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Equipment Discussions >> Mounts

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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: John Carruthers]
      #4399657 - 02/20/11 03:50 PM

Hello JC,

Quote:

a couple of points not mentioned so far, are the encoders perfectly concentric with the axes? does your optical axis coincide with the ota mechanical axis?




I think the encoders are perfectly concentric with the mnecanical axis, yes. Ditto about the optical/mechanical axis coincidence. But these are good points to check, nonetheless.

Thanks,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Skylook123]
      #4399796 - 02/20/11 05:10 PM

Hi,

Quote:

My two workhorse telescopes are 10" f/10 SCT, and 18" f/5 newtonian, so upon initial thinking I was a bit concerned about too small a field of view to get the target star into the field, balanced with the more power, the better. But my decision was taken away by my best friend and spouse for 44 years, who bought me an 8mm reticle eyepiece for Christmas four years ago. I guess that's why I married her! She saw I was looking over choices, and made it for me.




Nice! Can you mention the make/model? Also, what TFOV does that EP get on your scope?

Quote:

Since I don't use this eyepiece for any purpose other than star alignment, it works fine at 286X in the big newt and 318X in the SCT.




To have similar magnification, I will have to resort to a 5mm EP (250X on my scope), the only one I can find is the Orion 5mm Illuminated reticle EP... but it seems expensive at $180... and the TFOV will be really small at 12.5 arc minutes, which will make finding the stars for the alignment difficult (I have a no-magnification red-dot laser finder, which will make it necessary to use at least one intermediate EP to go from it to the 5mm EP).

Do you think that much magnification is really needed? Or would a less-powerful, bigger FOV EP like the Orion 12.5mm Illuminated Reticle EP (100X mag and 24 arc min TFOV on my scope) or even the Orion 20mm Illuminated Reticle EP (62X and 67.7 arc minutes) do the trick?

Quote:

One lesson I learned with the one I have is that when there is a reticle position adjustment capability for off axis guiding with the reticle, you need to make sure that the reticle figure remains centered. If it moves off center through use or any other reason, then the alignment suffers as the tube rotates to move from star to star. I find I need to eyeball the back side before I put it in the focuser to make sure the reticle plate is still centered and hasn't shifted.




Good tip, I did not know that the reticle could get shifted. Thanks!

Cheers,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: rmollise]
      #4399822 - 02/20/11 05:22 PM

Hi,

Quote:

Assuming your encoders are OK (nothing slipping, encoders properly mounted and concentric), the probable reason is your alignment procedure. It's hard to say much more since you don't mention the brand of DSCs, but, in general, stay away from stars near the horizon. 20 - 30 degrees up, minimum. And you want the two stars to be as far apart as possible. This time of year I use Polaris and Procyon.




Astromist picks the stars for me, always at 45 degrees or higher alt, and the 2nd star is always far apart from the 1st (the combo I've been using is Acrux and Denebola).

Quote:

Leave Astromist out of the mix till you get going. And be aware that DSCs, on most scopes, won't always put an object dead center. If you are getting 10' accuracy, that's about what to expect.




My DSC (David Ek's bluetooth version) has no keyboard/display and so depends on external software to be usable... but I will try PalmDSC as recommended by Lucy.

Thanks,


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Skylook123
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Reged: 04/30/05

Loc: Tucson, AZ
Re: DSC precision troubles new [Re: Durval Menezes]
      #4399903 - 02/20/11 05:51 PM

Quote:

Nice! Can you mention the make/model? Also, what TFOV does that EP get on your scope?






It's an inexpensive Meade 8mm Illuminated Reticle Eyepiece. As these devices go, it's pretty bottom of the barrel (the reticle plate shifts around very easily), but I only need it for the alignment. The FOV is like a soda straw; around 8 arcminutes in the 18", 5 arc minutes in the SCT. I get around this tiny view by using a low power eyepiece to get initially centered, then installing the reticle eyepiece for the fine tuning, of course never touching focus during the process.

