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Exposure times with 8SE mount

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#1 Startex

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Posted 14 February 2019 - 03:56 AM

Hello folks, I am considering a Nexstar 8SE for my next scope and am currently getting into EAA and eventually will expand into AP. I know AP is best with a fast refractor or Newt on a GEM and I actually have a GEM and F8 127 achro (I know, not fast) but was wondering if anyone has tried extended exposure times with a Nexstar 8SE tube and mount. What were the longest successful shoots?


Edited by Startex, 14 February 2019 - 03:59 AM.


#2 theoatc

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Posted 14 February 2019 - 05:08 AM

U can get away with 30 sec max. I owned the evo series, which has far more better tracking, since it has better motors, and I could achieve 30 - 40 sec tops.

#3 Noah4x4

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Posted 14 February 2019 - 06:15 AM

30 second is the maximum, but there is another option. 

 

When light pollution convinced me to convert to EAA/AP I had three choices with my Evolution;

 

a)   Buy a wedge

b)   Buy an AVX mount

c)   Buy Hyperstar.

 

I bought a wedge, a stupid decision given my health, an infernal device unless you have arms like an Orang-utan and the patience of (biblical) Job. I did get good results, but my sciatica eventually forced me to surrender. Options (b) and © demanded similar cost, and by then, I had had enough of tedious polar alignment and wrestling with mechanical instruments of torture, so I invested in Hyperstar. Now no polar alignment, no autoguiding; exposures of a few seconds because it reduces an SE8 from F/10 to F/2. Easy!

 

So my advice is buy a GEM if sure you are heading for AP, albeit an Alt-Az is much easier for 'visual', but once committed to an Alt-Az, consider Hyperstar rather than GEM.



#4 mclewis1

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Posted 14 February 2019 - 10:08 AM

Aside from focal length and your expectations (how picky you are) there are 3 things that will affect the length of exposures. 

 

1) The motor controllers in Celestron Alt Az goto mounts get a tracking "correction" or update about every 30 seconds or so. This can sometimes be visible as a small jump or shift in the object (depending on your focal length).

2) Periodic error (PE) and backlash from the gears. The PE is more from the smoothness of the gear surfaces and the backlash is from the amount of play between the gears. This is very scope dependent. Some scopes are better than others so it's often the luck of the draw. 

3) The area of the sky you are imaging. Field rotation is more prevalent in some areas of the sky than others so you can often notice better results on some objects at certain times of the year. http://calgary.rasc....ld_rotation.htm


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#5 TrustyChords

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Posted 14 February 2019 - 02:31 PM

 

1) The motor controllers in Celestron Alt Az goto mounts get a tracking "correction" or update about every 30 seconds or so. This can sometimes be visible as a small jump or shift in the object (depending on your focal length).

Having got a new NexStar mount recently, this is what I've suspected, I'm thrilled to hear it confirmed (well, sort of). I've been using 30 second exposures and having to toss 35% or so of them due to this correction.

 

My question is, is the 30 seconds strictly periodic-- meaning, does it occur precisely and only on 30 second intervals (and never, say, 35 seconds, 30.5 seconds, etc)?

 

If so, would it work accurately to set 25 second exposures, ad use a 30-second intervalometer to repeatedly "miss" the corrections? Working to sync the 5 second inactive camera time during the correction, of course.

 

Or am I being too naive this might work?



#6 deansjc

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Posted 14 February 2019 - 06:54 PM

Excellent document from the RASC Calgary Center - thanks for unearthing this Mark1!

 

I too am curious about periodic corrections that the mount undertakes.  I was not aware of that, in fact it is counter-intuitive since there is no guiding to create an input signal, and were the "required correction" movements known, I would expect them to already be programmed into the mount.

 

Mark, do the corrections occur in both Alt and Az?  Logically they would, meaning that even when operating on a wedge with the azimuth motion, any corrections would still be there.

 

Mark - or others - is  there is any indication in the HC of a correction being made, one would need to know that to even attempt to time exposures in between corrections.

