Unguided sub duration with respect to pixel scale/focal length

[Rodd]

I am currently shooting shooting at 2800 mm with a pixel scale of 0.28 arcsec/pix and am taking 30 second unguided subs with no elevation of eccentricity.  Stars look perfect.  I probably could go longer--but for the sake of this question, lets keep it at 30 sec.   If I changed scopes and sent to a widefield setup shooting at 2.46 arcsec/pix (same mount and camera), I think I could go much longer, but how much.  2.46 is almost 9 times 0.28.  Does this mean I could shoot 9 times longer for the same quality?  That would be 270 sec--close enough to 300 to call it 5 minutes.  And the drift would be very minimal.  When comparing the FOV at 2800 mm with the view at 318 mm, the amount of drift I get is really quite small on the widefield FOV.  If this is true--I think I will stop guiding with the FSQ and reducer.  300 sec is my maximum narrowband sub length for all scopes, and for broadband I will use 30 sec to 60 sec with the FSQ 106 and 2-3 minutes with the TOA 130--even that probably could go unguided.

But, first things first...what is the relationship between focal length, pixel scale, size of FOV, and tracking accuracy.   

[skybadger]

Image scale in "/pixel=206265/focal length in mm .

Turn that into "/pixel by multiplying by pixel size in mm

Field of view is "/pixel X # of pixels on sensor. 

Tracking accuracy is whatever you measured, so yes you could shoot for 9x longer but why not put the wide field camera on the narrow field guided mount and get the best of the guiding ?

 

 

 

 

[david_taurus83]

What mount? Periodic error will come into play with longer subs as well. Plus any imperfections on the gears will show up as sudden spikes in longer subs.

[Rodd]

24 minutes ago, skybadger said:

but why not put the wide field camera on the narrow field guided mount and get the best of the guiding

Well--That will require installing an OAG (I use guide scope on wide field scopes)--Finding the time to install the darn thing without eating up imaging time is the reason I am shooting unguided.  The camera is designed for short subs - the ideal sub length at F10 and gain 300 in my sky is 25-35 seconds.  I will install the OAG at some point because I want to shoot narrowband.  But for the wide field scope, if I can shoot 300 sec subs unguided (that's my typical narrow band sub length), I can eliminated potential errors associated with guiding.     Say good bye to worrying about guide graphs and tweaking the aggressiveness, and worrying about finding guide stars, and on and on.  It greatly simplifies the operation.   

[Rodd]

18 minutes ago, david_taurus83 said:

What mount? Periodic error will come into play with longer subs as well. Plus any imperfections on the gears will show up as sudden spikes in longer subs.

I use the AP Mach 1.  Maybe I would need to use a good PEC model.  I always thought that helped with guiding, but is there any reason why it couldn't help with simple tracking?  (I mean PEM I think)

Edited July 1, 2021 by Rodd

[david_taurus83]

Not familiar with AP mounts but they are a 'premium' mount so I would have thought you could shoot 5 minutes with a short focal length scope.

[Rodd]

6 minutes ago, david_taurus83 said:

Not familiar with AP mounts but they are a 'premium' mount so I would have thought you could shoot 5 minutes with a short focal length scope.

That's what I am thinking now.  

[wimvb]

PEC helps unguided tracking in RA. With good correction and a high end mount, you may very well be able to do 5 minutes unguided exposures. But with long exposures, you also need good polar adjustments, or you’ll get DEC drift. You should definitely try this:

record the periodic error and load it into your mount or its control software. Then with sidereal + pec tracking, use the phd guiding assistant to show residual periodic error plus polar alignment error. Let that determine your longest exposure time.

Edited July 7, 2021 by wimvb

[Rodd]

On 07/07/2021 at 10:20, wimvb said:

PEC helps unguided tracking in RA. With good correction and a high end mount, you may very well be able to do 5 minutes unguided exposures. But with long exposures, you also need good polar adjustments, or you’ll get DEC drift. You should definitely try this:

record the periodic error and load it into your mount or its control software. Then with sidereal + pec tracking, use the phd guiding assistant to show residual periodic error plus polar alignment error. Let that determine your longest exposure time.

Thanks Wim--just saw this--SGL not emailing me with responses.  Your idea sounds great - if only I knew how to do all that.  two problems--1) I don't know how to load the periodic error or turn on and off tracking in either axis.  I just let the mount run.  That's why I could never use an inferior mount--I am not skilled enough.   As for PHd guiding...I tried it once when Maxim DL wouldn't work (driver or USB on asi 1600 issue--I can't recall).  As it turns out, the PHD guide interface was so large I did not have access to controls beyond start/stop.  Amazingly, it worked and I got data--I guess "push here dummy" really is an appropriate adage for the software.  I have since learned how to change the scale of my laptop screen, so maybe I would not have that problem now.  Another problem I have in general is I find reviewing instructions and theory about how to operate the various things very difficult to do unless I am at the scope actually doing it--which in the dark on a clear night just can't happen successfully.  Much of the problem is I am not that familiar with computers, so changing USB traffic, diagnosing stuff, finding drivers--all that, is not in my wheel house.   On top of all of it, my equipment is finicky--cables are always suspect, the main camera will freeze requiring a computer reboot if I am not careful (that is why I always park the mount then slew to the other hemisphere when I do a meridian flip.  If I try to do that flip to the target--the camera freezes and I have to park and reboot anyway).   If I had 200 clear nights a year, I would happily take a few and learn all this.  Sadly--I get about 10/year these days.   

[wimvb]

Idon't use Maxim DL, nor do I have an AP mount, but my guess is that Maxim talks to the mount through an ASCOM interface. AfaIk, the ASCOM interface allows you to store pec data and should have a tracking mode called sidereal + pec. This mode can be accessible through Maxim. Some high end mounts even have a pec curve stored internally from the factory. Unfortunately I can't help you any further because I'm not familiar with either system. I'm a linux/INDI person. 😉

[Rodd]

5 minutes ago, wimvb said:

Idon't use Maxim DL, nor do I have an AP mount, but my guess is that Maxim talks to the mount through an ASCOM interface. AfaIk, the ASCOM interface allows you to store pec data and should have a tracking mode called sidereal + pec. This mode can be accessible through Maxim. Some high end mounts even have a pec curve stored internally from the factory. Unfortunately I can't help you any further because I'm not familiar with either system. I'm a linux/INDI person. 😉

Thanks Wim.  My mount came with a PEC--but I disabled it on advise of AP because I changed the mesh a while back when I realigned it.  I would have to develop a new one and use that - which would no doubt improve guiding.  The problem is I have no idea how to do it - though I know it can be done.  I have come across no method to access tracking rates in Maxim.  I would have to set that up in the AP Mount platform, though it probably has another name.   I can tell the mount to track using the stars (sidereal), the Moon, Comets, and one other--I forget what.  But that only pertains to tracking I think--not guiding.    Anyway--the real problem is clouds.  When its clear, I seem to be happy with my results.  When its cloudy, I question everything!

 

[wimvb]

1 hour ago, Rodd said:

Anyway--the real problem is clouds.

I understand the feeling. What is equally frustrating is a clear sky for weeks on end, but too bright to do any imaging. Not even narrow band up here, atm.

[Rodd]

4 minutes ago, wimvb said:

I understand the feeling. What is equally frustrating is a clear sky for weeks on end, but too bright to do any imaging. Not even narrow band up here, atm.

That kind of reminds me of a big full Moon when its perfectly clear.  I don't even bother - even Ha is crippled