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HELP - Narrowband problems


martintblack
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Hello

I am presently attempting to image M42 in narrowband. We are using a 16" LX200 (with a 6.3 focal reducer), and a SXVF-H16 CCD. We also have a C5 as a guide scope, with a Watec 120N camera. H-alpha filter at the moment.

We are fairly sure we have it focused, but our images are still coming out very badly. We think it's a problem with the guiding (using Phd guiding). Have played with the aggression setting, as well as exposure, and the rest, but seem to have a lot of movement between each exposure on the guide cam (about 1 second exposures). The movement is random in all directions.

Any suggestions are more than welcome!

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Hello Martin.

Have you biased the balance of the scope?

You should have it slightly weighted towards the east in RA(a couple of weights on the east fork arm), and slightly towards the rear in DEC.

Watch what is happening with your guiding corrections....in DEC, depending on which side of the meridian you are, you will notice that it is mainly guiding either north or south. When you've figured out which way, in the 'brain' settings, set it to this. This will then keep the gears meshed in that direction, as will biasing the balance.

Also, with 1 second exposures, you are probably chasing the seeing. With my 14 inch LX, I used to use 4 second guide exposures, and with my current setup this still seems to work well. You will need to experiment with this to find what suits your setup.

Make sure you have the graph enabled in PHD...it will show you the effect any changes in settings is having very quickly.

Cheers

Rob

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Hey

We haven't biased the balance, only tried to get it even on all sides. Will need to try that tomorrow when its light and I am a bit warmer. Taking images to test a theory just now at least. Have increased the exposure time in the guide camera to about 5 seconds, and that by eye at least looks better.

Will let you know in a few minutes how it works out!

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With a 16 inch scope, even with NB filters, you will have no problems getting a great result with 5 or 6 minute subs....to get it in the FOV though, you'll need a focal reducer.

Check out my website, there's a bubble on there done with a 14 inch LX200 and 6 minute subs.

Re. balance....if you have the scope perfectly balanced, you end up with the corrections going in both directions, and have to take up the backlash every time. Perfect balance is not a thing you want.

Cheers

Rob

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I find that if I resist the urge to pick the brightest(largest) star I get better tracking. If I turn the exposure up I end up with an even bigger bloated star. At first I thought this would be a good thing to track on. But PHD has to find the centre of that splodge to track, big splodge means potentially big error and drifting about. If you had a star that only illuminated 1 pixel then the maximum error would be '1'. See what I mean!

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I completely agree with Glider.

Try to pick a star that is fairly small, but distinct, preferably towards the centre of the FOV, although this isn't critical.

Also, when you calibrate, pick a star within a couple of degrees of your target.

Cheers

Rob

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Get the latest pre-release version of PHD guiding it will allow you to change guide settings onthe fly but in reality it will take around 11 guide cycles for the changes to take effect..

Use the star profile box to pick a star thats around 80-90% of the max exposure with an exposure time of around 3-6 s possibly longer... you want a nice "sharp" profile without a flat top...

Peter...

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The star we were guiding on definitely covered more than 1 pixel. It tracked fine in the RA, moving only a few pixels. Even in the dec, for most of the time (about an hours continuous tracking) the movement was only a few pixels. There is however a singular jump of 14 pixels at an arbitrary time. Compared to other runs I have examined, there seems to be regular similar jumps in dec. If I get time I will post the graphs produced.

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Rob,

Just a question on balancing in DEC, earlier in this thread you say slightly rear heavy, is there any reason for it being rear heavy as opposed to front (corrector end on a SCT) heavy ? Does it actually matter which as long as one end is slightly heavy ??

The reason I ask is I've read elsewhere that for a fork mounting on a wedge imaging South, DEC should be slightly heavy at the corrector end and for imaging North, DEC should be slightly rear heavy ?

Totally understand and agree on the RA setup.

Many thanks, Andy

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Hello Andy.

Good question, and one that I've been thinking about recently.

The received wisdom as far as I knew has always been to balance at the eyepice/camera end.

This of course doesn't mean it is right.

With german equatorial mount that I now use, I know that I need to switch guide commands from south only to north only when crossing the meridian, due to the fact that the N/S axis isn't perfectly aligned.

Tonight I watched the graph in PHD smooth out upon crossing when the mount had been pre-flipped, and the DEC guide mode pre set for after the meridian.

So what you say makes complete sense, but will depend on in which direction your mount is not perfectly N/Saligned, and therefore can't be a rule for all situations.

So....the rule probably is....

Figure out which end to bias on your own mount in DEC, then reverse it at the meridian.

Cheers

Rob

Edited by RobH
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Rob,

Thanks for the response.

As with all these things once you have a system that works then stick with it. My SCT scope at the moment is corrector end slightly heavy but still have some PHD issues so I'll give the camera end the weight bias and see what happens.

Best wishes, Andy

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