Bob's Knobs and SCT Collimation

[Albireo380]

I got a set of Bob's Knobs and fitted them to my LX90 a couple of weeks ago - the weather then promptly went rainy / cloudy until last night.

I set up the scope about 8pm last night and used the binoc's for half an hour. Then put in an 11mm EP (x182) and focussed on Altair. I saw a fuzzy star that looked like a headlight beam pointing 20 degrees off to my left. Hmm - definately well out of collimation after fitting the Knobs. I nervously adjusted the focus and ended up with a doughnut ring of light with a central dark blob well off towards one side - I tentatively turned one knob about a sixth of a turn - back at the eyepice the central dark circle had moved past the centre of the doughnut ring, but was definitely more central. I therefore turned the other two knobs about a couple of millimetres each, rechecked the eyepiece and was rewarded with a perfect doughnut of light, with the dark hole plumb in the middle. I racked the focus in, beyond the focus point and made very small adjustments again to the knobs. All seemed well.

I then repeated the process with a 9mm EP (x222) and finally a 6.4mm EP (x312). At the end of this I then went back to the 11mm EP and refocussed on Altair - the off centre headlamp was banished. A perfect star twinkled at me - "Wow !!!" I thought - this seems to work.

Interestingly, I could see the atmospheric movement (and perhaps tube currents) at the start of the process, when out of focus, as the scope was probably not properly cooled down. Also, seeing the diffraction rings was a more difficult process than I had expected - sure, a bright doughnut of light could easily be seen, but the diffraction rings were only evident at a certain amount out of focus - too far out and no diffraction rings, not far enough out and the star twinkling destroyed any chance of seeing them.

However, having been really nervous about firstly fitting the knobs, and then actually using them - it turned out I was worrying about nothing.

Question After collimating using a relatively bright star and working up through the magnification as far as possible - is there anything to be gained be repeating the process with a dimmer star?????

So ..... worried about collimating your SCT? - get "Bob's Knobs" - they make it a snap.

Tom

[Chubster]

Bobs Knobs ?????? Can I just go up to the counter and ask :shock:

[richard42]

thanks! i have been thinking about doing this for a while but not yet got round to doing it, will get some of them if i do, where did you get them from?

[Astroman]

Tom, et al,

I prefer using a dimmer star for very fine adjustment of collimation on my C8 SCT. I do exactly as you did with a bright star, then move to a dimmer star. The diffraction rings seem to stand out more on the dimmer star, at least to me, because they're not overpowered by the brightness.

In re Bob's Knobs, I have them too, and while they do make collimation much easier in the field, 'm not entirely happy that they stay in collimation as well as the philips head screws that came with my scope. This isn't a problem when the scope stays setup in the dome, (250 out of 300 clear days average in AZ), but I do have to check and recheck when I take it on the road. (Grand canyon, Kitt Peak, and other star parties etc.) I'd rate them "useable", but not all wine and roses. jmho.

[Albireo380]

Chubster - you just walk in "bold as brass" and pick them off the top shelf - they come in a brown paper bag

Richard42 - I bought mine from Moonfish - about £15. You can get something similar from BC&F #AC276 (Telescope House), costing about £16.

Astroman - Thanks for the advice, I will try graduating to a dimmer star, and see if it is easier not to have the diffraction rings overpowered. I will also recollimate every time I use the 'scope, as my dark sky site is about 45 miles from home, and the 'scope goes in the back of the car and is rattled about down the motorway. I will let you know how well the collimation holds.

Cheers

Tom

[MartinB]

To really nail the collimation Tom you need good seeing. You need to crank up the magnification to at least x600. I use x1000. However I have found that my NS8 stays in collimation quite well. I only tend to bother now when the seeing is really good esp if I am trying some planetary imaging.

[Albireo380]

Thanks Martin, I have a 4mm Ortho I can use to crank up the magnification to x500, after that it would have to be with the 6.4mm plossl + Barlow (x625). I rarely get really good atmospheric conditions in west central Scotland, just too much turbulence and humidity. But for that once or twice a year night, I will give it a go.

Tom

[MartinB]

Useless trying to observe anything at that magnification but it does amplify small collimation errors and allow good fine tuning.

[Astroman]

Martin B is right-use as high a magnification as seeing allows. Use of a Barlow isn't recommended, because then you're just testing the Barlow and not your main optics. Collimating in poor seeing is and exercise in futility. I check collimation a lot since I have good to excellent seeing much of the time.

[daz]

At the Kelling SP, one of the talks recommended using a webcam to record the diffraction rings on both sides of focus, and process the video's in Registax as you would do normally.

Not yet had a chance to put this to the test myself, but could be a good way to check and test

[richard42]

i'm a bit confused about this do i leave the diagonal in or not?

[Albireo380]

i'm a bit confused about this do i leave the diagonal in or not?

Good question, Richard - I wondered about that myself. I looked up several "SCT Collimation" web sites and they all said that if you used your SCT with a diagonal, then keep it in when collimating. So that is what I did - next time I am out (hopefully tonight or tomorrow night if the weather holds) I will try both with and without the diagonal and see if it makes any difference.

Any thoughts Astroman or Martin??

Astroman - your earlier point about the knobs not holding collimation as well as the Meade screws seems to be correct. I had my SCT out again on Thursday night. It had about 50 miles of travel in the rear of the car + up and down the stairs in a case twice - since collimated on Monday. It was definitely slightly out on Thursday - only a fraction, but there was very slight "flare" on bright stars. So I will have to "tweak" again when next out

Tom

[richard42]

that would be interesting to know.

I have not yet done my scope, have taken an image of a doughnuty star last time i was out, i'll try and find it, i'm very clumsy so would not like to risk anything if i dont need to.

[Albireo380]

that would be interesting to know.

I have not yet done my scope, have taken an image of a doughnuty star last time i was out, i'll try and find it, i'm very clumsy so would not like to risk anything if i dont need to.

I don't think you are risking much Richard. Remove one of the Meade screws, replace it with the "Bob's knob" before unscrewing the second Meade screw. Once all 3 screws have been replaced with the "Bob's knobs", follow the procedure outlined on this web site.

http://www.mira.org/ascc/pages/lectures/collim.htm

This site takes you through daylight (rough) collimation of your SCT.

You do the proper star collimation next time you are out under the stars.

The daylight collimation does seem reasonably good - at least, it seemed to me that I got reasonably well "roughly" collimated. It means that after fitting "Bob's knobs" (in the daylight and indoors) you can be reasonably sure to get rough collimation before you go out into the dark.

Tom

[radioactive]

I have had my bobs knobs for my C8 since Feb this year

As yet they are still in the envelope they came in as I have not had to col the scope yet I thought well why bother Will see how I get on at Kielder tomorrow thats a good journey for it to travel so will take them with me Again however it will be the fifth journey with them stowed ready

[MartinB]

DON'T use the diagonal when collimating even though it makes things more uncomfortable. At the high mags necessary it will introduce additional distortions. Also make sure the image is dead central. This is the advice of Ron Wodawski in his book New CCDastro which is the CCD imagers bible.