VintageBigBlue.org

 

Re: Periodic Error testing...


Mar 21, 2005

 


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

#24782 Mar 21, 2005

Hi Everyone,



I finally have a scope and am ready to test my mount :) ! Is there

someone out there willing to send me in the correct direction. I

have never done this before so I need all the help I can get. I am

planning to use my Sky-90/Ext-Q with a D100. I have picked up a

cable release so i can hold the shutter openfor the required 4

min. . It is my understanding that all I need to do is to just place

the mount on the ground, point it at a stat, focus the camera and

open the shutter. Is this how it is done or am I all wet?



Brian



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

#24783 Mar 21, 2005

you should do a rough polar alignment and then, using the altitude

adjuster, move the mount about 5 (if using a DSLR) or 10 (35mm)

degrees above or below the pole.

Next, attach the camera to your scope, focus it, and aim it at a star

about 20deg above the eastern horizon. Now open the shutter on "B" and

expose for about 8-32

minutes (IE multiples of 4min). Time the exposure precisely but do not

touch the scope during the exposure. When you have the film developed

you will see that the stars are trailed. Close inspection will reveal

that the trails will have a wavy appearance, this is PE. A perfect

drive would have straight line star trails. A good drive will show a

smooth sine wave appearance. A poor drive will have many jagged edges

in the star trail. You can then digitize the image and analyse it, or

you can project it on the wall. I use the GIMP (www.gimp.org)freeware

to analyse my star trails.



You will need to know the image scale of your OTA



Use the measuring tool in the GIMP and draw a line from the first

pixel to the last pixel of the star trail. The tool will report the

length in number of pixels and the angle between the first and last

points. Rotate the image in the opposite direction by the same angle.

You will now have a star trail that is completely horizontal. Dividing

the number of pixels in length by the time of the exposure will give

you the number of seconds per pixel and you can then see where each

cycle of the worm begins. Dividing the image scale by the number of

pixels across one dimension of the image will give you the number of

arc seconds per pixel. You find the total PE by finding the position

of the pixels in the star trail closest to the top of the image and

the pixel closest to the bottom of the image. The difference in the

number of pixels between these two points, is the total PE of the

drive. Multiply the this number of pixels by the image scale to give

you the PE in arc seconds. You can examine the trail to see if there

are any sudden vertical shifts in the trail, these represent rapid or

high frequency errors which will be a limiting factor in deciding how

many seconds you can allow between guiding corrections. If your drive

has a smooth sine wave appearance you can calculate the number of

pixels of vertical movement per unit of time which will represent the

longest time that you can guide without correction based upon the

focal length of your OTA.



cheers



Duncan





--- In Losmandy_users@yahoogroups.com, "astroimage2002" b_rich@j...>

wrote: >

> Hi Everyone,

>

> I finally have a scope and am ready to test my mount :) ! Is there

> someone out there willing to send me in the correct direction. I

> have never done this before so I need all the help I can get. I am

> planning to use my Sky-90/Ext-Q with a D100. I have picked up a

> cable release so i can hold the shutter openfor the required 4

> min. . It is my understanding that all I need to do is to just place

> the mount on the ground, point it at a stat, focus the camera and

> open the shutter. Is this how it is done or am I all wet?

>

> Brian



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

#24790 Mar 22, 2005

Duncan,

Thank you for your reply! WELL written and informative, it give me a

direction to attempt.

Brian

--- In Losmandy_users@yahoogroups.com, "dunmunro1" dmunro@s...>

wrote: >

> you should do a rough polar alignment and then, using the altitude

> adjuster, move the mount about 5 (if using a DSLR) or 10 (35mm)

> degrees above or below the pole.

> Next, attach the camera to your scope, focus it, and aim it at a

star > about 20deg above the eastern horizon. Now open the shutter on "B"

and > expose for about 8-32

> minutes (IE multiples of 4min). Time the exposure precisely but do

not > touch the scope during the exposure. When you have the film

developed > you will see that the stars are trailed. Close inspection will

reveal > that the trails will have a wavy appearance, this is PE. A perfect

> drive would have straight line star trails. A good drive will show

a > smooth sine wave appearance. A poor drive will have many jagged

edges > in the star trail. You can then digitize the image and analyse it,

or > you can project it on the wall. I use the GIMP (www.gimp.org)

freeware > to analyse my star trails.

