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Astro-Physics RAPAS or Revised QDA Method

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#1 FaronD

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Posted 09 May 2013 - 07:02 PM

The days are getting longer leaving less time for setup throughout the summer months. I would like to speed up the polar alignment process and get to imaging. There are two different PA options that I'm interested in for my Mach 1, both requiring some cash outlay. The first method and least expensive is the revised QDA (quick drift alignment). A purchase of an angle 9x50 finder scope is preferred for this method. The second option is to purchase the new Astro-Physics RAPAS. The RAPAS is a pricey PA scope and perhaps overkill if the QDA is as easy as described by Roland.

Any opinions on either method would be appreciated. I have no problems purchasing the RAPAS if it performs like the Tak PA scope.

Thanks,
Faron

#2 psu_13

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Posted 09 May 2013 - 07:51 PM

I used quick drift for about the first year I had my Mach-1. It works well as long as you can find the stars you need. But sometimes it can be frustrating if the scheme doesn't converge. I never measured how accurate the alignments were, but I was always able to do 2-5min exposures without any problems. I could get set up in about 20min with this scheme pretty consistently. It takes some planning because you have to pick good stars yourself.

I bought a RAPAS when they came out because it gets you close with one look rather than the iterative quick drift scheme. Then since I have the camera on the scope anyway, I usually do a pass or two of drift alignment with PemPro. This takes about 20min while it gets dark outside, which is about how long the QD method used to take me, give or take.

The tricky thing with the RAPAS is that you have to align it to you mount. Mine is close, but the alignment screws are not the easiest thing to manipulate, so I ended up just giving up and refining with PemPro, which I can do while doing the dishes in the house. :-)

If you are more mechanically adept than me (and everyone is) then it's perfectly possible to get this set up to be as accurate as the Tak scope, IMHO. This should allow you to get repeatable alignments in very little time.

#3 FaronD

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Posted 09 May 2013 - 08:31 PM

Thanks for the reply. That's exactly the real world info I'm looking for. I've been using Alignmaster, which works great when I have visual access to the listed stars, right now I don't. This as been an issue a few times already. I should look into Pempro as well. However, I don't think either of my cams will work with it. I have a QHY8 and a QHY5.

#4 Footbag

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Posted 09 May 2013 - 09:04 PM

I've been using the QD method recently. I just upgraded my finderscope for it. It works well, but takes some time. 10-15m if you do everything right. It's not so hard to do, but there are a few steps that can mess you up.

The reports on the RAPAS are good. It seems like it could replace drift alignment if properly aligned. If I wasn't building an observatory, I'd spring for the RAPAS.

#5 FaronD

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Posted 09 May 2013 - 09:41 PM

Thanks Adam. 10-15 mins is fine. I'm concerned about not having a recognizable star at times for the QDA. During the summer months where I'm located, I have to wait until 10:30 before it's completely dark. Precious time can't be wasted, I'm seriously considering the RAPAS.

Faron

#6 woodworkt

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Posted 10 May 2013 - 05:31 AM

For using a CCD with PEMPro, it seems more dependent on software selection than camera selection. It should run whatever through Maxim, but I don't think it supports nebulosity, for instance.

RAPAS is looking really interesting, though, esp. for those of us who have to drive out to a dark site and set up and polar align every time we want to do imaging.

--Ken T.

#7 psu_13

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Posted 10 May 2013 - 06:40 AM

One further note. When I did QD I used to be able to set up when it was not as dark out. I had the original Losmandy-based polar scope and would use it to get close to Polaris. Then I'd use QD after that. With the finder and bright stars you can do this well before the sky is even a dark gray. I find that with the RAPAS it needs to be a bit darker before I can see Polaris in the scope. Not sure why.

#8 Footbag

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Posted 10 May 2013 - 07:27 AM

One further note. When I did QD I used to be able to set up when it was not as dark out. I had the original Losmandy-based polar scope and would use it to get close to Polaris. Then I'd use QD after that. With the finder and bright stars you can do this well before the sky is even a dark gray. I find that with the RAPAS it needs to be a bit darker before I can see Polaris in the scope. Not sure why.


I believe the FL is longer on the RAPAS. From what I've seen the aperture is close, so the RAPAS must just be a bit slower.

