x2 Barlow v. Frame Rate

[mike shuter]

I'm imaging Jupiter with a Meade LX200 10" & an Altair 224c. With a ROI of 300 x 300 I achieve a frame rate of up to 125 f/s but with the introduction
of a x2 Barlow the ROI has to increase to 600 x 600. This drops the frame rate to about 38/39 f/s. With a finite capture period of say 30 seconds, is it best to have a larger image on the clip with the Barlow with a reduced number of frames or a high number of smaller images without the Barlow ?
Any advice much appreciated.
Mike.

[oopfan]

I'm not the expert on planetary but I think I'm in the ballpark with my analysis. The bottom line is that you are better off without the Barlow.

For a 10" aperture (254mm) Dawes' Limit is 0.46 arc-seconds, Rayleigh Limit is 0.54 arc-seconds. So given exceptionally stable skies and perfect optics you can resolve two objects separated by 0.46 arc-seconds.

Now let us factor in your camera. With the Barlow the resolution is 0.15 arc-second per pixel. Without the Barlow it is 0.30 arc-second per pixel. So your camera without the Barlow is a good match for your optics under ideal conditions (i.e. low-distortion optics and highly stable skies.)

In my opinion, you are not gaining anything by adding the Barlow, in fact, you are harming the image quality due to:

1. Lower frame rate.
2. Higher noise due to fewer stacked frames.
3. Potentially significant optical distortion due to the Barlow.

Brian

PS:
I made these calculations with the help of
https://astronomy.tools/calculators/tel ... pabilities
https://astronomy.tools/calculators/ccd_suitability

[mike shuter]

Thanks for your prompt reply. I'll put your recommendations into effect next time out, could be some time with our skies.

Regards,

Mike.

[oopfan]

Hi Mike,

The point I wanted to make is if your camera had larger pixels and the resolution without the Barlow was 1.0 arc-second per pixel, then yes definitely go with a high-quality Barlow, however, that is not the case. The other factor is your seeing conditions. If you are on a mountain top, then you have a good chance of hitting Dawes' Limit, but that is not an option for a lot of us.

Jupiter is high in the sky now, so give the Barlow a shot. The atmosphere might cooperate. However, I'd recommend keeping 300x300 ROI with the high fps even if it means not capturing the full disc.

Brian

[mike shuter]

Hi Brian,

At the moment Jupiter only reaches about 46 degrees max. from my location so atmos. will be a problem so will dispense with the Barlow. Next to dabble with is binning. Never tried it before so don't know the outcome. Experiments on a blank screen at 50% histogram using Altair Capture, x2 binning half the ROI & exposure time but surprisingly the frame rate is about the same. Seems lots to tinker with if only the clouds co-operate & the winds drop.

Mike.

[oopfan]

Mike,

Frame rate is primarily dependent on exposure, so if the exposure is 1 sec then frame rate is 1 per sec. However, 1 ms exposure doesn't yield 1000 fps! Why? It depends on the time it takes the camera to clock the pixels out of the sensor and over the wire to the laptop. That's why you will find that a smaller ROI yields a higher frame rate.

With regards to binning, I recommend playing with this calculator to get a feel for it:
https://astronomy.tools/calculators/ccd_suitability
Select "Meade - 10" LX200" for Telescope and "Altair Astro - GPCAM2 224C" for Camera and "Exceptional Seeing" for Seeing.

This is what you should see:

Screenshot 2023-09-30 at 12.32.39 PM.png (186.61 KiB) Viewed 4233 times

Notice the color spectrum below. You are in the green zone with Exceptional seeing. Next change the seeing to Good. Now you are in the blue zone. As it says you are now slightly over-sampling. Then change to OK, Poor, and Very Poor. More and more you go deeper into over-sampling. This is just saying that your optics and camera are limited by the seeing conditions. There isn't much you can do about it except move to higher ground!