>

> You will need to know the image scale of your OTA

>

> Use the measuring tool in the GIMP and draw a line from the first

> pixel to the last pixel of the star trail. The tool will report the

> length in number of pixels and the angle between the first and last

> points. Rotate the image in the opposite direction by the same

angle. > You will now have a star trail that is completely horizontal.

Dividing > the number of pixels in length by the time of the exposure will

give > you the number of seconds per pixel and you can then see where each

> cycle of the worm begins. Dividing the image scale by the number of

> pixels across one dimension of the image will give you the number

of > arc seconds per pixel. You find the total PE by finding the

position > of the pixels in the star trail closest to the top of the image and

> the pixel closest to the bottom of the image. The difference in the

> number of pixels between these two points, is the total PE of the

> drive. Multiply the this number of pixels by the image scale to

give > you the PE in arc seconds. You can examine the trail to see if

there > are any sudden vertical shifts in the trail, these represent rapid

or > high frequency errors which will be a limiting factor in deciding

how > many seconds you can allow between guiding corrections. If your

drive > has a smooth sine wave appearance you can calculate the number of

> pixels of vertical movement per unit of time which will represent

the > longest time that you can guide without correction based upon the

> focal length of your OTA.

>

> cheers

>

> Duncan

>

>

> --- In Losmandy_users@yahoogroups.com, "astroimage2002"

b_rich@j...> > wrote:

> >

> > Hi Everyone,

> >

> > I finally have a scope and am ready to test my mount :) ! Is

there > > someone out there willing to send me in the correct direction. I

> > have never done this before so I need all the help I can get. I

am > > planning to use my Sky-90/Ext-Q with a D100. I have picked up a

> > cable release so i can hold the shutter openfor the required 4

> > min. . It is my understanding that all I need to do is to just

place > > the mount on the ground, point it at a stat, focus the camera

and > > open the shutter. Is this how it is done or am I all wet?

> >

> > Brian







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

#24871 Mar 27, 2005

Duncan or anyone else,

I managed to get a 4 min. worm cycle shot off.... I put it in the

files section. The image scale is 1.71 arcsec/pixel. Could you let

me know what the error is?



Brian





--- In Losmandy_users@yahoogroups.com, "astroimage2002"

b_rich@j...> wrote: >

> Duncan,

> Thank you for your reply! WELL written and informative, it give me

a > direction to attempt.

> Brian

> --- In Losmandy_users@yahoogroups.com, "dunmunro1" dmunro@s...>

> wrote:

> >

> > you should do a rough polar alignment and then, using the

altitude > > adjuster, move the mount about 5 (if using a DSLR) or 10 (35mm)

> > degrees above or below the pole.

> > Next, attach the camera to your scope, focus it, and aim it at a

> star

> > about 20deg above the eastern horizon. Now open the shutter

on "B" > and

> > expose for about 8-32

> > minutes (IE multiples of 4min). Time the exposure precisely but

do > not

> > touch the scope during the exposure. When you have the film

> developed

> > you will see that the stars are trailed. Close inspection will

> reveal

> > that the trails will have a wavy appearance, this is PE. A

perfect > > drive would have straight line star trails. A good drive will

show > a

> > smooth sine wave appearance. A poor drive will have many jagged

> edges

> > in the star trail. You can then digitize the image and analyse

it, > or

> > you can project it on the wall. I use the GIMP (www.gimp.org)

> freeware

> > to analyse my star trails.

> >

> > You will need to know the image scale of your OTA

> >

> > Use the measuring tool in the GIMP and draw a line from the first

> > pixel to the last pixel of the star trail. The tool will report

the > > length in number of pixels and the angle between the first and

last > > points. Rotate the image in the opposite direction by the same

> angle.

> > You will now have a star trail that is completely horizontal.