#9 dawziecat

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Posted 10 May 2013 - 07:28 AM

Alignmaster was a failure for me. Too many trees to be able to get the star pairs required. I gather the A-P QD method might be the same, i.e. not really practical in a forest?

My RAPAS shipped two days ago. It will go on the AP1100 I will order as soon as A-P are actually accepting orders for them.

I have not been able to find anything on the RAPAS other than an illustration of what the reticle looks like. So, I don't know how to use it. Hopefully it comes with instructions?

I gather I will have to drift align the AP1100 as accurately as I can, then index the PAPAS to it?

I could not find any instructions on the A-P site as to how to use the RAPAS.

#10 Footbag

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Posted 10 May 2013 - 07:35 AM

This is the closest thing to instructions I've seen. From the AP Yahoo group. Ray also suggests it's good to level the mount in E-W first as well.

1. Mount the RAPAS on the adapter.
2. Polar align as best you can using any method you choose (drift, Pempro,
etc.).
3. Adjust the push-pins on the adapter to place polaris where it should be based
on the simulated reticle for a one time adjustment.
4. Next time, just use the reticle to align the scope and place Polaris in the
correct quadrant according to the app/driver graphical disply.

#11 Starhawk

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Posted 10 May 2013 - 10:01 AM

I'm at the point of just doing slow drift after the onboard polar routine in the Mach 1. It is far less dramatic than zipping back and forth across the sky, but it is always perfectly aligned, where my quick drifts have yet to be that good, so I keep trying for long enough to have been able to do a conventional drift.

I've talked to them about a precision alignment auto routine. Supposedly that's in Howard's queue, somewhere.

As tedious as the slow drift method is, you are assured of a precision alignment.

-Rich

#12 Footbag

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Posted 10 May 2013 - 10:50 AM

I'm at the point of just doing slow drift after the onboard polar routine in the Mach 1. It is far less dramatic than zipping back and forth across the sky, but it is always perfectly aligned, where my quick drifts have yet to be that good, so I keep trying for long enough to have been able to do a conventional drift.

I've talked to them about a precision alignment auto routine. Supposedly that's in Howard's queue, somewhere.

As tedious as the slow drift method is, you are assured of a precision alignment.

-Rich


Is that a drift with a reticle? I've been using PHD for my drift aligns, and the theory is the same, but PHD lets you see the movements much quicker. In seconds rather then minutes.

I'm back and forth over whether I prefer that or the QD method. It seems stupid, but one of the things I dislike about the QD method is that my finderscope is no longer aligned with my OTA. Since my orthogonality is so far off, aligning it under the stars isn't very easy.

#13 Peter in Reno

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Posted 10 May 2013 - 11:06 AM

That's why I have two finderscopes mounted on my OTA. One for QDA and other for aligned with OTA. Also, it seems to help to balance. Having only one finderscope mounted at one side can slightly affect the balance. It's probably not noticable with A-P mounts but was noticable when I had Atlas EQ-G mount.

Two finderscopes on C-8 EdgeHD

It's a good way to compare two finderscopes to see how much off the orthogonality is.

Peter

#14 Footbag

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Posted 10 May 2013 - 11:11 AM

That's why I have two finderscopes mounted on my OTA. One for QDA and other for aligned with OTA. Also, it seems to help to balance. Having only one finderscope mounted at one side can slightly affect the balance. It's probably not noticable with A-P mounts but was noticable when I had Atlas EQ-G mount.

Two finderscopes on C-8 EdgeHD

Peter


That thought had crossed my mind more then once. I actually have two finderscopes, my new one and the old one, sitting at home. I just listed one in S&S.

I really have to start getting into the "observatory" mentality and stop spending to make my setup easier. Hopefully, I won't be setting up as frequently.

#15 David Ault

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Posted 10 May 2013 - 01:31 PM

Faron,

I'm using a G11, not a Mach 1, but the technique I use should work with any mount as it's effectively a drift align, except using the motors to move the mount in RA. I don't use a polar scope at all, just a compass and the built in level in the mount to get close to the pole (and of course my latitude). This was not my idea and I saw it referenced on Cloudy Nights somewhere, but darn if I can find where (I wish I could find it again so I could credit them).

