Taraobservatory

Hello, 

The other night I was imaging Sh2-240 and I just cant see any detail. Even in 5min subs under bottle 2 skies. Has anyone experienced this before? 

I use these coordinates (see image)

Here is my equipment is QHY260M with Chroma 3nm filters and an Esprit 80ED 

Here is a screenshot of the image. What's going on here? Is my filter not working? Is it the orientation? 

In need of some assistance 

Any idea ? 

Merry Christams 

On 21/08/2022 at 21:37, dan_adi said:

The Mesu can handle a lot. 

I have a 8 inch refractor on it with all the accessories, maybe around 50 Kg. I know for sure the counter weights are 50 kg in total.

When the weather cooperates, guiding RMS is 0.2-0.3" with this load. Also, in my case at least, with a long-ish refractor, wind-gust can screw things up for a little while, but only in winds greater than 10 miles/hour. 

To me the price/performance ratio is incredible.

Im getting very curious about these Mesu mounts. 

I dont see much information accept that they are very good. But what software do you use to controll the mount etc? 

As far as I can see I could easily do 10m subs with this thing without any bloadting of stars. 

Do you have any complaints ? 

Clear skies 

Roger 

Thank you Vlaiv, 

again, thank you for the information. I think these will come in handy for this purpose. 

The Wavefront guys got in touch with me and told me bascically what you just did. Its not required to be very acurate, just within a resonable range as the the software dont actually measure FWHM but only concerns itself with difraction. 

Clear skies

25 minutes ago, vlaiv said:

They are terms that are of interest to astronomy enthusiasts?

SQM and Bortle scale are measures of object brightness.

SQM can measure both extended target and sky background (affected by light pollution), while Bortle scale is very crude measure often used in descriptive manner for sky brightness. Mostly used by visual astronomers.

Bortle scale ranges from 1 to 9 (best sky to worst sky).

SQM is abbreviation of "sky quality meter", but is often used as - magnitude per "square" element of sky / target (sq - square, m - magnitude) - be that arc second squared or arc minute squared - in any case, some surface. It is logarithm scale much like regular stellar magnitude - and has the same meaning - or rather, how bright patch of the sky / target would be if you took star of certain magnitude and "smeared" it over that surface (arc second squared or arc minute squared).

It is measured and besides being more precise description of the sky for visual astronomers is very handy for exposure / SNR calculations for imagers.

Here is conversion table between the two:

(not very precise, as I was able to glimpse MW at zenith from red - white border - which was about SQM18.5)

FWHM is measure of how good the seeing is at any given moment, but also - what is the resolution of long exposure image.

It stands for full width of half maximum and is measure related to Gaussian (or other similar) profile created by star in a telescope. When we talk about seeing FWHM - then definition is - FWHM of recorded star using very large aperture telescope for 2 seconds (or perfect tracking mount).

Seeing FWHM will not be equal to FWHM that you get in your image. FWHM in image is influenced in part by seeing FWHM, but mount performance and aperture size play a part. How precise is your focusing also plays a part and if telescope is diffraction limited or not (collimated well, or even optically degraded by choice - like inclusion of field flatteners / coma correctors that improve edge of the field but give away some of sharpness overall).

Image scale is simply conversion factor between angular units in the sky and linear units in focal plane - often expressed in units of pixels instead of microns - so you get arc seconds per pixel or "/px.

It depends on pixel size and focal length of telescope and formula is - image or pixel scale = pixel_size * 206.3 / focal length

(where pixel size is in micro meters and focal length in millimeters).

FWHM of the image and pixel scale are related like this - there is match between where pixels match what can be recorded in terms of sharpness. This is called optimum sampling.

If you sample with less pixels per sky angle - this is called under sampling and is in itself not a bad thing. In general, there are artifacts associated with under sampling - but those artifacts never happen in astrophotography (due to nature of blur that is imparted on image by atmosphere, mount and telescope). If you read that "stars are square" due to under sampling - well that is simply not true (they can be square if one uses improper interpolation algorithm - but that is whole another story).

If you sample with more pixels per sky angle than you need - you are over sampling. Over sampling is bad.

It produces poor results when image is viewed at 100% zoom - being soft with bloated stars, but there is much more important aspect of over sampling and why it is bad. We always want to take "faster" images of the night sky and produce smoother better looking images with less noise in given amount of time.

Over sampling prevents us from doing this - over sampled images have much lower SNR then they need to. This is because light is spread over more pixels (than it needs to be) - and signal per exposure is lower and hence signal to noise ratio is lower. This is very similar to using very high magnification visually - object just gets dimmer and you don't get to see additional detail.

Proper sampling is when you properly match FWHM and pixel scale and it lets you capture most detail with having best SNR for given time.

Hello Vlaiv, 

I was hoping to get an answer from you. I learn allot thank you! 

So in light of this information, what Skywave asks for is you lopcal seeing in FWHM. And I can get a rought idea from that though SQM reading or Meteoblue, right? an SQM reading of 21.78 would come to 1.3 somethig ? 

