Prime focus Problem

[Nortilus]

Hi,
I have a 8" Newt and my friend and I are trying to do a bit of astrophotography. The camera is a 450d and we have a 2 inch tube mount for the camera that goes into the focuser. However we can't seem to get anything to focus. I have the focuser all the way inwards as far as it goes but it still needs to go more.
Any suggestions on what the solution to the problem is.

[Ian Robinson]

Most likely the focal point is not far enough outside of the ota to reach prime focus at the focal plane of the camera.

You either need to replace the existing (rack and pinion ?) focusor with another with a lower profile (a crayford or a helical) or move the mirror up the tube an inch or 2.

[renormalised]

You need to shorten your tube to bring the secondary closer to the primary. Your focus is too far away from the chip in your camera and bringing the secondary to the primary will help to shift that focus outward, closer to the chip.

That's the extreme remedy...you may only have to purchase a low profile focuser in order to bring it into focus for your camera, but I say you'll likely have to shorten the tube like I said.

What brand of scope do you have??

[Nortilus]

Its a GSO 8" Newt I got from Andrews Comm a few years back, its on a dob mount and has served me well enough these past few years. Im looking at getting and EQ mount soon enough for it but we were just having a bit of a test run tonight, maybe get a shot or 2 of the moon, hence how we found this problem.

[Steve Pattie]

Everything said so far is true. However you might want to try to use a barlow lens - this should give you an image without the expense of a new focuser. If you move the mirror forward in the tube, you might need to use a larger secondary mirror to capture the entire light cone coming off the primary. Steve

[Tandum]

Alternately, a web cam should come to focus in the eyepiece range.

[Tandum]

Also check your t-ring. Pull it apart and look at it. I had one once that had a bit in the middle doing nothing and I removed it. That removed 10mm from the path. You can also screw the primary up the scope on it's screws to near end of travel. There's another 10 mm maybe.

Classic solution is to chop the tube and move the primary up. There is no way the secondary is too small for a 25-50mm chop.

[Nortilus]

I was looking at some solutions and a barlow might help to some degree, but the better solutions is what has been suggested here. Either get a low profile focuser or bring the primary up the tube. If i do the chop, is there a calculation i am able to do to work out how far up the tube i can go without losing and of those precious photos. As in how close can i get to the secondary.
Cheers.

[Nortilus]

found another solution. After a little investigation of the focuser, we realised that the bit where you would put 2" EPs (not sure what you would call it) actually unscrews out the the focuser. This will now get the camera at least 30mm closer to the secondary, hopefully fixing our problem. We just now have to get a Tmount for the camera that will screw into this new thread or make something up, not sure of the thread size, ID is about 60mm. My friend is a machinist so he is gonna make something up. Hopefully get some shots soon enough.

[mswhin63]

Not all Newts have this option. I posted some pictures to for my DOB but can't find the post. The 1.25" adaptor on mine unscrews to expose a "T" thread to insert the EOS to "T" adaptor without the 2" adaptor and I get about 30mm closer.

If someone could remember the post would be great.

[mswhin63]

Many thanks Peter (Tallstock) found the post.

http://www.iceinspace.com.au/forum/showpost.php?p=564371&postcount=55

[Nortilus]

those photos are handy to a degree, i have a 1.25" to 2" (for 1.25" EPs) then i can unscrew my 2" adapter (that holds 2" EPs) from the focuser. This inside diametre is about 60mm, which is bigger than 2" obviously. Having the 2" adapter unscrewed and just leaving the focuser only, now gets me about 30mm closer inwards. I dont know what I am really looking for but i need something that will screw into the focusers thread and attach to a 450d. But like I said, my friend is a machinist so we are just thinking of making something up ourselfs.

[mswhin63]

Unfortunately I was using the 450D to take the pictures and did not have a camera to take a pic of the final result. I will see what I can do.

[mswhin63]

I resurected my wifes old camera, not as good quality for close up but usable.

Picture 1 - 1.25" adaptor still together and EOS - "T" adaptor.
Picture 2 - Unscrew the 1.25" adaptor then lower half of 1.25" adapator screwed into EOD adaptor (upper half of the 1.25" adaptor not used).
Picture 3 - EOS adptor and lower 1.25" adptor inserted into the camera.
Picture 4 - Camera and attachments inserted into the scope.

This method provide me 30mm extra focus adjustment without making anything special, but this option is not available on all focusers. If your focuser does not have this thread capability then you will need to manufacture something to do the same job. Due to the size of the "T" thread it can only be done on a 1.25" focuser adaptor.

[graway]

What a great site this is
This info that you have shared has ended
much frustration for me.
Thank you
All I need now is for it stop raining and I
think I might get a picture:thanx:

[mswhin63]

The only problem with the arrangment is unable to place filters if I decide to modify the inbuilt filter with clear glass. I know astronimik make filters that go in. I am hoping this will be the answer although this will be a little way off.

[Nortilus]

I have a similar setup to your, except i have and extra bit that is a 2" adapter that screws into the focuser. I'll take some pics of my setup so others my benefit from this little discussion as well. I now know what i need to get made up as I cant seem to find anyone that sells or even makes what i need.