The Importance of Being Collimated

Previously, I had the focal reducer just before the ccd camera. After last night, I realised that it needed to be 110mm from the chip. In effect, this meant that the focal reducer had to be directly at the back of the OTA, then the William Opotics crayford focuser and then the chip. I adjusted this position until I had close on 110mm.

Despite the fact that I had collimated the night previously, my collimation needed to be adjusted again. This may be due to the repositioning of the focal reducer. Anyway, it became evident in my first image with the new configuration.
Then, I tweaked the collimation of one screw by a tiny fraction of a turn and took another image. This second image showed collimation to be more accurate and is shown below:



Now, if you compare this image to the second image above, you will see a slight difference in how the focal reducer has tried to flatten the image. Whilst it's different, I'm not sure if it's much better. Obviously, the collimation needs to be be tweaked ever so slightly, but apart from that, what do people here think?

Regards,

One of the things I just found out from analysing the images is the exact focal length of the scope.
With no focal reducer: 2252mm - I would have expected 3000mm
With focal reducer: 1610mm - I would have expected 0.63x3000=1890mm
I always knew that the quoted focal length figures were a general average, but didn't realise they could vary so much.

Regards,

To be honest Michael, I see very little difference. Maybe the sweet area is very slightly bigger ?
I cant believe the focal length is 2252mm, this makes the scope an F7.5.
I've heard of variances between 11 and 9, but 7.5 is an enormous difference !!
Remember that the focal length of the scope is a function of how far back the eyepiece/CDD is from the backplate of the scope. Maybe you are estimating from the wrong location??

I've never measured the focal length of the scope I got, it might be worth figuring it out.

Just to reword it a little - the focal lengths quoted above are the focal length of the *system*, rather than just the scope - as measured by the camera behind the crayford focuser at the back of the scope.
As you know, the focal length varies significantly depending on where the point of focus is taken.
As the primary is f/2 and the secondary is f/5, that *might* mean that every 1mm added onto the position of focus at the back of the scope reduces the focal length by 5mm as the primary has to move relative to the secondary to get the focus point further out the back of the ota.
The crayford focuser is approx 100mm in length and the chip is a further 23mm into the camera body - let's say 125mm in total. 3000 - 5*125 = 2375mm...which isn't a million miles off the figure I measured.
If however I had no crayford focuser on the back of the OTA, I can only presume the focal length would be much longer and closer to the "advertised" value.
Does this make sense Dave?
Just thinking, if I wanted to reduce the focal length of the system, it might be better to just use a simple extension tube to do the trick rather than a focal reducer. What do you think?

Regards,

I see where you are coming from there Michael, but my understanding of the SCT optical system is that the focal length of the scope increases as you get further away from the back of the scope, not decreases.

Remember this thread from 2.5 years ago.
www.irishastronomy.org/boards/viewtopic.php?t=1127

So, when not using a focal reducer I cant explain why you might get a shorter focal length the further away the eyepiece/camera is from the back of the scope.

When planetary imaging, I often deliberately push the toucam alot further away from the back of the scope more then I need to as it automatically increases the size of the planet in the cameras FOV as this action increases the scopes F ratio.