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still can not get a good view?



hi thanks for all your advice in my last post unfortunately am still having problems with the focus length


my setup  celestron c80ed with skywatcher eq5 goto

canon 450d with 2 extention tubes 7" in totel and 2 x barlow 


my problem is am using way to much extension tube just to focus (when i can get it in focus)


after reading all the comments yesterday on my last post i put them in to action i want out a did the polar alignment with just a 3" extension tube no lens or or angle lens. after this i spun the telescope to Saturn with the same set up as the polar alignment. it was a bit small as expected so i went and got my 20mm eyepiece projection kit and set it up but i could not see a thing just a black screen? tried different setups like just 3" tube then 7" tube with just a bralow  ect ect 


do i need a long barlow lens for this telescope on will a short 1 work just as well?


or is there something else am doing wrong? i can get it to focus on a tree line about 500ft away in the day time but that is using all 7" tubes and it is a the limit of the focal lengh on the telescope

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Sorry, but with a given extension tube between the drawtube and the camera, you will either be able to focus, or not.  If you can focus on a deep sky object sometimes with a given setup, but not others then your issue is one of technique, not hardware.


Yesterday you were asking about M31; today about Saturn.  Those 2 objects are way different in size.  You could use a 4X Barlow with a 600mm scope on Saturn and it would still be a small object, whereas M31 is too large to fit in your field of view even without the Barlow.


If you don't know where your focuser needs to be for a given setup (scope, camera, adapters, Barlow, etc.).  the best way to get close is to aim at the moon, or a very bright star, get down on your knees and look through the camera's viewfinder while adjusting focuser. If you are able to get close to focus then you know that the length of any adapters between the focuser and the camera are good.  If you cannot achieve focus then you should be able to see whether you have too much distance or not enough and adjust accordingly, by adding or removing adapters. For example, if you can see that focus gets better as you extend the drawtube, but you are not yet in focus when the drawtube is fully extended then you need to add an extension tube in front of the camera, rack the drawtube back in and try focusing through the viewfinder again.


Once you have the correct distance between the focuser and the camera, and you have the star or moon in rough focus by looking through the viewfinder, you should be able to switch to LiveView on the camera's LCD display (using 5X and 10X zoom as you get closer) to get the focus even closer.  Then you should be able to connect the camera to the computer and BYE to get final, critical focus.


I hope this helps.

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so how would i set it up ive watch a lot of videos on this and some ppl are saying but the barlow in 1st then the angled then my camera others have said angle barlow camera and others have said just barolw and camera? but no matter what way round i do i i still need a extension tube a long 1 at that? i was under the inpreshon that a barlow x the focal lengh by the given number? so my 2x barlow lens will take my focal lengh from 600 to 1200 and my 7" extention tube in mm is 177.8 in total 1377mm or 57 inch/ seams a bit much?as where as my 130p has a focal lengh of 650mm i can just put a bralow in and view things fine? i just dont get it i am tring to understand i really am 

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First, you are correct about the effect that a Barlow lens has on the focal length. A 2X Barlow gives you double the focal length of the scope alone, but, of course, it also reduces the field-of-view. This is not very important doing planetary imaging, since the planets are typically a very small part of the field-of-view even at focal lengths exceeding 4000 mm like an 8" f/10 SCT with a 2X Barlow.


Second, I would not use a diagonal when doing prime focus astrophotography. It is just extra glass in the light path.


Third, I would put some type of extension tube between your refractor's focuser and the Barlow to replace the light length of the removed diagonal. However, if your focuser has enough backward travel to reach focus without adding the extension, then I would not use the extension.


When I use a Tele Vue 2X Powermate, I typically need to put a 3" extension tube between the focuser and the Powermate to reach focus. BTW, Powermate is Tele Vue's name for their premium focal extenders (another name for a Barlow lens). They work for visual use or, with an optional T-thread adapter, for AP.

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The easiest trick for finding the number/size of Extenders necessary for you to Reach Focus:
1) Mount your Scope with the Optics that you intend to "Explore", but no Eyepiece or Camera or Extensions.
2) Point the Scope at the Moon (or failing that a Bright Streetlight several blocks away).
3) Hold a piece of Darkly Colored Paper behind the Optics, looking for an Image of the Moon to form on that surface.
4) Move the Paper inwards and outwards to find Best Rough Focus.
5) Note the Distance between the Optical Back of the Scope (or front of the Focuser) to the point where the Image came to Focus. 

THAT is where you need to place the Sensor of your Camera, by dint of Adding or Removing Extensions until you can reach that Distance.
If you are looking to find the Camera Focal Point for a Scope with a Barlow added:
6) Do the above, then place the Barlow so that the first Lens facing the Scope is located where you found Focus on the piece of Paper.
7) Now repeat Steps #3-#5.
THAT is where you need to place the Sensor of the Camera when you employ your Barlow.
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I cannot reach the focal plane of my Newtonian's primary mirror with my Rebel t1i.  I want to do prime focus imaging, but even with the focuser at its most inward position, I'm just shy of reaching the ideal spot for focusing.  Currently, I have a Celestron 11mm thick T-adapter with a 2" nosepiece attached to it.  A search for thinner T-adapters has led me to a Baader 2mm thick T-adapter, but its cost is more than I'd like to pay.  Have any of you found very thin T-adapters?

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