On the other end of the spectrum is under-sampling. It happens with large pixels and short focal lengths. That's the problem with my kit. You have the opposite problem with over-sampling. IMO, over-sampling is better to have than under-sampling. It can be easily remedied with binning.

Over-sampling isn't an issue if you are doing Lunar and Planetary since those objects are plenty bright, however, if the histogram tells you to increase the exposure, then I would opt for increasing binning since it won't harm your frame rate.

Over-sampling hurts when the object is faint as in nebulae and galaxies. Increase binning, however your Altair 224C isn't made for long exposures, so then consider purchasing a cooled camera for deep-sky imaging. You'll soon see, however, that there doesn't exist any single combination of telescope and camera that meets the needs of lunar, planetary AND deep-sky. Astrophotography is an expensive hobby if you want to do it all.

Brian

[turfpit]

Mike

To follow on from Brian's comprehensive description of using the CCD_suitability option on the astronomy-tools website to explain the effects of seeing/equipment; here is a practical example of a Jupiter capture based on my own experiences. The equipment used was a Celestron C8 SCT, Celestron 2x Barlow and ZWO ASI120MC (USB2). Using the 2x Barlow has rarely given me good results because of the atmospheric conditions where I live. I can understand why Damian Peach used to carry out planetary work in Barbados (renowned for excellent seeing) or at the Pic du Midi observatory (1m scope at the top of a mountain). An internet search for 'Damian Peach Jupiter' will turn up some useful information.

The description below was one of those rare occasions where use of the Barlow gave me good results.

Equipment: Celestron C8 SCT, Celestron 2x Barlow and ZWO ASI120MC (USB2)
Capture: SER, RAW8, 640x480, exposure 21ms, gain 70 (out of 100), 2700 frames (60s) @ 45fps
Software: SharpCap 3.0, Autostakkert, Registax

Jupiter.jpg (57.12 KiB) Viewed 4209 times

The exposure was not specifically chosen but was adjusted to achieve a 70% saturation on the histogram. The steps were:

• Set the gain to 70% (would use 60% if no Barlow)

• Adjust the exposure until the right hand side of the histogram was around 70%

• Capture the data

ASI120-histogram.png (6.91 KiB) Viewed 4209 times

2017-06-18-2220_4.CameraSettings.txt

(691 Bytes) Downloaded 71 times

With Saturn being a dimmer object, I set the gain to 90% and adjusted the exposure to achieve a histogram to 80%:

Saturn.png (66.55 KiB) Viewed 4209 times

With all of this, the histogram saturation was the driver. A pre-set gain value was used. The exposure was just the value required to achieve the desired histogram using a particular gain.

The figures I have quoted were for that particular set of equipment with the atmospheric conditions present that night. Treat them as guidelines.

For completeness, for lunar imaging I use a gain of around 30% and set the exposure to achieve a histogram of around 60%.

Dave

[mike shuter]

Thank you gentlemen, your advice is very much appreciated. I've noticed Dave's format is RAW8 whereas I've been using RAW12 & by doing a few test have just realised that the frame rate RAW8 produces is much greater. As regards Gain, SharpCap analysis of the 224c camera recommends 400, how this equates in other capture programs is guesswork for me.
Seeing at times can be quite reasonable when the skies clear & under a Bortle 1 sky helps but can't get out often enough to practice.

Again many thanks,

Mike.

[admin]

Hi Mike,

for Altair cameras, the lowest gain is always 100 in SharpCap, and gain 400 means that the gain is making the image 4x brighter. 900 means 9x brighter, etc.

Some other programs may show the Altair gain starting at 1, in which case a gain of 4 would be 4x brighter and equivalent to gain 400 in SharpCap.

If you hover the mouse over the gain slider in a recent version of SharpCap it will tell you this sort of info about how much effect the gain is having for most cameras.

Hope that helps,

Robin

[mike shuter]

Thank you Robin for your prompt reply. This has been a puzzle for me in the past but thankfully no longer.

Regards,

Mike.