L Enhance or L Pro factoring into exposure time

Anything that doesn't fit into any of the other forums
Post Reply
lasvideo
Posts: 11
Joined: Tue Oct 13, 2020 6:03 pm

L Enhance or L Pro factoring into exposure time

#1

Post by lasvideo »

Steve from Starizona suggested I view Robin Glover's video presentation on astrophotography. It was a revelation for me in several areas of interest. But I had one question that relates to my application of his principles.

I use a Celestron EDGEHD 8 " with a .7 focal corrector, ASiair, an ASI533 camera and both Optolong L Pro and L Enhance filters( depending on the subject). I live in suburban Las Vegas which is a Bortle 5/6 when shooting to the west.

On his Optimum Sub Length graphic, it is suggested in a Bortle 5 Suburban area, shooting at f/7 with my kind of camera, that 32 seconds is a good exposure time.At the bottom of the chart are specific use considerations

My ASI533 is OSC so the chart states for a color sensor to multiply by 3. It then suggests for a narrow bad filter to multiply by 25.

My question is this...Should I multiply 32 seconds exposure by 3 (since its a color camera) or by 25 (since I always use L Pro or L enhance filters), or by 28 since I use a color camera with those filters.

Thanks!
Attachments
Screen Shot 2020-10-12 at 4.18.12 PM.png
Screen Shot 2020-10-12 at 4.18.12 PM.png (461.91 KiB) Viewed 2836 times
Last edited by lasvideo on Tue Oct 13, 2020 10:53 pm, edited 3 times in total.
User avatar
admin
Site Admin
Posts: 13173
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: L Enhance or L Pro factoring into exposure time

#2

Post by admin »

Hi,

the answer might actually be to multiply by a factor of 75! That's because the filter is cutting out 95% or more of the sky glow and then the microfilters on each pixel will be throwing away two thirds of what's left (for instance on a green pixel only the green light goes through).

Realistically, unless your mount is professional grade, you are going to just have to pick an exposure time that is at the limit of your mounts tracking ability.

Cheers, Robin
lasvideo
Posts: 11
Joined: Tue Oct 13, 2020 6:03 pm

Re: L Enhance or L Pro factoring into exposure time

#3

Post by lasvideo »

Wow, after getting excited at the prospect of doing MORE exposures at a REDUCED exposure time (after seeing your presentation), that is disheartening news. :(

My Skywatcher EQ 6 -R on First Light a few days ago gave me solid tracking (less than 1 arc second deviation) for 10 - 10 minute exposures. (See attached). I had hoped, armed with your equations and science, to whittle them down to maybe 50 - 2 minute exposures and reap the benefits you extolled in your video.

But it sounds like that is not the case.
Attachments
NGC7380.PNG
NGC7380.PNG (883.76 KiB) Viewed 2829 times
User avatar
admin
Site Admin
Posts: 13173
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: L Enhance or L Pro factoring into exposure time

#4

Post by admin »

Hi,

it's the use of narrowband (or at least fairly narrowband) filters that pushes the required exposure time upwards. Overall you should reap the benefit because the signal-to-noise ratio in your final image will go down because you have excluded much of the light pollution, but the flipside of that shorter exposures are going to be less attractive and produce lower quality images to some extent than the longer ones.

Cheers, Robin
lasvideo
Posts: 11
Joined: Tue Oct 13, 2020 6:03 pm

Re: L Enhance or L Pro factoring into exposure time

#5

Post by lasvideo »

Thanks Robin! Very good to know for my future endeavors.
Maxchess
Posts: 4
Joined: Wed Sep 26, 2018 8:43 am

Re: L Enhance or L Pro factoring into exposure time

#6

Post by Maxchess »

Hi,
Robin (addmin) in your reply above you suggest that a factor of 75 may be necessary to calculate exposure times. 3x for the OSC and 25 for the filter.
I came across this https://www.researchgate.net/publicatio ... Comparison which contains the following table for the filters with the OSC camera ASI294MC:
OSC Filters.JPG
OSC Filters.JPG (128.31 KiB) Viewed 1344 times
This seems to suggest factors of 2.5 for the Lpro and 12.5 (RGGB ave Column) For the Lenhance giving a total of 7.5 and 37.5 which is not quite as onerous. if the base level for the sensor is 30 secs then with the Lpro the exposure becomes 225 secs but the LEhance gives 18 mins, which is still rather long, especially as aircraft trails often spoil my images. Does this seem right?

As the LEnhance is primarily used on objects with strong Ha emissions and on these wavelengths and others the transmission is 90%you could argue the factor is 1.1 so the 30 secs becomes 3x 1.1 * 30 = 100. As you are setting exposure time for the wavelengths of interest.

If I wanted to verify any this with my own ASI294MC and LEnhance, is it reasonable to do the sensor analysis and Histogram calcs with the Filters attached?

Max
User avatar
admin
Site Admin
Posts: 13173
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: L Enhance or L Pro factoring into exposure time

#7

Post by admin »

Hi Max,

yes, that makes sense - if you look at the transmission graph for the L-Enhance then my guess would be that the total bandpass is about 30nm (mostly in the wide peak that contains OIII and Hbeta). That estimate would give a x10 boost to exposure times, plus x3 from colour micro filters.

There is no need to run sensor analysis with the filter in place - the analysis measures the parameters of the sensor and can work with any sort of illumination as long as it is constant brightness. You would use the filter to perform the smart histogram measurements though.

Your point about the increase perhaps being excessive due to the light from emission sources passing the filter untouched is interesting. SharpCap tries to optimize for the darkest part of the frame, which (unless the target fills the frame) will be very black background due to the filter. In one sense that's correct, since you want to minimize the noise throughout the image. In another, you could argue that it's not too hard to squash noise in a nearly black background, it's actually the noise in the dimmest parts of the target that you care about. Of course, automatically (or even manually) selecting the dimmest part of the target and not some adjacent black background is hard, which is why SharpCap just goes for the simpler approach of optimizing for the darkest area in view.

cheers,

Robin
Post Reply