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Re: Imaging with 2 OTA's side by side

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  • Mike Dodd
    ... You ll need to make sure the two OTAs are precisely aligned with each other, or else the edges of the luminance and RGB frames will be lost. ... Well, no,
    Message 1 of 7 , Sep 16, 2009
      fernandorivera3 wrote:
      > What I would like to do
      > is sometime in the near future image with two small APO's, mounted
      > side by side, with one for RGB exposures, the other for Luminance or
      > H-alpha data. Now, assuming that (1) imaging setup is balanced
      > properly, (2) imaging setup is completely "orthogonal", & (3)
      > excellent polar alignment, what issues (besides differential flexure)
      > can arise with such a configuration?

      You'll need to make sure the two OTAs are precisely aligned with each
      other, or else the edges of the luminance and RGB frames will be lost.

      > Speaking of differential
      > flexure, it can be compenstated for by using an off axis guider, a
      > ccd camera with (a) built in off axis guider (QSI) or (b) built in
      > guide chip (SBIG), or physically connecting the guider itself to
      > telescope focuser.

      Well, no, not really. These methods will eliminate flexure for ONE of
      the OTAs, but the potential for it remains on the other OTA.

      In reality, what you are proposing is nothing more than a very large
      guide scope. The issue of differential flexure will remain, and because
      of the size and weight of the "guide scope," I believe it could be a
      non-trivial challenge to eliminate.

      Based on my experience with differential flexure
      <http://astronomy.mdodd.com/flexure.html> I would think twice before
      attempting what you're proposing.

      But are there other options? Is there something I
      > have not considered that would lead to issues/problems?

      How will you control and acquire images from two cameras simultaneously?
      Will you need two computers? I'd be surprised if you could have two
      instances of (e.g.) Maxim or CCDSoft (one for each camera) running at
      the same time on one computer.

      In terms of other options, you might want to consider sticking with one
      imaging OTA and a small external guide scope (60mm f/5 is what I use)
      Invest your money in automation software such as ACP instead of a second
      OTA. With automation software, you don't have to get up in the middle of
      the night to do the meridian flip and, since the software plate-solves
      pointing, framing is very precise, not only after the flip, but also
      from one night to the next. For example, I just completed two nights of
      imaging NGC6979 with ACP, and lost only about 5% of the image due to
      framing differences between the two nights.

      While automation software such as ACP is not a panacea, it certainly
      makes imaging much easier and enjoyable. There is a learning curve, but
      it's a heck of a lot smaller than the learning curve I went through with
      differential flexure.
      -----
      Mike

      Mike Dodd
      Montpelier, VA USA
      http://astronomy.mdodd.com
    • a1parkranger
      Hi Fernando, Here is a relatively easy fix for flexure: http://www.horizontalheavens.com/TMB80_3RingMounted.jpg *I eventually replaced this first ring around
      Message 2 of 7 , Sep 30, 2009
        Hi Fernando,
        Here is a relatively easy fix for flexure:
        http://www.horizontalheavens.com/TMB80_3RingMounted.jpg
        *I eventually replaced this first ring around the focus drawtube with a smaller 90mm ring on a DA. It works best when the three points are just barely snugged while still allowing the motorized focuser to work via FocusMax.

        FWIW, I also changed this setup to a double dovetail and extended a 3rd ring down and onto the NP-127 as well. Sorry, no images of that setup.
        Michael Howell
        http://www.horizontalheavens.com/