> Dividing

> > the number of pixels in length by the time of the exposure will

> give

> > you the number of seconds per pixel and you can then see where

each > > cycle of the worm begins. Dividing the image scale by the number

of > > pixels across one dimension of the image will give you the

number > of

> > arc seconds per pixel. You find the total PE by finding the

> position

> > of the pixels in the star trail closest to the top of the image

and > > the pixel closest to the bottom of the image. The difference in

the > > number of pixels between these two points, is the total PE of the

> > drive. Multiply the this number of pixels by the image scale to

> give

> > you the PE in arc seconds. You can examine the trail to see if

> there

> > are any sudden vertical shifts in the trail, these represent

rapid > or

> > high frequency errors which will be a limiting factor in

deciding > how

> > many seconds you can allow between guiding corrections. If your

> drive

> > has a smooth sine wave appearance you can calculate the number of

> > pixels of vertical movement per unit of time which will

represent > the

> > longest time that you can guide without correction based upon the

> > focal length of your OTA.

> >

> > cheers

> >

> > Duncan

> >

> >

> > --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> b_rich@j...>

> > wrote:

> > >

> > > Hi Everyone,

> > >

> > > I finally have a scope and am ready to test my mount :) ! Is

> there

> > > someone out there willing to send me in the correct direction.

I > > > have never done this before so I need all the help I can get.

I > am

> > > planning to use my Sky-90/Ext-Q with a D100. I have picked up

a > > > cable release so i can hold the shutter openfor the required 4

> > > min. . It is my understanding that all I need to do is to just

> place

> > > the mount on the ground, point it at a stat, focus the camera

> and

> > > open the shutter. Is this how it is done or am I all wet?

> > >

> > > Brian



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

#24878 Mar 27, 2005

Brian, I uploaded a file "losmpe.jpg" to the files section where I

took one of the fainter star trails and boosted the contrast. I guess

the total PE is 7 or 8 pixels which would be 14 to 16 arc seconds.

However, it appears as though this image is defocused to some extent,

which makes it very difficult to determine the centre of the star

trail. The bright star trail is almost 10 pixels high or about 20 arc

seconds. It is essential to get a fairly accurate focus.



cheers



Duncan



www.sfu.ca/%7Edmunro/petest.html



--- In Losmandy_users@yahoogroups.com, "astroimage2002" b_rich@j...>

wrote: >

> All,

> I gave the wrong image scale. Stupid mistake! The correct scale is

> 1.99 arc sec. per pixel

> Brian

>

> --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> b_rich@j...> wrote:

> >

> > Duncan or anyone else,

> > I managed to get a 4 min. worm cycle shot off.... I put it in the

> > files section. The image scale is 1.71 arcsec/pixel. Could you let

> > me know what the error is?

> >

> > Brian

> >

> >

> > --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> > b_rich@j...> wrote:

> > >

> > > Duncan,

> > > Thank you for your reply! WELL written and informative, it give

> me

> > a

> > > direction to attempt.

> > > Brian

> > > --- In Losmandy_users@yahoogroups.com, "dunmunro1" dmunro@s...>

> > > wrote:

> > > >

> > > > you should do a rough polar alignment and then, using the

> > altitude

> > > > adjuster, move the mount about 5 (if using a DSLR) or 10 (35mm)

> > > > degrees above or below the pole.

> > > > Next, attach the camera to your scope, focus it, and aim it at

> a

> > > star

> > > > about 20deg above the eastern horizon. Now open the shutter

> > on "B"

> > > and

> > > > expose for about 8-32

> > > > minutes (IE multiples of 4min). Time the exposure precisely

> but

> > do

> > > not

> > > > touch the scope during the exposure. When you have the film

> > > developed

> > > > you will see that the stars are trailed. Close inspection will

> > > reveal

> > > > that the trails will have a wavy appearance, this is PE. A

> > perfect

> > > > drive would have straight line star trails. A good drive will

> > show

> > > a

> > > > smooth sine wave appearance. A poor drive will have many

> jagged

> > > edges

> > > > in the star trail. You can then digitize the image and analyse

> > it,

> > > or

> > > > you can project it on the wall. I use the GIMP (www.gimp.org)

> > > freeware

> > > > to analyse my star trails.

