Meridian flip?

[Gasman]

Hi

I`ve never had a goto scope before but come across the term `meridian flip` now correct me if I`m wrong but is this where you are sending the scope to find an object the other side of the meridian from where you are then the mount goes the long way round to it instead of crossing the meridian?. My concern is possible damage to the scope hitting the pier and I gather there are `mount limits` in EQmod which I`m still sussing out but assume these can be used to protect the mount from going where it shouldn`t?. I have not actually used my NEQ6 yet and not sure if this `meridian flip` problem applies to this mount but I`d be interested in what others do to protect their scopes?.

Regards

Steve

[Martin-Devon]

Hi Steve, I think you have the main points in your summary. Essentially, if you start tracking an image that is on one side of the meridian, the scope & counterweight will assume the optimal position - however as you reach and then pass the meridian - unless you reset the mount you run the serious risk of the scope/weights getting into odd positions and quite possibly hitting the legs of the mount. This can cause damage if left for some time as the mount is continously trying to slew yet is prevented from doing so by the scope or weight being hard up against the legs.

If you're going to do any imaging session, it's a good idea to check first on Stellarium (or similar) exactly where your target is in relation to the meridian. Then fast-forward Stellarium and see the time when it reaches and passes across the meridian. This is when you just need to press the target name on the handset again - the mount will then reslew to it, but will adopt it's optimal position for the other side of the meridian, and will now be happy for the rest of the night with no fears of scope/weights banging up against the legs. You can usually go up to an hour past the meridian without re-setting if you don't want to reframe your target etc, but if you intend to do longer imaging than this you'll need to reset the mount to do the flip. If you are imaging with a CCD, then after flipping the mount the image is also flipped, so you also need to rotate the CCD 180 degrees to compensate for this - now it should look the same as before on your laptopn capture screen - you then just have to do the fine-tuning with the framing to get it exact. I usually do a screen shot before the flip to get a decent star recognition pattern, and then after the flip/CCD rotation match the new image back again using a few key dominant stars to the screen shot image.

All this may sound tricky at first, but believe me it's actually quite easy once you've done it a few times. Initially I used to always avoid targets that crossed the meridian, but now I have no problems with it all.

HTH.

[Martin-Devon]

sorry I should have added, your NEQ6 is an equatorial mount, so yes if you go past the meridian for any period of time you'll need to flip....

[Gasman]

Excellent reply Martin thanks for that!.

Regards

Steve

[perfrej]

On a side note... Set limits in EQMOD slightly before your gear hits the leg on both sides and EQMOD will flip it for you during imaging. If you have done a good polar align and a few alignment stars, you will most likely find your DSO in roughly the same position in your sensor, albeit upside down, after the flip. Works like a charm.

In EQMOD, there is a checkbox as to what it should do when reaching a mount RA limit. Park is one option, another is flip.

When you slew to a target, there is a checkbox in EQMOD "Force flipped goto", which will let you slew to a target with the gear on the not optimal side of the meridian (also called "weights up"). This is actually the best way to start an imaging session because you will get no flip and maximum imaging time.

When you set limits, remember to slew in RA slowly towards E and W, and then double check that DEC has free travel. Sometimes, the RA position for a leg hit is very dependent on where the DEC is, so check it thoroughly.

Once you have limits in effect, you will never hit a leg and you can enjoy auto meridian flips.

Make it so!

/per

[Gasman]

Thanks Per for the additional info. I must admit to wondering why the `Meridian flip` exists at all? is it something built in to the mount electronics to prevent damage and is it specifically for german type mounts as its not something I`ve come across before using my old C8 fork mounted scope?

Regards

Steve

[perfrej]

It has to do with maximizing image time and the fact that some don't like weights up conditions. Sometimes when I image, I will get a westerly pier position to start with because the easterly position hasn't cleared the right leg yet. After a while it is cleared, and when the mount approaches the left leg hit-point it will flip.

OzDave has a nice feature in his EQMac, where you can set the limit settings to maximize image time. It will then always slew to east of pier if the limits are not violated. It is a good setting that should be implemented in EQMOD as well. In fact, I'll post a feature request on that right now!

/per

/per

[chrisshillito]

OzDave has a nice feature in his EQMac, where you can set the limit settings to maximize image time. It will then always slew to east of pier if the limits are not violated. It is a good setting that should be implemented in EQMOD as well. In fact, I'll post a feature request on that right now!

Wouldn't get your hopes up!