        > From: tmboptical@yahoogroups.com [mailto:tmboptical@yahoogroups.com] On
        > Behalf Of fernandorivera3
        > Sent: Saturday, September 12, 2009 7:21 PM
        > To: tmboptical@yahoogroups.com
        > Subject: [tmboptical] Imaging with 2 OTA's side by side
        >
        > Greetings! This is my first post to the user-group, so I will be looking
        > forward to responses to my questions. What I would like to do is sometime in
        > the near future image with two small APO's, mounted side by side, with one
        > for RGB exposures, the other for Luminance or H-alpha data. Now, assuming
        > that (1) imaging setup is balanced properly, (2) imaging setup is completely
        > "orthogonal", & (3) excellent polar alignment, what issues (besides
        > differential flexure) can arise with such a configuration? Speaking of
        > differential flexure, it can be compenstated for by using an off axis
        > guider, a ccd camera with (a) built in off axis guider (QSI) or (b) built in
        > guide chip (SBIG), or physically connecting the guider itself to telescope
        > focuser. But are there other options? Is there something I have not
        > considered that would lead to issues/problems? I am asking this before
        > spending LOTS OF $$$ on a 2nd APO & 2nd ccd camera. So any comments,
        > suggestions, recommendations are greatly appreciated!
        >
        > Regards,
        > Fernando
        >
        >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
      • Mark
        Actually Terry there are a number of benefits to a side by side configuration compared to a piggyback one ... 1) The total equipment weight is closer to the
        Message 3 of 7 , Oct 8, 2009
          Actually Terry there are a number of benefits to a side by side
          configuration compared to a piggyback one ...

          1) The total equipment weight is closer to the mount which means a
          little less counterweight is required. This can be very useful if your
          setup requires close to the maximum available counterweight.

          2) Less weight on a single dovetail bar/plate (the one under the larger
          primary scope). The side by side spreads out the weight of the two
          scopes between the two plates. This is even more of an issue if you've
          got a primary scope with the smaller Vixen compatible dovetail bar.

          3) More flexibility in mounting multiple scopes, with rings you'll
          usually need multiple setups to handle more than two scopes. If you've
          got more than two scopes this can start to become a more important
          issues.

          4) More substantial mounting points compared to some 3 point rings. This
          is admittedly a bit of a two edged sword as the more substantial
          mounting also means less flexibility in pointing. You also need to
          spend some time aligning the saddles (usually a one time event) if
          you're not going to use 3 point rings.

          5) Increased weight carrying capability ... there are some practical
          limits for the piggybacked scope. Sometimes it's costs (really big rings
          and mounting hardware can be expensive), at other times it's weight
          (those bigger rings and mounting hardware can also end up weighing quite
          a bit more than the extra saddle and side by side bar). With a side by
          side configuration you can easily even mount scopes of the same weight
          together.

          Weight imbalance between the scopes is usually not an issue at all. Yes
          you do spend a bit more time on the initial setup getting everything
          balanced but once that's done there's no problem. With a SbS
          configuration you deal with the imbalance by sliding the connecting
          bar/plate sideways which places the primary scope closer to the mount's
          saddle and the lighter scope a bit farther away.

          I run a combination of C11, C6, TMB115 and a AT80ED scopes on a CGE with
          an ADM SbS configuration. For visual work it's usually the C11 and
          TMB115 (30 vs. 15lbs) but for imaging it can be just about any other
          combination. For autoguiding I've also started to use a converted 50mm
          finderscope instead of my 80mm refractor. This smaller guidescope
          configuration is mounted underneath the primary imaging scope on the
          dovetail bar/plate leaving the 2nd saddle open for other uses.

          Mark


          --- In tmboptical@yahoogroups.com, "Terry Tuggle" <tlt284@...> wrote:
          >
          > Fernando,
          >
          > There is no advantage to a side by side setup; in fact if the
          scopes
          > are not the same weight it sets up a built in imbalance; the
          traditional
          > over and under works the best. And for a guide scope the Mini Borg
          guiding
          > scope is top notch; its advantage is the guider can be hard mounted to
          the
          > scope without having to worry about focuser related issues.
          >
          > http://www.hutech.com/ImagAcce.htm
          >
          > The x-y positioner looks like a handy accessory as well.
          >
          >
          >
          > All the best,
          >
          > Terry
          >
          >
          >
          > _____
          >
          > From: tmboptical@yahoogroups.com [mailto:tmboptical@yahoogroups.com]
          On
          > Behalf Of fernandorivera3
          > Sent: Saturday, September 12, 2009 7:21 PM
          > To: tmboptical@yahoogroups.com
          > Subject: [tmboptical] Imaging with 2 OTA's side by side
          >
          >
          >
          >
          >
          > Greetings! This is my first post to the user-group, so I will be
          looking
          > forward to responses to my questions. What I would like to do is
          sometime in
          > the near future image with two small APO's, mounted side by side, with
          one
          > for RGB exposures, the other for Luminance or H-alpha data. Now,
          assuming
          > that (1) imaging setup is balanced properly, (2) imaging setup is
          completely
          > "orthogonal", & (3) excellent polar alignment, what issues (besides
          > differential flexure) can arise with such a configuration? Speaking of
          > differential flexure, it can be compenstated for by using an off axis
          > guider, a ccd camera with (a) built in off axis guider (QSI) or (b)
          built in
          > guide chip (SBIG), or physically connecting the guider itself to
          telescope
          > focuser. But are there other options? Is there something I have not
          > considered that would lead to issues/problems? I am asking this before
          > spending LOTS OF $$$ on a 2nd APO & 2nd ccd camera. So any comments,
          > suggestions, recommendations are greatly appreciated!
          >
          > Regards,
          > Fernando
          >
          >
          >
          >
          >
          > [Non-text portions of this message have been removed]
          >
        • Neil Fleming
          Your reasoning is pretty clear, Mark.  That s the way I thought it would work, too. I have tried both setups, and found that the piggyback worked better for
          Message 4 of 7 , Oct 12, 2009
            Your reasoning is pretty clear, Mark.  That's the way I thought it would work, too.
            I have tried both setups, and found that the piggyback worked better for me.  I had a solid Casady system, then found I had to move to piggyback.
               Cheers...Neil

