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I think I know the answer to this question, but would like your opinions. I am new to astronomy. I have a Williams Optics FLT 91 with a diagonal. I placed a star finder on it. During alignment of the star finder with a distant object ( a tree one mile away) looking through the diagonal with a 25MM eyepiece, I cannot focus the scope. If I remove the diagonal, I can focus the scope on the tree. I have no problem with the diagonal and eyepiece focusing on stars. I presume the diagonal added enough distance to the focal length, that the combination of the diagonal and eyepiece cannot focus on a tree one mile away. |
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Hi Dennis, yes you're right. If a target is close, the focuser's drawtube needs to be pushed further outwards. As targets get more distant, the focus point will go inwards. In your case, the diagonal pushed the focus point out past the telescope's drawtube limit. So there are three options: 1) Find a further daytime target 2) Get an extension tube 3) Pick a bright star at night to align the star finder. I usually do (3). Hope this helps. Joon Ren |
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thanks, Joon. I was going to refine the focus of the star finder on a bright star, the next time the clouds cooperate. A follow up question though: is the extension tube you mention (option #2) on the extension capability of the scope, so the focuser can extend the eyepiece further from the objective of the scopel? Just trying to understand optics. Dennis |
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| Do you have the visual back installed on the scope? Normally it’s removed for astrophotography. |
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| Sean I don’t know what the visual back is. Also, I’m not trying astrophotography yet. Just trying to align the star finder scope with the telescope so I can find stars for the telescope to focus upon. |
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| Hi Dennis, an extension tube is a separate accessory that is attached in between the focuser and eyepiece to give more backfocus. |
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| The visual back is the part that has the rotolock on it. If the rotolock is on the scope, you have the visual back on the scope. |
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| I see. A couple of questions. Does it simply screw off? If so, is it’s removal necessary to attach a DSLR to the diagonal? |
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The visual back screws off. Typically the visual back is removed for photography, but you need to determine your backfocus for whatever camera you’re using. There’s a good thread on cloudynights if you search flt91. Do you have the flattener? also, you don’t connect the camera to a diagonal. |
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Thanks. I’ve seen threads in the past about set backs for flatterers. I don’t have a flattened yet. Thanks for the tip on not using the diagonal with the DSLR. I didn’t know that. I will attach the DSLR to the scope without using the diagonal. When I attach the DSLR, I will approach whether or not to remove the visual back or not. Does the discussion you referenced indicate when to remove the visual back when using a Canon DSLR? |
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| I could not get my FTL91 to focus with the visual back installed using 2 inch eyepieces. I just removed it permanently and it works with everything. Its make the scope a bit shorter too. |
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| If you’re not using a reducer, the camera and t adapter would actually slide into the visual back. Keep the visual back on with that configuration. When you get a reducer, the visual back will come off and the reducer will replace it. |
