Views around orion and Mars

[MartinB]

Surprising break in the cloud last night. wonderful transparancy all evening but I had to go out to a social event until 10.30. Still ok when I got in at 10.30 but clouds starting to brew.

Chance for a quick session with my new 12" Skyliner dob. The collimation needed another tweak - seems to need this every session, including adjustment of secondary tilt.

although the transparency was intermittently affected by high cloud the seeing was superb - as good as I've ever had. Mars was rock steady with a 5mm Baader orthoscopic EP x300 The disc details don't stand out like the bands on Jupiter but they were crystal clear along with the polar cap. I preferred the view through a Pentax 7mmXL - warmer colour (probably an artefact produced by the extra glass) and the same amount of detail visible - just a bit smaller. Whereas previously I have had to peer down the EP for some minutes to make out all the details on the disc last night it was just there.

From Mars it was onto my first view of M42 and 43. A 30mm UWA Moonfish showed the nebular detail beautifully and included NGC1977 - the running man. I thought I could make out some of the nebulosity in the NGC but my brain might have been filling the gaps! The 4 main stars were clearly seen along with swirling clouds within the central nebulosity. There was also clearly some of the outer nebulosity visible as a faint mistiness. Orion was still low down and into some light pollution so I wasn't really expecting great things.

A 17mm T4 Nagler really pulled out the detail within the nebula with much more contrast. M43 now clearly visible. Upping the magnification further with a Pentax 10mm XW revealed the view of the night. Swirling clouds of nebula with terrific depth. The large aperture was certainly giving plenty of horsepower for imaging at x150 with no apparent diming of the view.

I then cranked things up further with the 7mm Pentax. Very close up now and it felt like a walk on the moors surrounded by swirling mist. I decided to have a good look at Trapezium and found the E star very clearly visible and fully split. The F star just appeared as a deformity on C.

I was unable to make out the horsehead or the flame neb but could see that small reflection neb that sits just beneath the horse's head (or to the east if you prefer).

The 12" aperture certainly allows you to crank up the magnification. I find the coma a bit irritating. It will be interesting to compare the views with my newly arrived LX200 10" but I'm waiting for a losmandy plate to allow me to get it mounted. It may be another year before I get seeing like last night though.

[themos]

The collimation needed another tweak - seems to need this every session, including adjustment of secondary tilt.

Secondary? My, I never touch mine. But I do tweak the primary with an ordinary cheshire.

[MartinB]

I'm using a Baader laser collimator which is great. You just look down the tube and see if the red spot is falling on the centre of the mirror. It was just a little bit off. I think I probably loosened the secondary a little too much when I first collimated. I think if I collimate by predominantly tightening (within reason) it will hopefully hold it's collimation better. The primary only needed a small tweak as well. It made a noticeable difference looking at Mars. The primary has some locking screws but tightening them throws out the collimation a very small amount so it is a fiddle getting it just right.

[themos]

I have been experimenting with the across-the-room collimation procedure. Basically, instead of putting anything on the focuser tube you mount it instead on a tripod across the room from the scope so that it faces towards the open focuser tube. The extra light travel should give you more accuracy but I haven't worked out the details yet. So far, I've taken a nice picture of my camera lens reflected in the scope's mirrors on the far side of the room.

WIth my 300P, I find that a quarter turn on a primary mirror bolt is usually enough to bring it into cheshire-collimation.

I hadn't thought about the "always tighten" rule. I guess that eventually your mirror will go further down the tube if you do it that way.

I've also found that the locking screws can mess up the collimation when tightened so sometimes I leave the mirror unlocked and check collimation more frequently. The dob is for observing so you can always pop the cheshire back in, you're not having to remove cameras and the like.

[MartinB]

All the secondary tilt adjustment screws were very tight when the scope arrived themos so I had to collimate by loosening. I guess the tighter the screws the more stable the collimation will be. I think I'm now at the point where I have to loosen 1 screw before I can tighten another so I'll see if it's stayed in place next time.

[tom.yates]

I always tweak the colli screw then tighten the locking screw then move to the next one and do the same again all through the sequence and find this way when it is collimated you have no need to use the locking screws .

[MartinB]

That makes sense, thanks Tom

[tom.yates]

Forgot to say nice Report Martin.

[lunator]

Martin

nice report.

How much of the field is affected by coma?

Cheers

Ian

[MartinB]

Depends very much on EP Ian but with 82deg apparent field of view with a 17mm nagler it is starting to become significant about 2/3 out from the centre. Less of an issue with my slightly narrower FOV Pentax's.