            www.flemingastrophotography.com  Direct from Boston - brilliant diamonds in pea soup Also check out the astro_narrowband Yahoo group!

            --- On Thu, 10/8/09, Mark <mclewis1@...> wrote:

            From: Mark <mclewis1@...>
            Subject: [tmboptical] Re: Imaging with 2 OTA's side by side
            To: tmboptical@yahoogroups.com
            Date: Thursday, October 8, 2009, 6:47 PM

            Actually Terry there are a number of benefits to a side by side
            configuration compared to a piggyback one  ...

            1) The total equipment weight is closer to the mount which means a
            little less counterweight is required. This can be very useful if your
            setup requires close to the maximum available counterweight.

            2) Less weight on a single dovetail bar/plate (the one under the larger
            primary scope). The side by side spreads out the weight of the two
            scopes between the two plates. This is even more of an issue if you've
            got a primary scope with the smaller Vixen compatible dovetail bar.

            3) More flexibility in mounting multiple scopes, with rings you'll
            usually need multiple setups to handle more than two scopes. If you've
            got more than two scopes this can start to become a more important
            issues.

            4) More substantial mounting points compared to some 3 point rings. This
            is admittedly a bit of a two edged sword as the more substantial
            mounting also means less flexibility in pointing. You also  need to
            spend some time aligning the saddles (usually a one time event)  if
            you're not going to use 3 point rings.

            5) Increased weight carrying capability ... there are some practical
            limits for the piggybacked scope. Sometimes it's costs (really big rings
            and mounting hardware can be expensive), at other times it's weight
            (those bigger rings and mounting hardware can also end up weighing quite
            a bit more than the extra saddle and side by side bar). With a side by
            side configuration you can easily even mount scopes of the same weight
            together.

            Weight imbalance between the scopes is usually not an issue at all. Yes
            you do spend a bit more time on the initial setup getting everything
            balanced but once that's done there's no problem. With a SbS
            configuration you deal with the imbalance by sliding the connecting
            bar/plate sideways which places the primary scope closer to the mount's
            saddle and the lighter scope a bit farther away.

            I run a combination of C11, C6, TMB115 and a AT80ED scopes on a CGE with
            an ADM SbS configuration. For visual work it's usually the C11 and
            TMB115 (30 vs. 15lbs) but for imaging it can be just about any other
            combination. For autoguiding I've also  started to use a converted 50mm
            finderscope instead of my 80mm refractor. This smaller guidescope
            configuration is mounted underneath the primary imaging scope on the
            dovetail bar/plate leaving the 2nd saddle open for other uses.

