LX200GPS and Hotech CT Collimator

[symmetal]

Only just remembered I bought a Hotech collimator not long after buying the scope over 20 years ago. 😁 I used the defocused doughnut method after recently putting it in the shed after 10 years without use, and it looked OK so left it. Viewing the Hotech YouTube video makes it easier to work out how to use it  than the written instructions. 

The lithium battery in the collimator package was still good. 🙂 I got the scope square on to the collimator by aligning the laser crosshairs, took about three interations of the alignment loop to do and was straightforward.. The Hotech video was done with a Celestron but I couldn't do the focuser tilt adjustment or the corrector plate alignment as the LX200 secondary isn't removeable and doesn't have the corrector plate collimation screws like the Celestron, and it would be a pain removing the corrector plate ring and inserting shims under the corrector and putting the ring back on to test. 😬 If the scope was looking straight up it would be easier to do this but I'm not going that far.

Looking at the inside tips of the laser crosshairs, which indicate the secondary cutoff the corrector plate looked very slightly off in the vertical direction but not enough to worry about I thought.

I turned on the three secondary collimation lasers and only saw one reflected on the target near the middle. 😲 Turning the focus to align the three dots on top of each other on the mirror inserted in the focuser, they were well off to one side rather than in the centre of the focuser. I adjusted the secondary collimation screws and got all three dots equidistant from the centre and the dot on the focuser mirror was a lot closer to the centre but not perfect.

Here's the final result. The three reflected dots are now equidistant but not on the radial lines opposite the lasers. If I put them on the radial lines they aren't equidistant. I suspect how I've got it is the preferred alignment. What does everybody think?

Looking at the photo again it looks like I could have got the vertical crosshairs a bit more equidistant from the centre so maybe it wasn't fully square on and the corrector plate actually isn't out like I thought. This may have brought the dots closer to the radial lines and the dot closer to the focuser center.

It's a lot better than the start though when all that was visible was the bottom dot on the 1 circle. 😁 It looks to be good for the next few days so will try it on the planets again and see if the adjustment gives actual better results. 🙂

Alan

[michael8554]

Given the amount of mirror flop in the average SCT, how consistent were the dot positions as you altered focus and then refocused ?

Michael

[symmetal]

7 hours ago, michael8554 said:

Given the amount of mirror flop in the average SCT, how consistent were the dot positions as you altered focus and then refocused ?

Michael

Good point. I tried it again today. I found that to see the horizontal crosshair positions correctly you have to position yourself below the scope looking up. From the side as yesterday you can't judge them precisely.

This time the corrector plate axial alignment looks spot on which is good. I found with focus you have to choose whether to collimate after focusing in or after focusing out. Continuously focusing in one direction doesn't change collimation but changing direction does. Also after repeatedly changing direction, the collimation consistantly swaps from one position to the other which is also good to know. So it's always to best to finally focus on your object in the same direction in which the scope was collimated. I found that anticlockwise turns on the focuser moves the mirror forward which should be more consistant, rather than the mirror possibly moving under gravity after clockwise turns.

Here's the results of  best collimation after focusing anticlockwise. Target looks similar to yesterday but the focuser mirror is more central. 🙂

here's the results after turning the focuser clockwise from the resul above. A noticeable change. Focusing to get the three dots coincident on the focuser mirror is not critical and has no effect on the collimation results as long as they are just roughly coincident.

Should be clear tonight so real images will hopefully tell. 😀

Alan

 

[symmetal]

The seeing was bad last night so won't be able to see if it's better on the planets. However I took videos of a defocused and focused star. The focused star didn't show any useable diffraction rings as it was too chaotic although the dancing patterns were not evened out overall and were brighter top left. The defocused star showed this off centre doughnut consistantly which doesn't look good and confirms the focused star brightness spread.

The doughnuts were much better than this before I used the Hotech. 🙁 Reports from people who have used the Hotech swear by its accuracy so I'm a bit surprised. Maybe the return dots not being exactly opposite the corresponding laser may have something to do with it. These star videos were taken with an ADC still in the path but set to null. Anybody have any thoughts?

I've found that the ADC doesn't affect the bahtinov optimum focus spike position when it's adjusted from min to max which is good to know, though the coloured fringes do change. I thought the ADC moves more or less of the prisms thickness in the path as it's adjusted which I assumed would alter the optical path length but it doesn't seem to as far as focus is concerned.

I suppose an artificial star is the next to use and see what it shows.

Alan