> > > >

> > > > You will need to know the image scale of your OTA

> > > >

> > > > Use the measuring tool in the GIMP and draw a line from the

> first

> > > > pixel to the last pixel of the star trail. The tool will

> report

> > the

> > > > length in number of pixels and the angle between the first and

> > last

> > > > points. Rotate the image in the opposite direction by the same

> > > angle.

> > > > You will now have a star trail that is completely horizontal.

> > > Dividing

> > > > the number of pixels in length by the time of the exposure

> will

> > > give

> > > > you the number of seconds per pixel and you can then see where

> > each

> > > > cycle of the worm begins. Dividing the image scale by the

> number

> > of

> > > > pixels across one dimension of the image will give you the

> > number

> > > of

> > > > arc seconds per pixel. You find the total PE by finding the

> > > position

> > > > of the pixels in the star trail closest to the top of the

> image

> > and

> > > > the pixel closest to the bottom of the image. The difference

> in

> > the

> > > > number of pixels between these two points, is the total PE of

> the

> > > > drive. Multiply the this number of pixels by the image scale

> to

> > > give

> > > > you the PE in arc seconds. You can examine the trail to see if

> > > there

> > > > are any sudden vertical shifts in the trail, these represent

> > rapid

> > > or

> > > > high frequency errors which will be a limiting factor in

> > deciding

> > > how

> > > > many seconds you can allow between guiding corrections. If

> your

> > > drive

> > > > has a smooth sine wave appearance you can calculate the number

> of

> > > > pixels of vertical movement per unit of time which will

> > represent

> > > the

> > > > longest time that you can guide without correction based upon

> the

> > > > focal length of your OTA.

> > > >

> > > > cheers

> > > >

> > > > Duncan

> > > >

> > > >

> > > > --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> > > b_rich@j...>

> > > > wrote:

> > > > >

> > > > > Hi Everyone,

> > > > >

> > > > > I finally have a scope and am ready to test my mount :) ! Is

> > > there

> > > > > someone out there willing to send me in the correct

> direction.

> > I

> > > > > have never done this before so I need all the help I can

> get.

> > I

> > > am

> > > > > planning to use my Sky-90/Ext-Q with a D100. I have picked

> up

> > a

> > > > > cable release so i can hold the shutter openfor the required

> 4

> > > > > min. . It is my understanding that all I need to do is to

> just

> > > place

> > > > > the mount on the ground, point it at a stat, focus the

> camera

> > > and

> > > > > open the shutter. Is this how it is done or am I all wet?

> > > > >

> > > > > Brian







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

#24879 Mar 28, 2005

Duncan,

I thank you for your help :). and I will try agan with a better

focus next time that I can see stars. The skys here have not been

good. I was just happy to be outside!



Brian



-- In Losmandy_users@yahoogroups.com, "dunmunro1" dmunro@s...>

wrote: >

> Brian, I uploaded a file "losmpe.jpg" to the files section where I

> took one of the fainter star trails and boosted the contrast. I

guess > the total PE is 7 or 8 pixels which would be 14 to 16 arc seconds.

> However, it appears as though this image is defocused to some

extent, > which makes it very difficult to determine the centre of the star

> trail. The bright star trail is almost 10 pixels high or about 20

arc > seconds. It is essential to get a fairly accurate focus.

>

> cheers

>

> Duncan

>

> www.sfu.ca/%7Edmunro/petest.html

>

> --- In Losmandy_users@yahoogroups.com, "astroimage2002"

b_rich@j...> > wrote:

> >

> > All,

> > I gave the wrong image scale. Stupid mistake! The correct scale

is > > 1.99 arc sec. per pixel

> > Brian

> >

> > --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> > b_rich@j...> wrote:

> > >

> > > Duncan or anyone else,

> > > I managed to get a 4 min. worm cycle shot off.... I put it in

the > > > files section. The image scale is 1.71 arcsec/pixel. Could you

let > > > me know what the error is?

> > >

> > > Brian

> > >

> > >

> > > --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> > > b_rich@j...> wrote:

> > > >

> > > > Duncan,

> > > > Thank you for your reply! WELL written and informative, it

give > > me

> > > a

> > > > direction to attempt.