The way the EQMOD flipped goto is currently implemented in EQMOD is entirely deliberate and is the result of much debate and careful thought. A counterweights up position is regarded as "exceptional" in terms of EQMOD control and should be avoided if at all possible. To reinforce this flipped gotos can only be achieved by manual intervention and if the user selects that option they must accept they are taking EQMOD out of its designed control mode -and this is this is why EQMOD automatically resets the flipped goto enable.

The N-Point pointing model that EQMOD currently uses is not designed for the situation where the mount is slewing around in a counter weights up position. Making such movements "routine" rather than special exceptions would, I'm sure, compromise the good reputation EQMOD has with regard to pointing accuracy and require a level of support and development that I'm personally not prepared to undertake.

With Astronomy you have to accept that not all targets are accessible at all times. and in my view astronomers need to carefully consider how their mounts, and the software driving them, work best and plan their observations accordingly - there is plenty up there to look at after all.

Chris.

[perfrej]

Chris,

I fail to see the problem about weights up. If the scope is balanced or intentionally a little weight-side heavy, how would performance be affected? Does it have to with something like alignment models use a different set of values for each side?

I agree on accepting the fact that you can't do all targets at all times. Heck, I'm at 59°N and some and i drool over the southern gems that the southerners get. Besides, my current site has no view to the south at all.

I find it irritating to be able to image for an hour and then have a flip when my mount limits would allow it to be reached flipped. The again, there is the checkbox

If yo uhave the time, please enlighten me! I live on knowledge!

/per

[chrisshillito]

When each point in the sky can be pointed at using two mount positions things get much more complicated. Currently there is no code in place to handle this. EQ mounts aren't supposed to move that way (you'll find the synscan doesn't permit it) and all the early coding effort was built around keeping the mount in a nice safe counterweights down position at all times.

Of course software can be changed / improved / expanded and EQMOD has been frequently updated in response to user feedback. Most changes - even with complex functionality generally turn out to be relatively self contained and therefore there are very few risks in implementing them. However, the alignment model is at the very core of EQMOD and is mathematically complex. If this were a minor change I would have done it. However, in my opinion, it would require a significant development/testing effort - one that I personally can find no motivation to undertake, especially as most manage without it. That's not to say others might not take it on.

Chris.

[philr]

Well said Chris, I think you are doing a magnificent job, in all our best interests and safety.

Thank you!

[ollypenrice]

Just to go back to Martin-Devon's point, surely you don't physically rotate the camera after a flip? The image is upside down but that doesn't matter. All you need to do is screen-rotate the pre flip image and use that to frame up the post flip one. Later you can rotate the post flip images in the software to stack them with the rest.

I'm surprised to hear all this worrying about going past the Meridian. Depending on the elevation of the object I run for as much as two and a half hours past the meridian without flipping. (That's on EQ6 and EM200 mounts.) At high elevations you can't do that. I end up with the weights way up in the air and no problems attending on it.

Olly

[chrisshillito]

I'm surprised to hear all this worrying about going past the Meridian. Depending on the elevation of the object I run for as much as two and a half hours past the meridian without flipping. (That's on EQ6 and EM200 mounts.) At high elevations you can't do that. I end up with the weights way up in the air and no problems attending on it.

I'm not sure why folks get worried about this either. Certainly I'd always recommend tracking through the meridian if it gets you the imaging time you need. I also hate the thought of automatic flips (I purposely made them difficult to enable within EQMOD so folks wouldn't "accidentally" turn them on).

This said, the flip feature folks most seem to want is the ability to pre-flip the mount. The ability to track two hours past the meridian is very useful to have, but if you want four hours of data from an object crossing in the next half hour then a 'pre-flip a handy feature to have in the box (I've tried suggesting delaying imaging until the object has crossed, waiting a month before trying again or picking something else to image - but they just won't be told! )

Chris.

[ollypenrice]

I'm not sure why folks get worried about this either. Certainly I'd always recommend tracking through the meridian if it gets you the imaging time you need. I also hate the thought of automatic flips (I purposely made them difficult to enable within EQMOD so folks wouldn't "accidentally" turn them on).

This said, the flip feature folks most seem to want is the ability to pre-flip the mount. The ability to track two hours past the meridian is very useful to have, but if you want four hours of data from an object crossing in the next half hour then a 'pre-flip a handy feature to have in the box (I've tried suggesting delaying imaging until the object has crossed, waiting a month before trying again or picking something else to image - but they just won't be told! )

Chris.

Yup, I totally agree.

Olly

[daz]

Also, if you rotate your camera post-flip, you would have to take flats BEFORE you rotate it....

[ianaiken]

If you really wanted to rotate, rotate the telescope in the rings without moving the camera, then you don't need new flats. But give yourself an easier life by just rotating them later in post production.