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Old 14-02-2021, 11:55 AM
MarkInSpace (Mark)
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Advantage of aperture (?)

I consider myself newish to astrophotography, I’ve been imaging for nearly a year, I started with a ZS71 and graduated quite a while ago to a 115 triplet refractor.
In spite of my online investigations, I have yet to fully understand the advantage of moving to a larger aperture telescope, particularly if I keep the f ratio the same.
As an example, I see that I can buy an 200mm f/8 RC with a reducer that gives me the same f/5 that I have with my current 115 refractor with a reducer flattener.
What practical advantage will this give me if I keep my same camera (asi294mc)?
Thoughts and images especially welcome.
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Old 14-02-2021, 12:37 PM
raymo
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The larger the scope, the smaller the detail that can be resolved. For instance
an 8" will show a lunar crater half the size that a 4" will. It is linear, so a 12"
will show one a third the size, and a 16" one a quarter the size.
Telescopes gather light according to the square of their diameter, so a 2"
gathers 4, a 4" gathers 16, a 6" gathers 36, and an 8" gathers 64, and so on.
that obviously means that an 8" [64] gathers light 4 times faster than a 4"[16], enabling four times as much data to be gathered in the same exposure time.
raymo

Last edited by raymo; 14-02-2021 at 12:44 PM. Reason: more text
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Old 14-02-2021, 12:54 PM
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What Raymo said plus,

The Field of View of the RC will be almost half of the Triplet.

Your image scale in acrsec per pxel is approx = px size x 200 / focal length

For the RC its = 4.63 x 200 / 1000 = 0.926 arcsec per pxel
As the camera is approx 4000pixels wide thats a Field Width of 0.926 x 4000 = 3704arcsec ( 61.73 arcmins).

For the Triplet its = 4.63 x 200 / 575 = 1.61 arcsec per pixel
Field Width = 1.61 x 4000 = 6440 arcsecs ( 107.3 arcmins)


There is also telesope resolution to consider

Dawes Limit = 120 arcsec / Objective Diameter (mm)

For the RC its 120/200 = 0.6arcsec
For the Triplet its 120/115 = 1.04 arcsec

The other difference is Star Magnitudes, a larger aperture can see fainter stars than a smaller aperture.
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Old 14-02-2021, 01:26 PM
Startrek (Martin)
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Mark,
I recommend you read an article by Craig Stark ( original developer of PHD guiding software) called “ Sampling Myths” circa 2010
Puts it all into perspective plus an attached image comparison

A bit technical but even an old dumbo like me can sort of understand it

Cheers
Martin
PS: I’ll take a 6” , 8” or 10” newt over an 80mm or 100mm refractor any day , no disrespect to anyone at all , it’s how deep you want to explore into an object and get the most resolution out of , that’s my challenge
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Old 14-02-2021, 01:42 PM
MarkInSpace (Mark)
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Thanks for the comments so far - I will read the whole series of signal to noise articles.
Here’s my problem: if the F number doesn’t change, I believe my ability to speed up capture does not increase. If my camera does not change, at some point, I loose any resolution benefit from the wider aperture. Am I on the wrong track?

Is there a graph or table that relates cameras to ideal apertures and focal lengths?
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Old 14-02-2021, 02:05 PM
Rerouter (Ryan)
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The imaging time changes a little. but your right that its practically the same.

6" 762mm vs 8" 1016mm = about 1.788x times the light collecting area but the image is 33.33% more zoomed in, so ends up about 0.5% brighter / less imaging time needed

Edit: I fogot that zoom deltas are a square. e.g. 5x zoom is 25x smaller area.

Last edited by Rerouter; 14-02-2021 at 03:23 PM.
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Old 14-02-2021, 02:09 PM
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In its simplest, a larger aperture allows you to image at a finer resolution without losing SNR.

