[AZ-Observing] Re: az-observing Digest V19 #67

From: "Dusty" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "dusty_85023" for DMARC)
To: "az-observing@xxxxxxxxxxxxx" <az-observing@xxxxxxxxxxxxx>
Date: Sat, 23 Mar 2019 03:11:14 +0000 (UTC)

Don't these astrograph reflectors have fixed primaries? Seems that would
address the moving mirror issue. Every SCT I've looked thru and focused had a
small shift when focusing. Those Edge mirror locks help but only if you finish
your focus pushing up against gravity on that third side of the support
triangle.
Sent from Yahoo Mail on Android

  On Thu, Mar 21, 2019 at 10:08 PM, FreeLists Mailing List
Manager<ecartis@xxxxxxxxxxxxx> wrote:   az-observing DigestÂ Â Â  Thu, 21 Mar
2019Â Â Â  Volume: 19Â  Issue: 067

In This Issue:
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion
Â Â Â  Â Â Â  Re: photography problems discussion

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

Subject: Re: photography problems discussion
From: Michael Collins <cal_donley@xxxxxxxxxxxx>
Date: Wed, 20 Mar 2019 23:15:43 -0700

On 3/20/19 8:41 PM, Padraig Houlahan wrote:

...the G11 just is not good enough for serious astrophotography in the sense
that exposures longer than about 2 mins are almost certainly
going to be flawed, and maybe 15% of those under 2 mins might not suffer from
uncorrectable (high frequency) periodic error.

Â  Not sure that I would agree with the almost certainly part of this
statement, at least not for every G11. When I had my observatory in the
Valley, I did a fair amount of speckle imaging with a C11 at an
effective focal length of about 5 meters. Since my camera had a very
small (808 x 608) sensor, the field of view was only 4 x 6 arcmin. It
was a challenge putting a target binary on the sensor since mirror shift
appeared to be about 15 arcmin, but I never had a problem keeping the
target within a few pixels even without guiding. While there certainly
was some periodic error evident at that image scale, it was quite smooth.

Â  On the few occasions that I did shoot some images at the native
Cassegrain focus using an 80 mm f/5 guide scope, I obtained what I
considered to be acceptable results.

Â  I wouldn't consider the G11 to be an optimal choice for long exposure
imaging with a C11, but I don't think it's fair to say that it's just
not good enough for serious astrophotography. If a given mount exhibits
high frequency periodic error as Padraig's evidently did, the design is
such that the problem can almost certainly be addressed. There are many
discussion threads on Cloudy Nights which thoroughly cover the topic.
--
<mailto: cal_donley@xxxxxxxxxxxx>

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

From: Padraig Houlahan <pjhmx1@xxxxxxxxx>
Date: Thu, 21 Mar 2019 06:53:27 -0700
Subject: Re: photography problems discussion

Sent from my iPhone

Â Not sure that I would agree with the almost certainly part of this
statement, at least not for every G11.

Iâm not sure I overstated thingsÂ  too much. I have a friend who has
successfully used the G11 and the Titan. I spent a few years trying all
different kinds of strategies using an Orion 80mm guide scope which should have
been adequate. I would get about a 5 to 10% success rate if I was lucky and
never with exposures greater than about 2-3 minutes. I upgraded the worm gears
etc.Â
PHD guiding always showed at least one sharp occasional discontinuity in the
tracking every 4 mins or so. Flexure is usually a continuous drift I believe.

As I said, I also think Lateral mirror shift is also a problem with the C11HD
Edge design - thereâs a website where a guy corrected this by drilling holes
and inserting nylon screws into the sides to support the mirror. I concluded
this was a factor when everything was locked down and some images in a series
were streaked significantly more than that from periodic error.

I eventually gave up on the configuration. Yes, I hear of some individuals
being successful usingÂ  a G11 at longer focal lengths, but I believe they are
the exceptions and in general the G11 isnât good enough and Iâm convinced
it is notÂ  accurate to characterize the G11 as being suitable for long fl
imaging.

I also think itâs too easy to blame user technique; there is a reason people
buy much more expensive mounts and why you rarely hear guiding complaints from
high-end mount users!

P
------------------------------

Date: Thu, 21 Mar 2019 09:11:53 -0700 (GMT-07:00)
From: Klaus Brasch <krbrasch@xxxxxxxxxxxxx>
Subject: Re: photography problems discussion

I have to agree with Michael and I presume I am your friend, Padraig, with the
G-11 and Titan, :)

I have used both mounts successfully for years with everything from C-11 to C14
HD, long FL refractors, etc. for exposures with PHD for up to 10 minutes with
no problems. Fortunately, I don't have to do that any longer shooting at high
ISO with modified DSLR. Typically my longest times now are around 5 minutes,
usually shorter. I also combine that with firmly attached guide scopes and make
sure the mount is always balanced with slight favor to the east, depending on
which side the instrument package is located of course.