            Mark


            --- In tmboptical@yahoogroups.com, "Terry Tuggle" <tlt284@...> wrote:
            >
            > Fernando,
            >
            >      There is no advantage to a side by side setup; in fact if the
            scopes
            > are not the same weight it sets up a built in imbalance; the
            traditional
            > over and under works the best. And for a guide scope the Mini Borg
            guiding
            > scope is top notch; its advantage is the guider can be hard mounted to
            the
            > scope without having to worry about focuser related issues.
            >
            > http://www.hutech.com/ImagAcce.htm
            >
            > The x-y positioner looks like a handy accessory as well.
            >
            >
            >
            > All the best,
            >
            > Terry
            >
            >
            >
            >   _____
            >
            > From: tmboptical@yahoogroups.com [mailto:tmboptical@yahoogroups.com]
            On
            > Behalf Of fernandorivera3
            > Sent: Saturday, September 12, 2009 7:21 PM
            > To: tmboptical@yahoogroups.com
            > Subject: [tmboptical] Imaging with 2 OTA's side by side
            >
            >
            >
            >
            >
            > Greetings! This is my first post to the user-group, so I will be
            looking
            > forward to responses to my questions. What I would like to do is
            sometime in
            > the near future image with two small APO's, mounted side by side, with
            one
            > for RGB exposures, the other for Luminance or H-alpha data. Now,
            assuming
            > that (1) imaging setup is balanced properly, (2) imaging setup is
            completely
            > "orthogonal", & (3) excellent polar alignment, what issues (besides
            > differential flexure) can arise with such a configuration? Speaking of
            > differential flexure, it can be compenstated for by using an off axis
            > guider, a ccd camera with (a) built in off axis guider (QSI) or (b)
            built in
            > guide chip (SBIG), or physically connecting the guider itself to
            telescope
            > focuser. But are there other options? Is there something I have not
            > considered that would lead to issues/problems? I am asking this before
            > spending LOTS OF $$$ on a 2nd APO & 2nd ccd camera. So any comments,
            > suggestions, recommendations are greatly appreciated!
            >
            > Regards,
            > Fernando
            >
            >
            >
            >
            >
            > [Non-text portions of this message have been removed]
            >




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

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            [Non-text portions of this message have been removed]
          • a1parkranger
            ... FWIW, I habitually run two instances of CCDSoft (1-guidescope, 1-main CCD) on a single computer and find this to be much more stable as one CCD does not
            Message 5 of 7 , Oct 13, 2009
              --- In tmboptical@yahoogroups.com, Mike Dodd <mike@...> wrote:
              ...<snip>...
              > How will you control and acquire images from two cameras simultaneously?
              > Will you need two computers? I'd be surprised if you could have two
              > instances of (e.g.) Maxim or CCDSoft (one for each camera) running at the same time on one computer.

              FWIW, I habitually run two instances of CCDSoft (1-guidescope, 1-main CCD) on a single computer and find this to be much more stable as one CCD does not take down the other if one hicups. However, the guidescope CCD does pause during main CCD downloads. For this reason a "better" choice would be using 2 CPUs, one for each CCD. This way guiding is not paused and main CCD images can be started sooner without having to let the guider "settle down" after regaining control(if it had actually drifted that much during the short download times). I use a Gemini controlled MI-250 with <+-1.5" PEC off error so this is not a problem in my situation unless shooting at +2000mm FL.

              As far as flexure on a Tandem Bar -- I have a 10"LX200R and my NP-127 mounted on a 14" Cassady Tandem Bar and after using a 3rd 3-point ring on the drawtube extension(need a long dovetail to do this), my flexure issues are gone. I have been able to take a series of 30min subs (ST-10XME) at 2100mm FL on the 10" while guiding on the NP-127(ST-402ME) and had 80% perfect frames. I attribute the bad frames mostly to atmospheric refraction since I was shooting a target that remained between 12-18 degrees above the deep southern horizon during acquistion.

              The comments that Mike D. makes about "added weight" is a biggy. I used to be able to mount my 10" on the left AND my TMB/A&M-80 piggybacked above the NP-127 on the right:
              http://www.horizontalheavens.com/Equipment%20Page.htm
              That was UNTIL I added a double-dovetail between the two refractors so that I could have that "3rd 3-point ring on both APOs. Flezure was cured but all that added weight now means that I can only have ONE refractor tandem with my 10" OTA so I am down to a 2-scope setup. And I am currently running 60.lb of counter-weights on the MI-250 though I have been as high as 70.lb.

              Michael Howell
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