CFW 1.73?
    FLI Proline/CenterlineFW 1.98?

    Some Guide Camera Backfocus GBF
    SBIG RGH, ST-402 0.69?
    SBIG ST-i -0.5? (guider inserted into helical focuser)*
    Starlight XPress Lodestar -0.5? (guider inserted into helical focuser)*

    * This means if the ST-i or Lodestar is inserted down into the helical focuser by 0.5, the focal plane will be at the top of the helical focuser. So, decrease 3? by the insertion depth of 0.5? = 2.5? and then ADD back the 0.5? recess of the detector 2.5? + 0.5? = 3?, for example.

    An Example Calculation – Apogee U16M/7-PositionFW and SBIG ST-402 Guider

    We will use the MMOAG-CFW-A (0.2?). CBF for the Apogee system with the 10-slot FW is 2.25?. Therefore, B becomes 0.625? + 0.2? + 2.25? = 3.075?.

    With the helical focuser (3?) and ST-402, A becomes 3? + 0.7? = 3.7?. Therefore the guider focal plane will be 3.7 – 3.075? = 0.675? above the focus determined from B.

    So, we can either ADD 0.75? between the MMOAG and U16M with the MMOAG-3-L. Or, if we want to keep the distance between the MMOAG and imaging focal plane as short as possible (my choice), we can replace the helical focuser with the included short, manual nose cup. In this case, A becomes 2? + 0.7? = 2.7?. All you need to do is slide the ST-402 up 0.3? using its 1.25? nose piece and it will be close to focus. You can fine tune the focus with star images. This process takes only a few minutes and once locked down, the focus does not change. It is a little less convenient than the helical focuser, but provides this important option for configuring your system.

    Please note that camera manufactures often specify the MECHANICAL back focus of their products. 3 mm thick filters will ADD 1 mm (0.04?) to that mechanical backfocus as measured from the telescope or from the MonsterMOAG prism. The camera window, often 3 mm thick will add another 1 mm (0.04?). So, You may have to add 2 mm (0.09?) to the mechanical backfocus.
    There may be some confusion as camera manufacturers measure backfocus from the focal plane of the CCD to the outer surface of the camera. When they account for the thickness of the filters, the SUBTRACT the 0.04?, which is correct as measured from the CCD. However, most people measure backfocus from the back of their scope of from a corrector, and then add/subtract spacers to arrive at the correct backfocus. In this case, as measured from the scope, the 0.04? must be ADDED. A subtle point, but does get people in trouble from time to time.
    Also, please note that different filter wheels from the same manufacturer may have different backfocus, as indicated in the list above.

    SBIG ST-i or SX Lodestar

    These guiders are 1.25? cylinders with the focal plane recessed about 0.5? up from the end. So for the example considered above where B = 3.075?, you can use the helical focuser. So, by inserting either guider ~0.5? down into the top of the helical focuser brings the focal plane of the guider to the TOP of the helical focuser. A = 3? – 0.5? insertion distance + 0.5? internal backfocus = 3?. The +/-0.2? travel of the helical focuser allows you to reach optimum focus. Please see our Optical Layout and Scope-Side Connections pages to help configure your MMOAG with the correct adapters for your system.

    For largest CCDs (KAF16803, KAF9000)
    Small backfocus 1.23? of the body
    Total backfocus with both provided adapters is 1.45?
    Solid body to hold large camera, filter wheel, filters
    Large central opening of 2.55?
    Guide in front of your filters
    More and brighter guide stars with unfiltered light
    Guide in the main optical beam
    Avoid differential flexure of guide scope
    Fixed guider position for automated data acquisition
    Helical focuser for precision focusing of guide camera
    Shorter 1.25? manual holder for guide camera also provided if the helical focuser is too tall to reach focus
    Male 2.7?-24 scope-side adapter (0.1? thick) included
    Female 2.7?-24 / Male 3?-24 camera-side adapter (0.1? thick) included
    Adapters have dovetail connections to prevent tilt
    Adapters can be rotated and locked to align with telescope and camera
    Wide variety of adapters and different backfocus
    Additional guide port assemblies come with prism, prism spacers, oval holder, helical focuser and 1.25? manual holder

    We work very hard to ensure that we offer the absolute best prices online.  If you find another online store that offers a lower price then us within six months of your purchase date please let us know and we will refund your original payment for the difference.  We want you to feel confident that you are getting the absolute best price for the product you are ordering.  If you find that our own website has a lower price for the same item you have ordered within six months of your purchase date will refund the difference as well.