I have owned three different SCT's and always made sure the mirror's were fully
locked down after focusing. The HDs are supplied with lock down knobs but the
older models were not. I fixed that by inserting two long wing bolts in place
of the lock down screws used during transport. That did the trick, although
refocusing between takes was also important if ambient temps changed
frequently.

Finally, after a few years of use, both my G-11 and Titan were sent back to
Losmandy for cleaning and "tune up". So, I have been very happy with both
mounts now after 15 years of use.

Klaus

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

From: Michael Collins <cal_donley@xxxxxxxxxxxx>
Sent: Mar 20, 2019 11:15 PM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: photography problems discussion

On 3/20/19 8:41 PM, Padraig Houlahan wrote:

...the G11 just is not good enough for serious astrophotography in the sense
that exposures longer than about 2 mins are almost certainly
going to be flawed, and maybe 15% of those under 2 mins might not suffer
from uncorrectable (high frequency) periodic error.

Â  Not sure that I would agree with the almost certainly part of this
statement, at least not for every G11. When I had my observatory in the
Valley, I did a fair amount of speckle imaging with a C11 at an
effective focal length of about 5 meters. Since my camera had a very
small (808 x 608) sensor, the field of view was only 4 x 6 arcmin. It
was a challenge putting a target binary on the sensor since mirror shift
appeared to be about 15 arcmin, but I never had a problem keeping the
target within a few pixels even without guiding. While there certainly
was some periodic error evident at that image scale, it was quite smooth.

Â  On the few occasions that I did shoot some images at the native
Cassegrain focus using an 80 mm f/5 guide scope, I obtained what I
considered to be acceptable results.

Â  I wouldn't consider the G11 to be an optimal choice for long exposure
imaging with a C11, but I don't think it's fair to say that it's just
not good enough for serious astrophotography. If a given mount exhibits
high frequency periodic error as Padraig's evidently did, the design is
such that the problem can almost certainly be addressed. There are many
discussion threads on Cloudy Nights which thoroughly cover the topic.
--
<mailto: cal_donley@xxxxxxxxxxxx>
--
See message header for info on list archives or unsubscribing, and please
send personal replies to the author, not the list.

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

From: Michael McDonald <mikemac@xxxxxxxxxxx>
Subject: Re: photography problems discussion
Date: Thu, 21 Mar 2019 12:19:39 -0700

Iâve come to 2 tentative conclusions:

1) Expecting single pixel guiding (0.5 arc seconds) on an entry level mount and
in the light polluted environment of Chandler is unrealistic.

2) Mount manufacturers are selling snake oil.

Letsâ address #2Â  first since thatâs a provocative statement. In an effort
to figure out whether Iâm getting everything out of my (AVX) mount as it is
capable of, I went look to see what the manufacturer (Celestron) said about its
pointing and tracking specifications. When the answer turned out to be nothing,
I looked at other mounts specifications.

And with the exception of iOptron, Celestron, Losmandy, and Star Watcher
donât mention anything in the online specs about pointing or tracking
accuracy. iOptronâs EC modules claim a PEC of less than half of an arc
second. Their non EC models claim a +/- 5-10 arc second tracking accuracy,
depending on the model (bigger models have the lower error). All of the
manufacturers are more than willing to tell you irrelevant info like the
diameter of the shafts. I assume they donât give tracking info either because
itâs a lot worse than theyâd like you to believe or it varies so much
between units that the resulting number is meaningless.

There does seem to be some justification for the general assumption that more
expensive mounts are more accurate ones. But there doesnât seem to be much,
if any, objective data to back that assumption up. Is a mount twice as
expensive as another twice as accurate? Do all mounts in the same price
category have similar accuracies? Before I pay $2-4K for a new mount, Iâd
like some objective numbers that would give me the warm fuzzies that the money
was worth it.

So back to #1. Given that my AVX costs 1/3 of the low end iOptron CEM25EC, I
assume the AVX has at best the +/-10 arc second tracking accuracy. Thatâs 20X
the 0.5â/pixel I was hoping for. Not even close.

And an arc second is a lot smaller than I had appreciated. And it appears that
achieving tight enough mechanical tolerances to achieve sub arc second tracking
is hard, which equates to expensive.

So, given my tentative conclusions, what do I do? Well, Iâm not giving up on
getting the best images I can with what I have or reasonably can afford. Iâm
going to experiment with the changes I mentioned before (add the Barlow to
increase the guide scopeâs focal length, add the reducer to decrease the main
scopeâs focal length, remount the guide scope in a firmer location). Iâm
also going to start experimenting with binning. If my system truly isnât
capable of exploiting all of the resolution of my ASI1600, then that high
resolution would be better used by combining adjacent pixels into larger
âsuperâ pixels at are more light sensitive, aka binning. By being more
sensitive, that will shorten my exposure times, putting less demand on my
systemâs tracking ability. And since Iâm just using the resulting images on
my computer monitor, I really donât need 4500x3600 pixels.

Time to find a clear night so I can try some of this out before having to pack
everything up for the AAMM next weekend.