> > > > Brian

> > > > --- In Losmandy_users@yahoogroups.com, "dunmunro1"

dmunro@s...> > > > > wrote:

> > > > >

> > > > > you should do a rough polar alignment and then, using the

> > > altitude

> > > > > adjuster, move the mount about 5 (if using a DSLR) or 10

(35mm) > > > > > degrees above or below the pole.

> > > > > Next, attach the camera to your scope, focus it, and aim

it at > > a

> > > > star

> > > > > about 20deg above the eastern horizon. Now open the

shutter > > > on "B"

> > > > and

> > > > > expose for about 8-32

> > > > > minutes (IE multiples of 4min). Time the exposure

precisely > > but

> > > do

> > > > not

> > > > > touch the scope during the exposure. When you have the

film > > > > developed

> > > > > you will see that the stars are trailed. Close inspection

will > > > > reveal

> > > > > that the trails will have a wavy appearance, this is PE. A

> > > perfect

> > > > > drive would have straight line star trails. A good drive

will > > > show

> > > > a

> > > > > smooth sine wave appearance. A poor drive will have many

> > jagged

> > > > edges

> > > > > in the star trail. You can then digitize the image and

analyse > > > it,

> > > > or

> > > > > you can project it on the wall. I use the GIMP

(www.gimp.org) > > > > freeware

> > > > > to analyse my star trails.

> > > > >

> > > > > You will need to know the image scale of your OTA

> > > > >

> > > > > Use the measuring tool in the GIMP and draw a line from

the > > first

> > > > > pixel to the last pixel of the star trail. The tool will

> > report

> > > the

> > > > > length in number of pixels and the angle between the first

and > > > last

> > > > > points. Rotate the image in the opposite direction by the

same > > > > angle.

> > > > > You will now have a star trail that is completely

horizontal. > > > > Dividing

> > > > > the number of pixels in length by the time of the exposure

> > will

> > > > give

> > > > > you the number of seconds per pixel and you can then see

where > > > each

> > > > > cycle of the worm begins. Dividing the image scale by the

> > number

> > > of

> > > > > pixels across one dimension of the image will give you the

> > > number

> > > > of

> > > > > arc seconds per pixel. You find the total PE by finding

the > > > > position

> > > > > of the pixels in the star trail closest to the top of the

> > image

> > > and

> > > > > the pixel closest to the bottom of the image. The

difference > > in

> > > the

> > > > > number of pixels between these two points, is the total PE

of > > the

> > > > > drive. Multiply the this number of pixels by the image

scale > > to

> > > > give

> > > > > you the PE in arc seconds. You can examine the trail to

see if > > > > there

> > > > > are any sudden vertical shifts in the trail, these

represent > > > rapid

> > > > or

> > > > > high frequency errors which will be a limiting factor in

> > > deciding

> > > > how

> > > > > many seconds you can allow between guiding corrections. If

> > your

> > > > drive

> > > > > has a smooth sine wave appearance you can calculate the

number > > of

> > > > > pixels of vertical movement per unit of time which will

> > > represent

> > > > the

> > > > > longest time that you can guide without correction based

upon > > the

> > > > > focal length of your OTA.

> > > > >

> > > > > cheers

> > > > >

> > > > > Duncan

> > > > >

> > > > >

> > > > > --- In Losmandy_users@yahoogroups.com, "astroimage2002"

> > > > b_rich@j...>

> > > > > wrote:

> > > > > >

> > > > > > Hi Everyone,

> > > > > >

> > > > > > I finally have a scope and am ready to test my

mount :) ! Is > > > > there

> > > > > > someone out there willing to send me in the correct

> > direction.

> > > I

> > > > > > have never done this before so I need all the help I can

> > get.

> > > I

> > > > am

> > > > > > planning to use my Sky-90/Ext-Q with a D100. I have

picked > > up

> > > a

> > > > > > cable release so i can hold the shutter openfor the

required > > 4

> > > > > > min. . It is my understanding that all I need to do is

to > > just

> > > > place

> > > > > > the mount on the ground, point it at a stat, focus the

> > camera

> > > > and

> > > > > > open the shutter. Is this how it is done or am I all wet?

> > > > > >

> > > > > > Brian







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