Re: Recommendation on guider focal length


Feb 17 4:21 PM

 


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

#27569 Feb 17 4:21 PM

Hi,



I am using as primary imager a DSLR (20Da) with pixels being about 6.5

microns.

My main scope is having FL 1280mm, at F6.4.

I'd like to guide with my FS60 (FL 360mm, F6), using my SXV-H9 (pixels

are also 6.5 microns).

I am not too satisfied so far of the results.

My mount has PEC on, and without guiding has a remaining PE of +-4

arcsec.

Seeing is about 2 arcsec.



I tried to use the FS60 at native focal length and also with TV

PowerMate (2.5x); the results are not bad but on 10 min exposures,

stars are elongated. Results do not look much better with the PowerMate

than at native focal length. When looking at the RMS error, I am below

0.5 pixels RMS over 4 minutes in each direction, when using the

PowerMate.



Any hint ? Is there a rule for matching guide scope focal length with

that of the primary/imaging scope ? I am confused as I remember having

obtained far better results with my STV (now sold, too bad) with the

FS60 at native FL.





Pierre



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

#27570 Feb 17 4:46 PM

phenrotay wrote: > Hi,

>

> I am using as primary imager a DSLR (20Da) with pixels being about 6.5

> microns.

> My main scope is having FL 1280mm, at F6.4.

> I'd like to guide with my FS60 (FL 360mm, F6), using my SXV-H9 (pixels

> are also 6.5 microns).

> I am not too satisfied so far of the results.

> My mount has PEC on, and without guiding has a remaining PE of +-4

> arcsec.

> Seeing is about 2 arcsec.

>

> I tried to use the FS60 at native focal length and also with TV

> PowerMate (2.5x); the results are not bad but on 10 min exposures,

> stars are elongated. Results do not look much better with the PowerMate

> than at native focal length. When looking at the RMS error, I am below

> 0.5 pixels RMS over 4 minutes in each direction, when using the

> PowerMate.

>

> Any hint ? Is there a rule for matching guide scope focal length with

> that of the primary/imaging scope ? I am confused as I remember having

> obtained far better results with my STV (now sold, too bad) with the

> FS60 at native FL.

>

>

>

>



Pierre,



Could it be that you are seeing differential movement ? You don't

mention what type of OTA but unless it's a refractor you could be

setting yourself up for failure. It's hard enough to keep the guider and

main imaging stable relative to each other but a lot of optics are prone

to movement too.





360mm *should* guide 1300mm assuming that there is no unwanted movement.



Regards



Bill



--



William R. Mattil.:.www.celestial-images.com



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

#27577 Feb 18 8:27 AM

Pierre,



For sure you are dealing with a flexure issues. I too have a VC200L and

struggled for 3 months trying to use an external guider chasing my tail -

even buying an ST402 to see *fix* my guiding problems.



I can prove it's flexure: Point the scope at a part of the sky that is no

higher than 45 degrees in alt and pointing due east. Now take a 10 min

guided shot. I think you will find perfectly guided stars. Now point the

scope near the meridian but with dec tilted over so it's no higher in alt

than 45 degrees and take a 10 min guided shot. You should get elongated

stars.



The VC200L has many points where it can flex. The OTA itself is not stiff,

the primary mirror cell might shift a little and the worst offender is the

focuser - it's not a tak! The FC60 you are using is a relatively heavy

little scope and you are going to notice differential movement. In my case,

the math showed that a 1mm flexure differential between the guide scope and

main OTA would result in a 3 pixel shift on the main imaging camera. There

is no winning this battle.



The solution is to use an off-axis guider. I've tried 4 of them and the one

I recommend is the Lumicon Easy guider for Newtonians. It is the thinnest

OAG available off the shelf so you can just barely use the 6.4 vixen reducer

with this scope. The metal back focus for this reducer is 86mm and with the

OAG in place, my placement resulted in just over 90mm. You can also get a

custom adapter machined to hold the reducer in the focuser draw tube to

bring the reducer within spec. I've never tried this combination with a

DSLR. I'm using an SBIG ST series with SWFW8 which has a backfocus of 46mm.

The DSLR has a backfocus of 56mm if I'm not mistaken which might make it

impossible to rack the focuser in far enough to reach focus - so be

forewarned if you try this route.



Cheers,

Stef.



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

From: MaxImDL@yahoogroups.com [mailto:MaxImDL@yahoogroups.com] On Behalf Of

phenrotay

Sent: Sunday, February 18, 2007 3:01 AM

To: MaxImDL@yahoogroups.com

Subject: [MaxImDL] Re: Recommendation on guider focal length



Bill,



thanks for your inp. hereby some precisions.



