Recently, I got myself a new Meade 80mm F/11 guidescope. Just for fun, i decide to image through it last night. First target Keyhole Nebula consist of 10 x 2 minutes 49 sec . I never used a refractor before and didn't expect the Keyhole will be so closeup? i thought i will be getting a bigger FOV than my 8" F/4 scope. doh!
Nice one Eric. F/11 certainly isn't going to give you a wide field of view that's for sure. It's would be like imaging down a drinking straw after using your f/4.
Yep, hartmann mask or diffraction strings across the dew shield for focusing will help, but if you are only guiding with it absolutely correct spot on focus isn't necessary as your not going to be processing it. Your stars still need to be focused but you don't need to be pedantic about it.
I use another option for getting within the ball park, especially as I may be swapping things around during the night on my refractor. For each item I might be attaching (300D, ToUcam, ToUcam and barlow, Eyepiece projection, DSI etc) I have a line marked and labelled on the focuser tube to save me searching for focus. I'm in the process of doing something similar for my SCT on a label to be attached to the side of the OTA. But not being able to see the mirror distances change I'm using 1/3 turns CW or CCW from focus on my 12mm reticle. Seems to be working Ok.
After you have got focus using a Hartman Mask or software etc, try a couple of eyepieces in your focused position and try to get one parfocal or if it is just out, use a parfocal ring and draw a pencil line on the refractor focuser.
Will save you trying to find focus if you remover the quider and use the refractor for visual or whatever. Whenever you put the guider back in, just use the parfocal eyepiece first and get focus, then put your guider in and your away. You can also use the eyepiece to find a suitable guidestar as well.
Eric the only way to get perfect focus is by test exposures. The problem then is if the temperature changes or the scope or lens is still cooling down you lose perfect focus. I have been battling with this from day one. That is why I built the stepper focuser. This allows me to calibrate the lens or telescope for true focus position as a function of temperature. It is a lot of trouble initially but saves a lot of time in the long run. I found that by the time you notice focus going 'off' during a series of sequential exposures and then get dark adapted and refocus anything up to an hour is lost.
This is what makes astrophotography 'fun' so many variables you have to try to control. Everybody has their own 'methods' of coping.
I am afraid there is no simple or correct answer.
Even with the 80mm F/11 thats still one very good image. Well done Eric for thinking out side the square and showning how good a 80mm F/11 scope can be.
After you have got focus using a Hartman Mask or software etc, try a couple of eyepieces in your focused position and try to get one parfocal or if it is just out, use a parfocal ring and draw a pencil line on the refractor focuser.
Will save you trying to find focus if you remover the quider and use the refractor for visual or whatever. Whenever you put the guider back in, just use the parfocal eyepiece first and get focus, then put your guider in and your away. You can also use the eyepiece to find a suitable guidestar as well.
Cheers
JohnG
Great idea John. I do have some parfocal rings with came with the DSI II which i don't use at all.
Eric the only way to get perfect focus is by test exposures. The problem then is if the temperature changes or the scope or lens is still cooling down you lose perfect focus. I have been battling with this from day one. That is why I built the stepper focuser. This allows me to calibrate the lens or telescope for true focus position as a function of temperature. It is a lot of trouble initially but saves a lot of time in the long run. I found that by the time you notice focus going 'off' during a series of sequential exposures and then get dark adapted and refocus anything up to an hour is lost.
This is what makes astrophotography 'fun' so many variables you have to try to control. Everybody has their own 'methods' of coping.
I am afraid there is no simple or correct answer.
Bert
Cheers Bert. thanks for the idea. Any pics of your stepper focuser?
Even with the 80mm F/11 thats still one very good image. Well done Eric for thinking out side the square and showning how good a 80mm F/11 scope can be.
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It's would be like imaging down a drinking straw after using your f/4. 