 

(Happy to be naive too :-)  Great questions.

 

John



#7 Michael_Swanson

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Posted 14 February 2019 - 07:01 PM

Having got a new NexStar mount recently, this is what I've suspected, I'm thrilled to hear it confirmed (well, sort of). I've been using 30 second exposures and having to toss 35% or so of them due to this correction.

 

My question is, is the 30 seconds strictly periodic-- meaning, does it occur precisely and only on 30 second intervals (and never, say, 35 seconds, 30.5 seconds, etc)?

 

If so, would it work accurately to set 25 second exposures, ad use a 30-second intervalometer to repeatedly "miss" the corrections? Working to sync the 5 second inactive camera time during the correction, of course.

 

Or am I being too naive this might work?

I don't know if anyone has ever tried to time it in practice, but Celestron programmed the hand control to issue the corrections every 30 seconds.  The processor in the hand control would likely delay the interval if a higher priority task occurs (user accessing the menu, a flood of commands via the port on the bottom of the hand control, etc.).  Any delay should be very brief but would be totally unpredictable.

 

All that said, I've always recommended exposure duration of no longer than 20-25 seconds, take a good number of them, and throw out the bad ones where the tracking correction occurs during an exposure (any image with a little 'jump' in it).  Since you will not exactly know when the start of the update sequence occurs (basically it is right after the last movement was sent to the MC), you won't benefit by trying to create a sequence of:

  • 25s exposure
  • 5s wait
  • 25s exposure
  • 5s wait
  • repeat

it is more likely you will experience nearly 100% bad exposures as the tracking update will occur during the 25s exposures, not the 5s wait periods.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com


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#8 Michael_Swanson

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Posted 14 February 2019 - 07:11 PM

Excellent document from the RASC Calgary Center - thanks for unearthing this Mark1!

 

I too am curious about periodic corrections that the mount undertakes.  I was not aware of that, in fact it is counter-intuitive since there is no guiding to create an input signal, and were the "required correction" movements known, I would expect them to already be programmed into the mount.

 

Mark, do the corrections occur in both Alt and Az?  Logically they would, meaning that even when operating on a wedge with the azimuth motion, any corrections would still be there.

 

Mark - or others - is  there is any indication in the HC of a correction being made, one would need to know that to even attempt to time exposures in between corrections.

 

(Happy to be naive too :-)  Great questions.

 

John

Hi John,

 

I detailed this in a post a few months ago but it is hard to find things sometimes in the forums.

 

The hand control has the "brains" to determine the correct rates for each axis to simulate smooth sidereal tracking in alt-az mode.  The motor control (the processors inside the mount - one processor for each axis) receive the tracking rate for each axis and actually control the motion of each motor.  So, every 30 seconds the hand control calculates the correct rate for each axis according to the current RA-Dec and sends those rates to motor control processors.  As described in my immediately previous post, there is no practical way to determine when the tracking updates are occurring.  There is nothing on the hand control display and the hand control doesn't send anything out the external port on the bottom.  Technically, you could interface a PC/laptop via the AUX port and monitor all the AUX bus traffic to see the messages being sent to the motor control, but that would be a very custom solution as there is nothing out there pre-written for this.  Also note that the AUX port is a TTL interface (https://en.wikipedia...ransistor_logic) so a small bit of electronics would be required to convert levels to RS-232 and deal with flow control.

 

EDIT - I missed your question about tracking on a wedge.  When you complete an EQ alignment on a wedge, this series of events does not occur.  After any movement (for example, manual arrow button slewing or a GoTo), the hand control sends a single tracking command to the RA (azimuth) axis processor in the motor control to tell it to track at sidereal rate indefinitely.  There are no 30 second updates in EQ mode.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com


Edited by Michael_Swanson, 14 February 2019 - 07:14 PM.


#9 TrustyChords

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Posted 14 February 2019 - 07:13 PM

I don't know if anyone has ever tried to time it in practice, but Celestron programmed the hand control to issue the corrections every 30 seconds.  The processor in the hand control would likely delay the interval if a higher priority task occurs (user accessing the menu, a flood of commands via the port on the bottom of the hand control, etc.).  Any delay should be very brief but would be totally unpredictable.