The main scope is a Vixen VC200L (essentially a Cassegrain), with 8in

aperture.

The guiding scope is a Takahashi FS60C refractor.



What amazes me is:

- the RMS is 0.5, which I believe to be on the small side

- when I used (notice the past tense) my STV, with equivalent

equipment, I had much more consistent results.





Pierre

--- In MaxImDL@yahoogroups.com, "William R. Mattil" wrmattil@...>

wrote: >

> phenrotay wrote:

> > Hi,

> >

> > I am using as primary imager a DSLR (20Da) with pixels being

about 6.5 > > microns.

> > My main scope is having FL 1280mm, at F6.4.

> > I'd like to guide with my FS60 (FL 360mm, F6), using my SXV-H9

(pixels > > are also 6.5 microns).

> > I am not too satisfied so far of the results.

> > My mount has PEC on, and without guiding has a remaining PE of +-

4 > > arcsec.

> > Seeing is about 2 arcsec.

> >

> > I tried to use the FS60 at native focal length and also with TV

> > PowerMate (2.5x); the results are not bad but on 10 min

exposures, > > stars are elongated. Results do not look much better with the

PowerMate > > than at native focal length. When looking at the RMS error, I am

below > > 0.5 pixels RMS over 4 minutes in each direction, when using the

> > PowerMate.

> >

> > Any hint ? Is there a rule for matching guide scope focal length

with > > that of the primary/imaging scope ? I am confused as I remember

having > > obtained far better results with my STV (now sold, too bad) with

the > > FS60 at native FL.

> >

> >

> >

> >

>

> Pierre,

>

> Could it be that you are seeing differential movement ? You don't

> mention what type of OTA but unless it's a refractor you could be

> setting yourself up for failure. It's hard enough to keep the

guider and > main imaging stable relative to each other but a lot of optics are

prone > to movement too.

>

>

> 360mm *should* guide 1300mm assuming that there is no unwanted

movement. >

> Regards

>

> Bill

>

> --

>

> William R. Mattil.:.www.celestial-images.com

>











Yahoo! Groups Links







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

#27589 Feb 19 7:59 AM

Stef,



Thnaks for your ideas. This sounds bad.

What puzzles me is that the results with my STV guider were

consistently OK.

The FS60 is what I use as guider (not the FC60); it is very

lightweight.

I thought of going the off-axis way as well but indeed with the FR6.4

and a DSLR, I believ no one will work because of focus range.



Pierre --- In MaxImDL@yahoogroups.com, "Stef Cancelli" scancelli@...> wrote:

>

> Pierre,

>

> For sure you are dealing with a flexure issues. I too have a VC200L

and

> struggled for 3 months trying to use an external guider chasing my

tail -

> even buying an ST402 to see *fix* my guiding problems.

>

> I can prove it's flexure: Point the scope at a part of the sky that

is no

> higher than 45 degrees in alt and pointing due east. Now take a 10

min

> guided shot. I think you will find perfectly guided stars. Now

point the

> scope near the meridian but with dec tilted over so it's no higher

in alt

> than 45 degrees and take a 10 min guided shot. You should get

elongated

> stars.

>

> The VC200L has many points where it can flex. The OTA itself is not

stiff,

> the primary mirror cell might shift a little and the worst offender

is the

> focuser - it's not a tak! The FC60 you are using is a relatively

heavy

> little scope and you are going to notice differential movement. In

my case,

> the math showed that a 1mm flexure differential between the guide

scope and

> main OTA would result in a 3 pixel shift on the main imaging

camera. There

> is no winning this battle.

>

> The solution is to use an off-axis guider. I've tried 4 of them and

the one

> I recommend is the Lumicon Easy guider for Newtonians. It is the

thinnest

> OAG available off the shelf so you can just barely use the 6.4

vixen reducer

> with this scope. The metal back focus for this reducer is 86mm and

with the

> OAG in place, my placement resulted in just over 90mm. You can also

get a

> custom adapter machined to hold the reducer in the focuser draw

tube to

> bring the reducer within spec. I've never tried this combination

with a

> DSLR. I'm using an SBIG ST series with SWFW8 which has a backfocus

of 46mm.

> The DSLR has a backfocus of 56mm if I'm not mistaken which might

make it

> impossible to rack the focuser in far enough to reach focus - so be

> forewarned if you try this route.

>

> Cheers,

> Stef.

>



Contact Us
This Site's Privacy Policy
Google's privacy policies

S
e
n
i
o
r
T
u
b
e
.
o
r
g