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Re: Polar Alignment with a GM-8


Jan 25, 2005

 


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#24048 Jan 25, 2005

Back in December, I did the two star alignment with the polar scope.

I was able to do 30 sec unguided prime focus exposures but in the

future I'd like to get up to a minute or longer. If I did a three

star alignment will this be accurate enough to do 3-5 minute

exposures with guiding? I'm working with focal lengths of 880mm or

less. Will the three star alignment be accurate enough for prime

focus astrophotography or should I invest in the Gemini Goto to get

better results? Any help would be appreciated.



Debbie



----------------------------

#24049 Jan 25, 2005

Debbie,

To get the best out of your mount it is wise to do a drift alignment after your polar. This will fine tune your alignment. If you are planning to do unguided imaging then you should do a PEC to correct for drive error. The better your alignment and PEC the longer exposure times you can go.

John



-----Original Message-----

From: Debbie astrodeb@...>

Sent: Jan 25, 2005 1:04 PM

To: Losmandy_users@yahoogroups.com

Subject: [Losmandy_users] Polar Alignment with a GM-8







Back in December, I did the two star alignment with the polar scope.

I was able to do 30 sec unguided prime focus exposures but in the

future I'd like to get up to a minute or longer. If I did a three

star alignment will this be accurate enough to do 3-5 minute

exposures with guiding? I'm working with focal lengths of 880mm or

less. Will the three star alignment be accurate enough for prime

focus astrophotography or should I invest in the Gemini Goto to get

better results? Any help would be appreciated.



Debbie















Yahoo! Groups Links



















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----------------------------

#24050 Jan 25, 2005

Hi Debbie,



Three star alignment should be better than two, but how much better

depends on your polar scope and how accurately it is adjusted.



With guiding, 3-5 minutes at that focal length should be achievable,

especially if you can go for 30 seconds unguided already. For longer

astrophoto exposures (typically 30 minutes or more), you may need to

refine the polar alignment further to avoid field rotation issues.



You don't need Gemini to improve your polar alignment. Use the drift

alignment method: all it requires is a telescope and a crosshair

eyepiece. Here is a good description of the process:



www.darkskyimages.com/gpolar.html



With some practice, this should take 15-20 minutes after using the

polar scope.



Regards,



-Paul









--- In Losmandy_users@yahoogroups.com, "Debbie " astrodeb@c...>

wrote: >

> Back in December, I did the two star alignment with the polar scope.

> I was able to do 30 sec unguided prime focus exposures but in the

> future I'd like to get up to a minute or longer. If I did a three

> star alignment will this be accurate enough to do 3-5 minute

> exposures with guiding? I'm working with focal lengths of 880mm or

> less. Will the three star alignment be accurate enough for prime

> focus astrophotography or should I invest in the Gemini Goto to get

> better results? Any help would be appreciated.

>

> Debbie



----------------------------

#24051 Jan 25, 2005

Hey Debbie,



--- Debbie astrodeb@...> wrote: > Back in December, I did the two star alignment with the polar scope.

> I was able to do 30 sec unguided prime focus exposures but in the

> future I'd like to get up to a minute or longer. If I did a three

> star alignment will this be accurate enough to do 3-5 minute

> exposures with guiding? I'm working with focal lengths of 880mm or

> less. Will the three star alignment be accurate enough for prime

> focus astrophotography or should I invest in the Gemini Goto to get

> better results? Any help would be appreciated.



As John suggested, do a drift alignment. As far as Gemini goes, I think

it's a great investment. I've found that drifting can take up a fair

amount of time. Gemini has features (Polar Axis Correction) that allow

me to bypass drifting completely and be ready to image as soon as it's

dark enough. Because all I do is astrophotography, I also thought

Gemini would be a time saver allowing GOTOs to objects too faint to see

visually. I think it all works together beautifully, for me at least.



Alan







=====

SCT Astrophotography: www.pbase.com/avoetsch/astrophotography

FS-102 G-11/Gemini: www.pbase.com/avoetsch12952/tak_fs102

& www.pbase.com/avoetsch12952/fs102







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#24053 Jan 25, 2005

--- In Losmandy_users@yahoogroups.com, "Debbie " astrodeb@c...> wrote: >

> Back in December, I did the two star alignment with the polar scope.

> I was able to do 30 sec unguided prime focus exposures but in the

> future I'd like to get up to a minute or longer. If I did a three

> star alignment will this be accurate enough to do 3-5 minute

> exposures with guiding? I'm working with focal lengths of 880mm or

> less. Will the three star alignment be accurate enough for prime

> focus astrophotography or should I invest in the Gemini Goto to get

> better results? Any help would be appreciated.

