Question about meridian flip

[Martin-Devon]

I plan to do a long imaging session of NGC 6960 using a CGEM mount, and note on Stellarium that the target crosses the meridian at around 11.45pm - can I just ask for feedback about what to do when this happens.

From reading several threads it looks like I do the following when NGC 6969 crosses the meridian:-

a) Reset the GoTo to NGC 6960 (I guess this flips the mount?)

Turn the Atik CCD round 180 degrees since the image is now inverted (is this correct)

c) Reset the PHD guider to undergo a new calibration routine on a new guide star

Is the above a correct summary, or do I need to do anything else?

[Psychobilly]

If you rotate the camera in the scope you will need new flats... the stacking software should be able handle the 180 degree rotation ...

Peter...

[martin_h]

Leave the CCD where it is....DSS and the like will stack the "upside down" with the "right way up" subs ok.

The only thing to look out for is when the flip is completed you may not be exactly in the same position as before, so will have to tweek the position a bit...remembering that your new image will be rotated 180 deg.

Or you could leave it tracking as long as possibe past the Meridian to see how far you can get. I have an ED80 on a NEQ6, pier mounted and can go 2 hours past Meridian.

[Martin-Devon]

Thanks for the quick & helpful feedback. So all I need to do then post the meridian flip is to re-check the framing (and focus) of the target again, knowing that it will be a mirror-image of what I had before. It's good to also know that I can go a little time beyond the actual meridian crossover before I actually do the flip - this gives me more flexibilty setting up the initial runs.

I must admit, in my imaging runs to date I've never checked on Stellarium whether or not my target crosses the meridian during the run - I'll make a point of checking this every time now.

[lukebl]

Like Martin, I've managed to get satisfactory guiding till about 2 hours after transit without a meridian flip. Generally, I try to avoid the hassle of a flip!

[Earl]

Like Martin, I've managed to get satisfactory guiding till about 2 hours after transit without a meridian flip. Generally, I try to avoid the hassle of a flip!

Interesting, I had not thought it wise to do so, but Ill give it a go now and grab some more subs

[lukebl]

Interesting, I had not thought it wise to do so, ....

I know. It's probably not ideal, but I find it saves a lot of hassle at 2 am when you're tired and would rather be in bed.

[Earl]

I know. It's probably not ideal, but I find it saves a lot of hassle at 2 am when you're tired and would rather be in bed.

Yeah agreed

[ollypenrice]

I go miles past the meridian as well (and hate the flip!)

Don't rotate the camera and don't rotate the image in the capture software. You can get in a mess with darks as well if you're not careful! For reframing, post flip, just rotate a copy of one of the original frames in whatever software will show it and set the new framing to match the inverted original.

Hunky dory.

Olly

[great_bear]

Interesting, I had not thought it wise to do so

Assuming you've checked nothing will bash against the mount, what's unwise about going past the meridian?

[ollypenrice]

Assuming you've checked nothing will bash against the mount, what's unwise about going past the meridian?

Nothing.

Olly

[lukebl]

Assuming you've checked nothing will bash against the mount, what's unwise about going past the meridian?

I thought it was something to do with the fact that the scope balance changes, and the guiding changes from 'pulling' rather than 'pushing' or something. Or maybe I'm talking rubbish.

[Uranium235]

With my mount (CG5-GT - same Nexstar HC I think), im lucky if i can go 15-20min past the meridian before PHD throws a serious wobbly. When/if you flip you will notice the positioning isnt quite the same, so you will have to spend time on re-framing so you dont lose too much FOV after stacking.

[Earl]

Assuming you've checked nothing will bash against the mount, what's unwise about going past the meridian?

I have been under the assumption that a) weights should never go above the scope and guiding needs to be calibrated for the pier side it is on..

Obviously this is not the case, which is good

[Psychobilly]

This is the one advantage of a fork mounted scope on a wedge - no meridian flip marlarkey .. my best skies are around the meridian and I just carry on imaging straight through it...

[martin_h]

Last time out I noticed oblong stars after passing the Meridian....wayyy past. So I just stopped PHD and forced a re-calibrate....All was good again.

Just a note....If you are tripod mounted just be aware of the legs!!! I'm on a pier so no problem there. Also it will depend on the length of your scope and where you are imaging, with my ED100 I cant go to far past, the filter wheel hits the top plate, ED80 - no problem!

[ollypenrice]

I thought it was something to do with the fact that the scope balance changes, and the guiding changes from 'pulling' rather than 'pushing' or something. Or maybe I'm talking rubbish.

Yes, true, but if your guidng goes AWOL in RA just slide the CW outwards a tad. If you flip you will need to rebalance anyway so there is no difference.

Olly

[RikM]

I can go about an hour past meridian then the scope gets too close to the tripod legs.

With the large mirror clips in the 150P I get dark triangles in the diffraction pattern but if I have sets of subs either side of the meridian, the pattern seems much reduced after stacking ???

[MartinB]

How far you can go past the meridian varies with scope dec. You can get a rough idea by playing around during the day. Try different decs and see how far round you can go in RA. You do have to watch your cables though, make sure you have enough slack and that nothing is going to snag.

If you do perform a meridian flip you need to adjust your counterweight so that the set up stays weighted slightly to the east. Again, you can prepare beforehand by marking the positions for correct balance for both counterweights east and west.

After a flip you don't need to recalibrate the guiding but you do need to reverse the dec corrections.

[Martin-Devon]

Thanks everyone for the helpful comments & discussion - I now understand what I need to do.

MartinB – just one small query - how do I actually reverse the dec corrections in PHD for the guiding? I will also do a dummy-run to see what happens with the scope set-up and cables by pointing initially at the RA/Dec of my target and then moving the scope round just in RA to see whether anything collides or cables are too short - thanks for the advice.

[MartinB]

Sorry Martin, I can't help with PHD since I use Maxim which just has a little box to tick. I'm sure there will be something there!

[shaunster]

In PHD, click the brain then you can select dec guiding either Auto, north or south.