Eye piece hole

[justinshort010]

Hello mankind I’m Justin I have a question about the eye piece hole measurement from my primary mirror my focal length is 2.46 feet does my secondary mirror stop right at the end of the focal length any help would be appreciated I got lost watching those videos 

[Ags]

The focal plane of your telescope is at your eyepiece, not at your secondary mirror.

[NGC 1502]

Sounds like you have a Newtonian reflector?
 

Your focal length of 2.46 feet is the total distance measured from the primary mirror’s optical surface to the middle of the secondary mirror then to where the focal plane is located at the focuser. The focal plane is the location of the eyepiece.

[Albir phil]

Are you asking where you should drill the hole for the focuser. If so take the mirror F/L minus half it's diameter however that will put your focus point at the tube side now you need to allow for your back focus when you fit the focuser, about 3or 4inch so you deduct this as well, this the brings the focus outside the tube wall.

[AstroMuni]

5 hours ago, justinshort010 said:

Hello mankind I’m Justin I have a question about the eye piece hole measurement from my primary mirror my focal length is 2.46 feet does my secondary mirror stop right at the end of the focal length any help would be appreciated I got lost watching those videos 

In the diagram above the 2.46 feet is at the point marked 'prime focus'. The secondary mirror moves that point 90 degrees to the bottom in this diagram. The eyepiece will need to be at a suitable distance away from this point (depending on the power of the lens in the eyepiece) to achieve focus. Does this answer your question?

[M40]

Hello Justin, assuming you are using a concave primary mirror in a Newtonian reflector, the focal length in your case is 2.46 feet from your primary mirror. This is where the light goes to a single focal point. By inserting the secondary mirror you then move the focal point to hopefully outside or near the outside of the tube. The position of the secondary mirror will determine the location and size of the hole in the tube.

As different eyepieces have different focal lengths, you then need the focuser to position the eyepiece at the correct focal length from your relocated focal point. Hope that makes some sort of sense. Sketch out your thoughts as it could help with our answers. All the best. 

[Albir phil]

2 hours ago, AstroMuni said:

In the diagram above the 2.46 feet is at the point marked 'prime focus'. The secondary mirror moves that point 90 degrees to the bottom in this diagram. The eyepiece will need to be at a suitable distance away from this point (depending on the power of the lens in the eyepiece) to achieve focus. Does this answer your question?

 

8 hours ago, justinshort010 said:

Hello mankind I’m Justin I have a question about the eye piece hole measurement from my primary mirror my focal length is 2.46 feet does my secondary mirror stop right at the end of the focal length any help would be appreciated I got lost watching those videos 

The diagram posted explains it more clearly, depending on the diameter of your mirror you deduct half it's diameter plus allowance for focus,that is where your focuser hole must be.Draw it out on paper full scale before you do any drilling.

[Mandy D]

@justinshort010 the link below might provide some additional help, as there is no simple answer to your question:

https://www.cloudynights.com/topic/925547-can-prime-focus-location-be-determined-from-eyepiece-size/