Issues Plate Solving at a longer focal length

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musicmatters
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Joined: Sat Aug 28, 2021 1:44 am

Issues Plate Solving at a longer focal length

#1

Post by musicmatters »

I have a C14 and I have only ever used the Hyperstar unit on it. I am using Sharpcap/ASTAP as my plate solver. I have never had an issue using the C14/Hyperstar setup as it always platesovled every time and always within 1-2 seconds.

I recently purchased a Starzonia .63 SCT Reducer to experiment at a longer focal length. My thought was as we get into galaxy season the targets are smaller then the larger planetary nebula I was shooting with the Hyperstar in the winter.

I changed the focal length to 2365mm in the settings window of Sharpcap to account for the new .63 reducer setup, and when I try to plate solve I only get success like 10% of the time. I often get a "not enough stars error". I have seen it say that it can see anywhere from 10-35 stars, but its saying that's not enough. I understand that the smaller FOV inherently will see less stars, but I'm wondering if I'm doing somehting else wrong that its not plate solving with a higher success rate. Any ideas on things I can look for to increase this success rate?

I have tried 2 sessions with this .63 reducer and I can obviously see the difference in shooting at F6.7 with the .63 reducer vs F2 with the Hyperstar. I was able to eventually get it to plate solve and I did both sessions on M51, and was was made fully aware at how much "slower" the experience is at F6.7 with needing to take much longer subs and total integration time to get the same image I was able to get at F2.

My 2nd question is, I am wondering if my thought process is correct in even trying to shoot these smaller galaxies like M51 using the .63 reducer. Would I be better suited by leaving the Hyperstar on and trying to get a more properly framed shot a different way? Would it be the same thing to just use the Hyperstar and use a smaller resolution or ROI setting for the camera, or a binning setting, or just shoot it and crop the final image? I am wondering if I can still have the benefit of the speed of F2/Hyperstar and still get the best image I can of these smaller targets and maximize "pixel efficiency". Is the .63 reducer really giving me any benefit here?
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admin
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Re: Issues Plate Solving at a longer focal length

#2

Post by admin »

Hi,

so, in moving from f/2 to f/6.7, you have made a change that means the amount of light falling on each pixel has reduced by a factor of about 11, and the area of sky in view has similar reduced by a factor of about 11. Both of these can hurt you when it comes to plate solving...

The reduced amount of light means that you need to take longer exposures when plate solving to get the same sort of clarity in terms of signal/noise ratio, which is key to star detection. If you were previously taking 10s frames to plate solve, try 100s now!

The reduced area of sky in view means that even if you increase the exposure and detect plenty of stars in the image, fewer of them will be in Astap's star database. You may need to install one of the more extensive star databases to help (see this info on star database usability in Astap - http://www.hnsky.org/astap.htm#database_usability)

Hopefully a combination of the longer exposures and the larger database will help. It's also worth making sure your version of Astap is up to date - Han is frequently updating and fixing issues.

On the subject of a 'trick' to take advantage of f/2 hyperstar but also to zoom in on smaller targets, unfortunately there isn't one. If you want finer detail stith the hyperstar then you would need a sensor with smaller pixels - say 1 micron instead of 3 microns would give you 3 times finer view. The only problem is that the 1 micron pixels would collect roughly 10 times less light than the 3 micron ones, so would require 10 times longer exposures...

cheers

Robin
musicmatters
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Re: Issues Plate Solving at a longer focal length

#3

Post by musicmatters »

Thanks. I am using a 294MC-Pro camera for this. Would I be better suited with a different camera for the .63 reducer setup?
roelb
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Re: Issues Plate Solving at a longer focal length

#4

Post by roelb »

With my 290MM (small FOV) ASTAP doesn't always find a solution, reporting "Too small FOV".
Using "Astrotortilla" will almost find a solution.
When using ASTAP, be sure to set the correct "Field of View HEIGHT" into the Stack menu.
Roel
Celestron Nexstar Evolution 8 - Celestron Nexstar 6 SE - OTA 5 SE - StarSense
Starlight Xpress Lodestar X2 Mono -- ZWO ASI290MM -- ZWO ASI224MC -- ZWO ASI533MC-Pro
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Re: Issues Plate Solving at a longer focal length

#5

Post by admin »

Hi,

at very long focal lengths (for deep sky), you might want to consider using big pixel cameras - the 4.6 micron pixels in your 294 will be oversampling at 2000mm+ ... that is the resolution of the camera is considerably higher than the typical seeing limit of 1-2 arc seconds. Unless you have very steady skies, you might want to consider 2x2 binning?

The ASTAP field of view height thing should be handled automatically - SharpCap will ask Astap to solve once using the saved field of view height, and if that fails it will re-run the solve telling ASTAP to try a range of field of view heights. If the second one succeeds then the correct height will get saved for the next time around. Of course, if you have specified the focal length of the telescope then SharpCap will calculate the FOV and tell ASTAP via the command line parameters, bypassing the saved value.

cheers,

Robin
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Menno555
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Re: Issues Plate Solving at a longer focal length

#6

Post by Menno555 »

For me with 2032mm, ASTAP is also not working well. Tried All Sky Plate Solver (ASPS) and that works all of the time.
I had a 294Mc Pro in the past too and with ASPS and SharpCap that worked just fine.

Menno
ChrisR Oz
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Re: Issues Plate Solving at a longer focal length

#7

Post by ChrisR Oz »

For me at 1488 mm (2125 mm x 0.7 reducer) it seems sensitive to noise using ASTAP. If I try gain=300 it will not work. But at gain=200 or less and a little more exposure it works every time. Certainly, the reducer helps too.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, AVX mount and Focuser, CPWI, Starlight Xpress AO, OAG and Filter Wheel, ZWO ASI 294MC Pro and 120MM mini, SharpCap Pro, PHD2, Powermate 2x, Baader Moon & Skyglow, Astronomik CLS-CCD, ZWO Duoband filters.
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Re: Issues Plate Solving at a longer focal length

#8

Post by admin »

Hi folks,

check which star database you have installed in Astap - for FOV much under 1 degree, you need the larger H18 database.

cheers,

Robin
ChrisR Oz
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Re: Issues Plate Solving at a longer focal length

#9

Post by ChrisR Oz »

Hi Robin,

Yes, I use the H18 database. It’s essential to find enough stars in the small FOV.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, AVX mount and Focuser, CPWI, Starlight Xpress AO, OAG and Filter Wheel, ZWO ASI 294MC Pro and 120MM mini, SharpCap Pro, PHD2, Powermate 2x, Baader Moon & Skyglow, Astronomik CLS-CCD, ZWO Duoband filters.
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