>

> Debbie



Why not try it and see? Lots cheaper than buying Gemini...



My last prime focus work was with three star alignment at an effective

focal length of 1600 mm (1000 x 1.6, telescope + Digital Rebel). My 5

minute exposures were successful, with only the occasional nudge on

the guide star.



Laura Halliday VE7LDH "Que les nuages soient notre

Grid: CN89mg pied a terre..."

ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte







----------------------------

#24055 Jan 25, 2005

I'll try the three star alignment next time out and practice the

drift alignment method. The drift method seems pretty

straightforward. With practice, hopefully 15-20 minutes is all it

will take. I have the Digital Rebel as well so I don't think I'll be

taking exposures longer than 5-7 minutes due to noise from the

camera. Thanks for the help.



Debbie



--- In Losmandy_users@yahoogroups.com, "laurahal42"

laura.halliday@t...> wrote: >

> --- In Losmandy_users@yahoogroups.com, "Debbie " astrodeb@c...>

wrote: > >

> > Back in December, I did the two star alignment with the polar

scope. > > I was able to do 30 sec unguided prime focus exposures but in

the > > future I'd like to get up to a minute or longer. If I did a

three > > star alignment will this be accurate enough to do 3-5 minute

> > exposures with guiding? I'm working with focal lengths of 880mm

or > > less. Will the three star alignment be accurate enough for prime

> > focus astrophotography or should I invest in the Gemini Goto to

get > > better results? Any help would be appreciated.

> >

> > Debbie

>

> Why not try it and see? Lots cheaper than buying Gemini...

>

> My last prime focus work was with three star alignment at an

effective > focal length of 1600 mm (1000 x 1.6, telescope + Digital Rebel).

My 5 > minute exposures were successful, with only the occasional nudge on

> the guide star.

>

> Laura Halliday VE7LDH "Que les nuages soient notre

> Grid: CN89mg pied a terre..."

> ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte



----------------------------

#24056 Jan 25, 2005

Deb,

A good polarscope is a great thing. Don't know if you're aware

that the reticle is collimatible. Start with a perfecty levelled

mount. Check the polar scope in daylight with a distant object

centered, to establish that the scope's optical axis is well aligned

with the polar axis. Then, at nightfall, do a polar scope sighting,

then do a careful drift alignment. Once you've established no drift

for say 15 minutes, go back to the reticle and check the orientation

of stars relative to the reticle. if they're now off, you want to

carefully collimate the reticle to match the stars. There are 3

allen grub screws for this. I believe the newer scopes have a

reasonably sized head which comes in standard Allen kits.



Debbie, be VERY careful. Do it iteratively, loosening two screws

slightly and push with the third. When done, just barely snug each

screw to hold the adjustment. Torqueing down will crack the glass

reticle!



To really take advantage of your cameras's 8-minute capability,

especially at a fairly long f.l. of 880mm, you'll need to guide.



Truly, warren



--- In Losmandy_users@yahoogroups.com, "Debbie " astrodeb@c...>

wrote: >

> I'll try the three star alignment next time out and practice the

> drift alignment method. The drift method seems pretty

> straightforward. With practice, hopefully 15-20 minutes is all it

> will take. I have the Digital Rebel as well so I don't think I'll

be > taking exposures longer than 5-7 minutes due to noise from the

> camera. Thanks for the help.

>

> Debbie

>

> --- In Losmandy_users@yahoogroups.com, "laurahal42"

> laura.halliday@t...> wrote:

> >

> > --- In Losmandy_users@yahoogroups.com, "Debbie " astrodeb@c...>

> wrote:

> > >

> > > Back in December, I did the two star alignment with the polar

> scope.

> > > I was able to do 30 sec unguided prime focus exposures but in

> the

> > > future I'd like to get up to a minute or longer. If I did a

> three

> > > star alignment will this be accurate enough to do 3-5 minute

> > > exposures with guiding? I'm working with focal lengths of

880mm > or

> > > less. Will the three star alignment be accurate enough for

prime > > > focus astrophotography or should I invest in the Gemini Goto

to > get

> > > better results? Any help would be appreciated.

> > >

> > > Debbie

> >

> > Why not try it and see? Lots cheaper than buying Gemini...

> >

> > My last prime focus work was with three star alignment at an

> effective

> > focal length of 1600 mm (1000 x 1.6, telescope + Digital Rebel).

> My 5

> > minute exposures were successful, with only the occasional nudge

on > > the guide star.

> >

> > Laura Halliday VE7LDH "Que les nuages soient notre

> > Grid: CN89mg pied a terre..."

> > ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte



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