 

All that said, I've always recommended exposure duration of no longer than 20-25 seconds, take a good number of them, and throw out the bad ones where the tracking correction occurs during an exposure (any image with a little 'jump' in it).  Since you will not exactly know when the start of the update sequence occurs (basically it is right after the last movement was sent to the MC), you won't benefit by trying to create a sequence of:

  • 25s exposure
  • 5s wait
  • 25s exposure
  • 5s wait
  • repeat

it is more likely you will experience nearly 100% bad exposures as the tracking update will occur during the 25s exposures, not the 5s wait periods.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com

Thanks for the input, Mike.  Good point about other operations coming in, potentially shifting the 30 second period. I suppose it would depend on how the timer was implemented (poll/sleep + execute, versus a true timer).

 

If it were a strict 30 seconds, it should be fairly trivial to land the 5s window through a couple trial and errors (e.g. if star trails are seen, stop and offset another few seconds; rinse, repeat); and assuming the intervalometer was also accurate, one would then simply set the repeat for an hour or so and hope everything lines up. I'm not too confident both systems would be this accurate together, however.

 

Followup question, coincidentally I found another one of your posts that suggested in wedge mode (EQ tracking), this track correction is omitted:

 

>> When on a wedge (even with less than perfect polar alignment), the situation is different.  As long as you perform an EQ Alignment, or manually set Tracking Mode to EQ North/South, tracking is handled differently.  In that case, the motor control makes no movement in the dec (alt) axis and tracks at a set sidereal (default) rate in RA (azm).  There are no rate updates from the hand control and thus no minuscule jumps.

 

(EDIT: link https://www.cloudyni.../#entry7760751)

 

In this case, if I'm doing polar alignment something else is up with my star trails entirely.

 

Either way, once rain clears up in a couple days I'll try it out and report back.

 

Thanks again for the input!


Edited by TrustyChords, 14 February 2019 - 07:20 PM.


#10 Michael_Swanson

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Posted 14 February 2019 - 07:16 PM

Thanks for the input, Mike.  Good point about other operations coming in, potentially shifting the 30 second period. I suppose it would depend on how the timer was implemented (poll/sleep + execute, versus a true timer).

 

If it were a strict 30 seconds, it should be fairly trivial to land the 5s window through a couple trial and errors (e.g. if star trails are seen, stop and offset another few seconds; rinse, repeat); and assuming the intervalometer was also accurate, one would then simply set the repeat for an hour or so and hope everything lines up. I'm not too confident both systems would be this accurate together, however.

 

Followup question, coincidentally I found another one of your posts that suggested in wedge mode (EQ tracking), this track correction is omitted:

 

>> When on a wedge (even with less than perfect polar alignment), the situation is different.  As long as you perform an EQ Alignment, or manually set Tracking Mode to EQ North/South, tracking is handled differently.  In that case, the motor control makes no movement in the dec (alt) axis and tracks at a set sidereal (default) rate in RA (azm).  There are no rate updates from the hand control and thus no minuscule jumps.

 

In this case, if I'm doing polar alignment something else is up with my star trails entirely.

 

Either way, once rain clears up in a couple days I'll try it out and report back.

 

Thanks again for the input!

Yep, that was the post I was alluding to a moment ago :-)



#11 Startex

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Posted 14 February 2019 - 09:50 PM

Well fellas, I ordered the 8SE with the WiFi dongle tonight, should be here next week. Is there a image gallery here specific to the 8SE?



#12 Noah4x4

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Posted 15 February 2019 - 02:49 AM

I don't know if anyone has ever tried to time it in practice, but Celestron programmed the hand control to issue the corrections every 30 seconds.  The processor in the hand control would likely delay the interval if a higher priority task occurs (user accessing the menu, a flood of commands via the port on the bottom of the hand control, etc.).  Any delay should be very brief but would be totally unpredictable.