If you have a 4” F/5 and an 8” F/5 both of them put the same amount of photons per pixel (using your ASI294). The 8” captures 4x the amount of photons but it has twice the focal length; 4x less photons per pixel. So, you have the same amount of photons per pixel but at twice the resolution; twice the focal length.
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Old 14-02-2021, 02:27 PM
Startrek (Martin)
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Mark
Here’s a few notes ( not in order ) from Craig Starks document

F ratio determines photon density which is explained as follows -
Aperture = total photons
Focal length = spread of photons
F Ratio = density of photons

Photon density per pixel well rather than total photons collected is more important
There is a balance between Aperture and focal length. As we go up in focal length for same Aperture past the point of over sampling, you will lose SNR without gaining any spatial information.
Scopes that have higher focal ratios of say f8 to f10 spread photons thinly which reduce the photon count per pixel and reduces SNR. Running a scope at a lower F ratio will make each pixel cover more Sky which provides better images.

Finally
Quote - Photon count for an extended object is driven by F ratio. Image scale is driven by focal length. Want more resolution at the same pixel wise SNR ?, boost the Aperture but keep the same F ratio. Want more SNR in your images and your willing to trade of some spatial information ( area ) or your already asking for more spatial information ( area ) than your conditions will allow ? Then drop your F ratio !!

Cheers
Martin
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Old 14-02-2021, 02:42 PM
Startrek (Martin)
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Quote:
Originally Posted by Startrek View Post
Mark
Here’s a few notes ( not in order ) from Craig Starks document

F ratio determines photon density which is explained as follows -
Aperture = total photons
Focal length = spread of photons
F Ratio = density of photons

Photon density per pixel well rather than total photons collected is more important
There is a balance between Aperture and focal length. As we go up in focal length for same Aperture past the point of over sampling, you will lose SNR without gaining any spatial information.
Scopes that have higher focal ratios of say f8 to f10 spread photons thinly which reduce the photon count per pixel and reduces SNR. Running a scope at a lower F ratio will make each pixel cover more Sky which provides better images.

Finally
Quote - Photon count for an extended object is driven by F ratio. Image scale is driven by focal length. Want more resolution at the same pixel wise SNR ?, boost the Aperture but keep the same F ratio. Want more SNR in your images and your willing to trade of some spatial information ( area ) or your already asking for more spatial information ( area ) than your conditions will allow ? Then drop your F ratio !!

Cheers
Martin
Forgot to mention
Camera type , pixel size, well depth , Gain, sensor temperature, QE , colour or mono etc etc... is a separate issue not discussed but is directly associated with the optical physics above
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Old 14-02-2021, 03:21 PM
MarkInSpace (Mark)
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You all are great! Thanks for the info - enough links to keep me reading for the rest of the weekend!
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Old 14-02-2021, 10:28 PM
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Camelopardalis (Dunk)
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On top of all this good stuff, you need to consider seeing...

All the modern CMOS cameras with small pixels can make for some impractical resolutions (per pixel) with a larger scope when coupled with “average” seeing.

The limits at which you’re no longer capturing extra detail but just further characterising the blur can quite easily be reached with modest gear.

Of course, on a better than average nights, the results can be gobsmacking...it’s just that these nights are inversely proportional to your aperture
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Old 15-02-2021, 01:48 PM
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Its an interesting topic. Traditionally the answer would be aperture rules and its still true but perhaps to a lesser degree.

I think it comes down to what you want to image.

You can quote Craig Stark but try to image most spiral galaxies and you'll see the advantage of aperture and longer focal length.

With advances in sensitivity of cameras and higher quality modest aperture scopes I would say 70-130mm is good for widefield, 8-12 inches is probably a practical limit to galaxy and close in view imaging. There are quite few modest aperture APOs now that are not super expensive and are capable of a great deal.

I would class astro scopes into 4 categories:

Up to about 106mm aperture APO for widefield imaging.

Around 130mm APO for higher resolution widefield imaging.

Medium focal length wider aperture like up to about 12 inches and often F4 or faster. This for smaller objects but also can do fairly large objects. Its quite flexible. Focal length around 1260mm.

Longer focal length large aperture to get the small dim galaxies with decent detail.

There are tons of superb 10-12 inch Newts around F4 that show the power of aperture and a fast F ratio. A large aperture allows a faster F ratio whilst also maintaining a decent focal length needed for smaller dimmer objects.

As pointed out the latest CMOS sensors tend to have smaller pixels than the usual crop of CCD cameras which are typically 9 micron and as small as 4.54 micron. Whereas CMOS smallest is 2.3 micron and 3.76 is a common size now and 6 micron being one of the largest (also an older less sensitive sensor).