In this case, the reticle is more important than the magification; you want to repeat an accurate centering. So, yes, I think that a longer focal length reticle eyepiece would make the alignment a bit easier, without loss of precision. High power is necessary if no reticle is used; the smaller field and a defocused star can bring some precision to the process. A reticle eyepiece does the same thing, but better. Both high power and reticle may be a bit of overkill. In my opinion, of course!


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Durval Menezes]
      #4523573 - 04/17/11 04:50 AM

Hello Folks,

It's been almost 2 months, so I'm posting an update.

Thanks in large part to your help, I've been able to diagnose and mostly fix my alignment troubles. Here is a q


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Durval Menezes]
      #4523640 - 04/17/11 06:20 AM

Hello Folks,

It's been almost 2 months, so I'm posting an update.

Thanks in large part to your help, I've been able to diagnose and mostly fix my alignment troubles. Here is a quick summary.

The main problem, as Jim (Skylook123) correctly predicted, was innacuracy when centering the alignment stars; I've fixed that almost completely via his technique of using a (relatively) high-power EP and defocusing the star to cause it to almost fill the field.

For this, I've tried at first my highest-power EP (a Baader Hyperion 5mm, which on my scope gives 249x on a 16' field) but found it difficult to find the star, as my finder is a simple 1x red led; I've settled on using my Nagler T6 9mm (138x, 36" field) with the focuser almost all the way out, which makes it relatively easy to find a bright star after sighting it on the finder, and causes it to fill 3/4 or so of the field. Also, I keep to stars away from the horizon, like Rod (rmollise) recommended.

Another spot-on tip I'm using is Greg's (Gnowellsct): instead of trying to use the star pairs pre-selected by Astromist (which, due to the many obstructions on my observing place, is always time-consuming and sometimes even impossible), I first choose any bright star approximately in the middle of the visible area of the sky, and then bypass Astromist's list for the 2nd star and directly pick another one nearby: I've found this gives me very good precision (certainly under 10') in that area of the sky, while wasting very little time.

If I then start observing on another area of the sky, it's enough to use the "Improve pointing" Astromist menu command to add 4 more alignment stars in that region (it doesn't matter how far apart they are) and then command a "N-Star Alignment": this quickly gives me not only very good precision on that new area of the sky, but also on the previous area, and on anything in between. Just by repeating this procedure as the sky turns over my head for each new region, I keep getting great precision from the DSC the whole night.

These two techniques together allow me to have very precise positioning with almost no hassle: I've just finished a quick observation session (less than one hour, from just after the Cheese Devil was low enough on the sky to stop spoiling it, to just before the pre-dawn light made it impossible to continue), and I was able to observe a lot of small DSOs (mostly "sub-Messier" NGCs) in the Libra-Scorpio-Ophiucus area: with the Nagler T6 9mmm on the focuser almost all the way out, first I used Jim's technique on Antares and Sargas in Scorpio for Astromist's "2-Star Alignment", then I did as Greg suggested and added 4 more bright stars on Scorpio itself and did a "N-Star Alignment"; from that point on, I just refocused the eyepiece (the Nagler never left the scope) and then dialed each DSO on Astromist: after pushing the scope to the indicated position, the DSO was always on the field and, most often than not, at its very center.

I didn't need to fix my azimuth encoder's non-orthogonality, and neither to replace Astromist with better error-modeling software: just the above two techniques did it for me.

I'm planning on acquiring a illuminated reticle EP on my next trip to the US, so as to get even more precise alignment (I will post a thread about that later, on the Eyepieces forum), but this is almost perfectionism on my part, as I already have the problem mostly fixed.

Thanks again for all the great help. Jim, Greg, Rod and all the others, you rock, folks!

Cheers,


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skybsd
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Reged: 02/01/08

Re: DSC precision troubles new [Re: Durval Menezes]
      #4523658 - 04/17/11 06:44 AM

I've been following your thread, here.,

So pleased to hear that you've worked out the kinks - hope you now get some great nights to work your magic

Congrats!!!! :waytogo;

Regards,

skybsd


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