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A couple of questions about Ha


RobertI
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Tried my new 7nm Ha filter last night but not much success. Had a number of challenges; first could not get the alignment stars on the Lodestar chip when aligning, eventually discovered I was still in Zoom mode in LL! (Paul - a 'zoom mode ON' indicator in LL would be useful). Still no luck with finding stars, then realised I had repositioned my finder during the week and had not re-aligned it with the main scope. After all this I only had a small time to test the filter before the clouds finally terminated the session.

The plan was to use the 60mm finder/guider at F3.75, but thought I would try the main 150mm scope at F9 first - probably not a good idea. Using 30 second exposures with the Ha filter I managed to see lots of tiny stars but never my intended targets of the Bubble Nebula and the Cocoon Nebula. This was probably due to alignment issues (outlined in another thread) but would 30 seconds at F9 be enough to see these objects?

Finally I had some problems stacking. Although multiple exposures were being taken they were not being stacked (according to the progress stats at the bottom of LL). For example 4 x 30 second lights were taken but only 1 had been stacked. Does LL have an issue with the tiny stars in Ha exposures?

Thanks

Rob

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Hi Rob,

Sounds like one of those nights where everything goes wrong. I think you have a handle on your alignment problem. 150mm at F9 is a FL of 1350mm. That's really long for the LS. I think you saw my recent captures (tanks for the like) of the Bubble and Cocoon that were taken at a 800mm FL. Both of those objects are tough. The Bubble is a little easier to find, because of the bright area in the Bubble itself. At F9, I think you will need 45 or 60 seconds, but you may be able to start seeing something at 30 if you increase the contrast slider more. I would suggest trying again either with the 60mm scope or using a focal reducer with the 150mm scope. The stacking problem is probably due to a combination of long FL and Ha filter. The filter reduces the number of readable stars as does the narrow field. I usually don't start stacking until I have the object adjusted to my liking. Then I turn on mean stacking to reduce the random noise.

You may want to try an easier object for the first one. One of the summer nebulae like M20 for example.

Good luck. Hope things go better for you next time.

Don

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I always do the alignment with a 25-50mm eyepiece and no filter.

With NB filters I don't think I have ever used any optics slower than f4.

Depending on the processor of your computer, stacking may take some time. I have an older computer and if there are many stars, stacking would not complete during a 30sec exposure. But if one is patient and is waiting longer, then stacking will take place. Look at the number of Raw Stars on the stacking tab. If there are more than 100 of them, then stacking will be slow. Will work but on my computer, it may take closer to 60 secs. This is one of the reasons I have no real incentive to go for exposures shorter than 60sec. Btw Paul is making some refinements of the stacking routine with particular consideration for scenarios with many tiny stars.

Clear Skies!

--Dom

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Thanks for the great answers guys.

Don - i think I will start will the 60mm as per your suggestion, I think i overestimated my abilities! if I still can't see anything there must be something more fundamental wrong. Sadly i cannot see any further south than +30 Dec without transporting the scope (a situation I hope to change when I eventually move house) so cannot see M20, but I think there are other bright emission nebulae I can try.

Dom - i will check the number of raw stars - stacking has not been a problem before so led me to believe is something to do with the Ha, but will look at that anyway. Aligning with an eyepiece sounds like a very sensible and hassle free solution, especially at longer FLs, although I do not have an illuminated reticle eyepiece (do you use one?).

Thanks

Rob

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Rob,

The illuminator of my 25mm Stellarvue crosshair ep doesn't work. I defocus stars slightly to make them look like cheerios (tiny doughnut shaped cereal). That way I see well, when they cross the crosshair lines and it is easy to place their intersection in the middle of the "bull's-eye". I sometimes use a 50mm ep that doesn't have cross hair. Somehow the eye has the ability to see, if an object is at the center of a circle. Even more with the concentric circles of a doughnut star.

--Dom

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Thanks for the great answers guys.

Don - i think I will start will the 60mm as per your suggestion, I think i overestimated my abilities! if I still can't see anything there must be something more fundamental wrong. Sadly i cannot see any further south than +30 Dec without transporting the scope (a situation I hope to change when I eventually move house) so cannot see M20, but I think there are other bright emission nebulae I can try.

Dom - i will check the number of raw stars - stacking has not been a problem before so led me to believe is something to do with the Ha, but will look at that anyway. Aligning with an eyepiece sounds like a very sensible and hassle free solution, especially at longer FLs, although I do not have an illuminated reticle eyepiece (do you use one?).

Thanks

Rob

Sorry, Rob. I keep forgetting that I'm at a 20 degree latitude. How about Dumbbell? That a pretty bright emission nebula? Looks pretty neat in Ha, too.

I haven't seen the problem with stacking that Dom has. If mine doesn't stack within about five seconds, it won't stack. Dom needs to upgrade from that Commodore. Just kidding, Dom.

Don

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Hi folks,

Yep there a few performance and upgrades to the stacking routine. I have found a few issues with the multi spectrum datasets provided by Dom so plan on sorting them before the next release. Time hasn't been with me recently but should be on the case a bit more this week!

Rob, do you have any raw FITS files from your issues with Ha and stacking? If so I can use them to aid in debugging.

Paul

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