Plus the trend for these sensors is going to be smaller and smaller pixels so less and less suitable for astrophotography.
These later CMOS can be binned 2x2 but its not a hardware CCD-like binning but rather done after the fact of an image by software so the gains are less.

Greg.
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Old 15-02-2021, 02:26 PM
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Quote:
Originally Posted by gregbradley View Post
These later CMOS can be binned 2x2 but its not a hardware CCD-like binning but rather done after the fact of an image by software so the gains are less.
I thought some of the more recent CMOS cameras could offer hardware binning, e.g. ASI6200/QHY600 ?
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Old 15-02-2021, 06:58 PM
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Quote:
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I thought some of the more recent CMOS cameras could offer hardware binning, e.g. ASI6200/QHY600 ?
No its software binning only.

The 294M bins 2x2 giving 11mp. It can be run 1x1 giving 47mp. its the only one I am aware of that you can bin.

Greg.
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Old 15-02-2021, 10:54 PM
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According to the PDF manual for the ASI6200 (section 5.8), it supports hardware bin for bin2 and bin3 (software for bin2, bin3 and bin4).

I'd presume the QHY600 would be similar?
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Old 16-02-2021, 01:20 PM
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Quote:
Originally Posted by lazjen View Post
According to the PDF manual for the ASI6200 (section 5.8), it supports hardware bin for bin2 and bin3 (software for bin2, bin3 and bin4).

I'd presume the QHY600 would be similar?
Oh OK.. The sensor is the same in the ASI and QHY except the QHY photographic version uses the industrial grade sensor whereas the ASI uses a lesser consumer grade sensor.

I see there is some reference to binning here on the QHY website but its not done the same way as CCDs to its detriment. I am under the understanding that 2x2 binning does not raise the signal to noise ratio much at all compared to CCD where it does. It does increase the full well depth though.
FAQs
1. Does QHY600 support hardware binning?
The CMOS sensor itself has some binning function but it should not be the hardware binning (FD binning). And also the binning in the sensor is based on the location of the bayer color . it means it will binning with the same position of the same color.And for monochrom QHY600 sensor, it is still use such a position to do binning. So we think it is not a good solution for the monochrom binning.
And since the very low readout noise of the QHY600, so the digital binning may bring more advantage. First , it can increase the fullwell. Binning at 2*2 will gives four times of the fullwell. Second, it will increase the AD sample depth. Binning at 2*2 will give 18bit data range. For readout noise, the N*N digital binning will cause the readout noise become SQR(N*N)= N times. For example, if the readout noise is 1.9e at 1*1 binning. The 2*2 digital binning readout noise will become 1.9*SQR(2*2)=3.8e.


Greg.

Last edited by gregbradley; 16-02-2021 at 02:32 PM.
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Old 28-02-2021, 11:22 PM
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Assuming my ASI2600 is using a similar approach to the 6200, I think it’s all software binning. Even the so called increased FWCs are “virtual” (in that if one pixel overflows, you’re non-linear).

Turtles all the way down sadly.
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Old 01-03-2021, 12:55 AM
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Its a bit misleading when they state the Full Well Capacity increases 4 times when 2x2 binning.
The 2x2 binning also captures photons 4 times faster, in other words the stars will reach saturation at the same rate of time if it was not binned.
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Old 02-03-2021, 12:09 PM
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The ASI6200/QHY600 apparently have some hardware binning.

On the QHY600 there are different readout modes. One of these extends the full well depth. I don't think that is only virtual in that case. This is one of the main QHY advantages, these different readout modes.

But its something I will keep an eye on as I started using the extended full well depth and it seemed to hold highlights better and reduce background banding.

Greg.
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Old 02-03-2021, 06:51 PM
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From my understanding with astrophotography many use small apertures compared to visual astronomy. They use refractors, easier to hand, dont need as heavy duty mounts, no collimation and better sharpness and no centre obstruction. If aperture was the main issue then all astrophotographers would have hugh apertures, but simply don't.

The aperture comes into play with planetary, with astrophotography you put your money into the mount first, and aperture less concerning.
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