 

All that said, I've always recommended exposure duration of no longer than 20-25 seconds, take a good number of them, and throw out the bad ones where the tracking correction occurs during an exposure (any image with a little 'jump' in it).  Since you will not exactly know when the start of the update sequence occurs (basically it is right after the last movement was sent to the MC), you won't benefit by trying to create a sequence of:

  • 25s exposure
  • 5s wait
  • 25s exposure
  • 5s wait
  • repeat

it is more likely you will experience nearly 100% bad exposures as the tracking update will occur during the 25s exposures, not the 5s wait periods.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com

Great information Mike. 

 

I use Hyperstar, software focussing and stacking tools. When I use extremely short exposures (5s) I suffer few rejected subs. The closer I get to 30 seconds the higher the automated rejection rate. I guess this explains why. I abandoned my wedge (too much effort) but it does track in RA Only. Now that CPWI includes a 'Starsense Auto on Alt-Az wedge align' I might attempt it again. 



#13 mclewis1

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Posted 15 February 2019 - 10:03 AM

Well fellas, I ordered the 8SE with the WiFi dongle tonight, should be here next week. Is there a image gallery here specific to the 8SE?

Because the optics are the same across all the C8s you'll find lots of images. Look for any taken on Alt Az mounts (8i, 8SE, NexStar 8, NexStar 8 GPS, CPC800, or Evolution). Yes most of these scopes have the better gears (worm and spur) but if they are used in Alt Az orientation (and not on a wedge) they will be a good example of what you can do.

 

I'd also poke around in the gallery in the EAA forum. There you'll find lots of 8SE images taken with short exposures, but remember that the folks in that form are more focused on observing the image from the scope rather than working to make pretty pictures (so little or no post processing).



#14 roelb

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Posted 16 February 2019 - 05:27 PM

Well fellas, I ordered the 8SE with the WiFi dongle tonight, should be here next week. Is there a image gallery here specific to the 8SE?

Take a look here (for 6 SE): https://www.cloudyni...-6-se-asi290mm/



#15 carolinaskies

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Posted 16 February 2019 - 07:17 PM

When in Alt-Az the maximum exposure is quite limited because the mount has to move both axis.  Field rotation starts to set in.  You don't notice it visually for a while because there isn't any orientation in the image circle. You would notice it if you used an illuminated reticule.  As was said earlier, depending on where the object being imaged is located the amount of correction varies for both RA and DEC and thus the amount of maximum exposure has to be compensated.   

In EQ mode properly polar aligned (mount is placed parellel to the celestial pole at the local latitude using a wedge) only the RA drive is required to run at sidereal rate and no standard corrections are sent.  The mount in EQ mode arrives at it's final pointing position and then simply never re-checks it's position.  If the polar alignment is PERFECT the telescope will track the center of the field until the power is turned off or something alters the drive system.  PE, periodic error, however causes the telescope to shift slightly, speeding up or slowing down like a car climbing a hill or going down the other side. 

PE is caused by the mesh of the worm gear with the driven gear.  The error occurs at a constant interval based on the gear ratio of the two for the worm gear period to complete a 360 revolution. It is the precision of the worm gear that dictates periodic error and is most important because it is cut at a severe 'corkscrew' angle. If a mount has PEC, periodic error correction, the mount can be trained to adjust for the error during one session until power is switched off.  If you have PPEC, the mount can be trained for permanent periodic error, storing a profile of when to advance or coast the motor to maintain the proper positioning from session to session.  

Using a illuminated reticule the PEC can be adjusted by eye using the hand control and setting it to PEC train. Alternately this can also be adjusted if you use external guide programs and a camera attached to the telescope or via a guide camera to make those training corrections.   I am more familiar with the Meade PEC/PPEC of my LX200, but essentially the mount is set to record the changes during a set interval according the the specific mount(on my LX200 it was 3min as I recall) That 3 minutes was a full revolution of the worm gear in the telescope.  PPEC can also be trained into guiding software btw.     




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