Here are the directions I've written down for myself to make this go quickly:
Using Artemis Capture with my ATIK 314E:
- North is to the right
- South is to the left
- East is to the bottom
- West is to the top

1) Correcting East-West
A) Choose a set of faint stars at the intersection of
the celestial equator and the meridian. This needs
to be with 30 arc-minutes of RA and 5 degrees of
declination. Make sure some stars are visible at
a 0.5 second exposure binned 3x3.
B) Set the exposre to 65 seconds and binning to 1x1.
C) Set the tracking rate to "Guide"
D) Start the exposure
E) Leave the star in place for 5 seconds
F) Press the "East" button for 30 seconds
G) Press the "West" button for 30 seconds
H) Note the dot where the star started out with the 5
second exposure.
- If the star drifted to the south turn the knob
on the left counter clockwise (turn the top
towards the mount)
is too far west
- If the star drifted north then turn the knob on
the left clockwise (turn the top away from the
mount)
I) Adjust the east-west appropriately.
J) Repeat steps D through I. As the 'V' shape narrows
increase the exposure time to 125 seconds, then 245
seconds (guiding for 60 and then 120 seconds in each
direction respectively) to increase the precision.
Eventually the 'V' shape should become a line.

2) Correcting North-South
A) Choose a set of faint stars within 5 degrees of the
celestial equator and around 20 degrees above the
eastern (or western) horizon.
B) Set the exposure to 65 seconds and binning to 1x1.
C) Set the tracking rate to "Guide"
D) Start the exposure
E) Leave the star in place for 5 seconds
F) Press the "East" button for 30 seconds
G) Press the "West" button for 30 seconds
H) Note the dot where the star started out with the 5
second exposure
- If the star drifted to the south,
the polar axis is too high
(turn the knob CCW)
- If the star drifted to the north,
the polar axis is too low
(turn the knob cw)
I) Adjust the altitude axis appropriately.
J) Repeat steps D through I. As the 'V' shape narrows
increase the exposure time to 125 seconds, then 245
seconds (guiding for 60 and then 120 seconds in each
direction respectively) to increase the precision.
Eventually the 'V' shape should become a line.

3) Go back and repeat steps 1 & 2 until there is no deviation.
In practice, I've only had to do this once in each direction provided the mount was level.

I'll add that if I am doing under 10 minute exposures I don't go past 65 seconds in either direction. If I'm doing narrowband and need to hit 30 minute subs then I'll go for 2 or sometimes 4 minutes in each direction. I've also found that as long as I am consistent with my leveling I rarely need to make an adjustment to the altitude for the North-South correction (the two primary locations I image from are my back yard and my clubs dark sky site which are at the same latitude).

There are a couple of things I really like about this method. First, I know that for 65 seconds I have less than 1 pixel of drift, regardless of focal length. Second, because you are exposing for an extended time, you don't need to have a bright star. Occasionally I try to start the process early while there is some light out, especially if I'm in an unfamiliar location. This usually limits me to 35 second exposures or less before the sensor saturate, but even then I always find some star trails and am able to identify the V shape.

I'm not sure how the A-P mounts work, but with the G11 I don't have to do any modeling before it starts tracking and I can do gotos, so I usually do this drift alignment method first and then do the modeling. Usually, the whole process takes me under 6 minutes (under 15 if I am being picky for narrowband).

Occasionally, I do the calculation to determine how far off the pole I am (with my 90mm f=630mm scope it's about 1.5 to 2 arc-minutes and with my 8" SCT @ F/10 it's typically under 30 arc-seconds), but I generally don't worry about it as I've never seen any field rotation as long as I drift for the appropriate times.

All this being said, I've never used PemPro, Alignmaster or QDA so I can't really say how they compare, other than the fact that this method is free for anyone with a CCD camera or DSLR.

Regards,
David

#16 Starhawk

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Posted 10 May 2013 - 02:41 PM

I'm a 45 minute drive away from a dark sky, so I really need mobility. So, a lot of the Mach 1 issue for me is in terms of eliminating equipment and then performing setup in a way guarranteed to get alignment. The idea of having an observatory in a dark sky site where I could observe night after night without starting setup from scratch would a dream come true.

So, I'm trying to do it all without a PC deployed in the field.

-Rich

#17 FaronD

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Posted 10 May 2013 - 04:03 PM

Thanks everyone for the replies. Thanks David for the detailed procedure. I went ahead and placed an order for the RAPAS, it's in stock and shipping. This will be the quickest method for me after the initial alignment. Anyone looking for a Pasill4 ;)

Faron






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