Mike McDonald
mikemac@xxxxxxxxxxx

PS None of this is meant to disparage the AVX. It was designed to be an entry
level mount used primarily for visual observations and wide field (Milky Way)
photography. Itâs yourâs trulyâs fault for trying to use it for something
it wasnât really designed for.

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

Date: Thu, 21 Mar 2019 14:00:42 -0700
Subject: Re: photography problems discussion
From: Ray Heinle <ray.heinle@xxxxxxx>

I don't think anyone mentioned this but SCTs focus by moving the mirror up
when the main focus knob is turned counter-clockwise.Â  Thus if you
deliberately turn the focus knob clockwise to bring it below best focus,
then slowly approach focus by slowly turning the knob counter-clockwise the
mirror will be firmly resting on its stops at focus, gravity keeping the
mirror in place.
Ray

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

Subject: Re: photography problems discussion
From: Scott <srstiers@xxxxxxxxx>
Date: Thu, 21 Mar 2019 17:37:16 -0700

Has anyone ever tried cementing the primary mirror in place and using an
external focuser?Â Â  It could be reversible as long as only a few dabs of
silicone were used to 'freeze' the mirror.Â  It would take some trial and
error to figure out exactly where fix the mirror but I would think with
a focuser with enough travel, a position could be found that would work
for visual and photography.

I have a Crayford style for my C11 and I will see next weekend if I can
find a positioin to leave the primary and not touch it the rest of the
weekend.

Scott R. Stiers, W7BIT
Gilbert, Az
--
On 3/21/2019 2:00 PM, Ray Heinle wrote:

I don't think anyone mentioned this but SCTs focus by moving the mirror up
when the main focus knob is turned counter-clockwise.Â Thus if you
deliberately turn the focus knob clockwise to bring it below best focus,
then slowly approach focus by slowly turning the knob counter-clockwise the
mirror will be firmly resting on its stops at focus, gravity keeping the
mirror in place.
Ray

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

Subject: Re: photography problems discussion
From: Michael Collins <cal_donley@xxxxxxxxxxxx>
Date: Thu, 21 Mar 2019 18:07:03 -0700

On 3/21/19 5:37 PM, Scott wrote:

Has anyone ever tried cementing the primary mirror in place and using an
external focuser?

Â I've seen reports of people doing this, but it leaves you with a
somewhat specialized telescope. The big advantage of the moving primary
focus strategy is a very wide back focus range (distance from the
backplate to the focal plane). Since SCTs are only minimally sensitive
to separation between the corrector, primary and secondary elements,
focus is achieved by moving the primary toward or away from the
secondary. Because of the magnification of the secondary, adjusting the
separation by only a few millimeters displaces the focal plane by a much
larger value. This makes it possible to focus a 1-1/4" eyepiece at the
minimum possible distance from the backplate or a camera or binocular
viewer far behind the telescope.

Â Fixing the separation, as would be the case if the primary were
cemented in place, means that any back focus requirements must be
accommodated by the focuser or with a combination of extension tubes.
Feasible, but not as convenient.

I have a Crayford style for my C11 and I will see next weekend if I can
find a positioin to leave the primary and not touch it the rest of the
weekend.

Has anyone ever tried cementing the primary mirror in place and using an
external focuser?

I have a Crayford style for my C11 and I will see next weekend if I can
find a positioin to leave the primary and not touch it the rest of the
weekend.

Â  You will probably find success if you stick with one diagonal and
simply change eyepieces. If you are going back and forth between visual
and photographic configurations, it may be more of a challenge. Of
course, anyone who owns a refractor or similar telescope that provides
fixed back focus is in the same boat. This is why many refractors are
equipped with focusers that offer 75 to 100 mm focus travel while a
typical Crayford for an SCT is limited to 25 mm.
--
<mailto: cal_donley@xxxxxxxxxxxx>
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------------------------------

Date: Fri, 22 Mar 2019 01:01:53 -0400 (EDT)
From: Paul Lind <pulind@xxxxx>
Subject: Re: photography problems discussion

Guys,

Regarding external focusers for SCTs, double star observers do this to be able
to know the exact focal length of theirÂ  SCT (which varies as the primary is
moved). I believe they somehow lock the primary, not glue it.Â  One person that
does this is Richard Harshaw.Â  His email from a previous sac-forum posting
was: Richard Harshaw <rharshaw2@xxxxxxx>.Â  I don't know if it's current.

Paul Lind

----- Original Message -----
From: Michael Collins <cal_donley@xxxxxxxxxxxx>
To: az-observing@xxxxxxxxxxxxx
Sent: Thu, 21 Mar 2019 21:07:03 -0400 (EDT)
Subject: [AZ-Observing] Re: photography problems discussion

On 3/21/19 5:37 PM, Scott wrote:

Has anyone ever tried cementing the primary mirror in place and using an
external focuser?

I have a Crayford style for my C11 and I will see next weekend if I can
find a positioin to leave the primary and not touch it the rest of the
weekend.

[AZ-Observing] Re: az-observing Digest V19 #67

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