    To request your partial refund simply e-mail us a link to the same product on our website, or on our competitors website within six months from the date of your order and we will process the credit accordingly.

    Our 100% Price Guarantee has some limitations:

    • You must purchase the item from our website before requesting your Price Match Guarantee
    • Promotions such as rebates and buy one, get one free offers are not eligible
    • The item must be in stock on the competitors website
    • The competitor must be an online store, they may not have a retail location
    • The website can not be a discounter or auction website (ie; eBay, overstock, etc..)
    • The competitor must be an Authorized Retailer of the product in question
    • The Price Match Guarantee includes the item price and the shipping charges, it excludes sales tax

    Order Confirmation As soon as you place your order you will receive an order confirmation e-mail.  This means that we have received your order in our system and pre-authorized your credit card for the purchase.  As soon as we receive your order we automatically reach out to our suppliers to confirm that it is in stock and available for immediate shipment.  If your item is on backorder or unavailable we will void the pre-authorization and reach out to you via e-mail.  If your item(s) are available for immediate shipment (within 5 business days) we will process the charges and submit the order for shipment.

    Order Shipment If your order is received, the inventory is in stock and your credit card information matches the information, In most cases, we can ship within 5 business days. We will send you tracking information within 24 hours of your order leaving the warehouse to the e-mail address your provide when checking out.  If you do not receive tracking information from us within six business days of your order please email us at

    Miscellaneous Shipping Information

    • We cannot ship to PO boxes.
    • Non-standard shipping surcharges will apply to isolated service areas, such as remote areas, areas not readily accessible by a delivery vehicle, islands, Hawaii & Alaska etc., as defined by the shipper. These circumstances occur less than 1% of the time.

    Delivery Times

    You can estimate the deliver time to your home or business by viewing the "Ships Within" information listed with every product. This refers to the time from when you place your order, to the time it leaves the warehouse. The total delivery time for your order is calculated from the time your order is placed, until you receive it. The diagram below shows how the total delivery time is broken down into pre-ship processing time and shipping time. 

    Processing time is the time it takes from when you submit your order to when the product leaves the warehouse. The "Ships Within" time frame listed with each of our products on the website applies to this time frame. For example, if your order a product that reads "Ships Within: 2 Weeks", this means that the product typically leaves the factory within two weeks. If there will be any additional time required to process your order, we will notify you immediately to confirm the delivery period. This time is needed for products that are shipped to you directly from our manufacturers. The manufacturers need this time to fulfill your order. In some cases, the items are custom built.  Shipping time is measured when your order leaves the warehouse to when it arrives at your address. Actual shipping time can vary between 2-5 business days. Contact our helpful customer service experts at (888-264-2244)  for more information.


    Did your shipment arrive damaged or defective?

    Please report damaged or defective merchandise to us immediately by email phone. Photos of the items and the packaging would be greatly appreciated. This will speed up our resolution process. We strongly recommend at the time of delivery, if your notice any cartons that look dented, crushed or just not right, open them immediately and inspect the contents. If the contents appear to be damaged, please notate it on the delivery ticket, refuse that item only and contact us as soon as possible for instructions.

    Brands/Products that do NOT qualify for our 30 Day Return Policy:


    APM products do not qualify for our 30 day return policy.

    Custom Orders

    Additionally, items that are special ordered, customized, or engraved - cannot be returned or refunded under our 30 day return policy. 

    If your engraved item needs to be repaired within 30 days of purchase, please reach out to us and we will have the product repaired or replaced, re-engrave if necessary, and return it to you in new condition.

    Customer Reviews

    No reviews yet


    Have a Question?

    Be the first to ask a question about this.

    Ask a Question