Here is a link if anyone is interested. Software looks pretty interesing. 

Kind regards

Clear Skies 

Hello, 

I tried to search this online and didnt find anyting. Im not using FB so how can I get some info on this? How to look and maybe book?

Cheers

Roger

Hello,

Probably a stupid question, but here goes.

What is the relationship between FWHM, Bortle, SQM and image scale?

I know what they are by themselves, but can we use a number from SQM and an image scale to determine FWHM? Etc etc.

The reason for the question is that a collimation software (Skywave) needs you to input FWHM to do a correct calibration, but how can you know FWHM without taking a picture to measure? If the scope is uncollimated, I assume your measurement of FWHM will be wrong.
Please correct me if I'm wrong.

Clear skies 

Roger 

Hello, 

Im currently in the process of making my own test jigs for ccd tilt and collimation of telescopes. 

Im new to this so I have a few questions. 

The CCD tilt is fairly simple as I dont need allot of distance to do this test. But lets say I want to do a star test (using artificial stars) I would need allot of distance here. My question is this; can I just increase the distance of the filed flattener from the telescope to achieve a closer focal distance ?

Kind regards

Roger 

1 hour ago, Mandy D said:

Ooh, pretty! But, what is it?

I know, I know: It's a telescope ...

A bit more detail for those of us drooling who have no idea beyond it being a telescope, please. 😊

Hello, 

This is the Sharpstar SCA260 V2 FLO sent it from the UK yesterday and it arrived here in Norway today. 

Ill get stuck in soon and send some pictures of some alignment tests ill be doing. 

Clear skies 

Roger 

Here is what my postman brought me today

Cant wait to set it up! 

Clear skies 

4 hours ago, Adam J said:

Really you need to post an image. 

Adam 

Hi Adam,

Ill post next time as I didn't save any of my tests and now the clouds are rolling in. 

Kind regards

Roger

Hello,


I have some issues with my QHY 268. The back focus seems to not be uniform. I get elongated stars in 3 corners which they are pin sharp in another. This leads me to believe that the sensor not uniform with the optics. I have also tested with another telescope and the same issues occur so its definitely a sensor misalignment in the camera. I understand that I can adjust this at the front if the camera but I wonder if there are some tips on how to do this successfully ? As my attempts to tackle this has so far failed. Is there some kind of software or an easy way to get this done ? 

Kind regards

Roger 

Hei, this looks interesting. 

Stupid question here, but when you say desired SNR? I think I understand this but I better ask for clarification. 

What is my desired SNR under 21.78Mag sky ? (Pick a random target) I see that 20 has been put there as a preset but isn't that dependant on the target and sky brightness ? And how does on get to that number? 

Kind regards

Roger 

Hello, 

Thanks for the information Need to look into this. Is it normal for people to travel to other countries for this ? 

Clear skies 

Hello, 

I came across this forum and been wondering how one goes about creating a star party ?  What are the rules and conditions that needs be met ? 

Cheers 

Roger

16 hours ago, dan_adi said:

Small pixels are not really a problem, as you can bin your camera and make larger pixels and optimize you image scale. I guess the new ASI 6200 or 2600 series don't have amp glow. But given your top notch skies the cameras you have will do a great job as is. I use a CCD KAF 16200 sensor and it's ok in Bortle 4. Bellow is a work in progress, about 20 hours worth of subs in luminance with a CCD. I'm ok with it.

MasterLightAbell2218_ABE.tif 62.57 MB · 5 downloads

Reassuring to say the least. I can honestly not see any noise in your picture. What is your exposure pr sub here? I have never been able to put my gear to proper use yet. The images I have done is no more than max 10subs pr filter at max 300sec for M101 and 180 for the clusters. Not nearly as much time as I actually need. Just waiting to get started again

Clear skies 

7 hours ago, dan_adi said:

For example galaxy M33, surface brightness 23 mag/arcsec^2, and a desired SNR of 20 in each band:

Luminance: 1.55 hours

R: 5.38 hours

G:5.72 hours

B: 6.3 hours

For a SNR of 40 in each band:

Luminance: 5.98 hours

R: 21.7 hours

G: 22.6 hours

B: 24.9 hours

As a side note I don't know how the SNR adds in each band, I've had a conversation with Vlaiv some time ago about this, but I could not find some equations in this regard. The conclusion beeing the SNR in each band is not additive, like the final SNR is not the sum of SNR in L + RGB.

In your case I would aim for a SNR of 40 in Luminance, and a SNR of 20 in each RGB, so I would expose for 5.98 hours in Luminance, 5.38, 5.72,6.3 in RGB. Round that to 6 hours in each band, so 24 hours in total exposure time. Hope this gives you an idea about your setup and imaging time.

Another note, for extended objects like galaxies and nebulae, you need the surface brightness for computation, not the simple magnitude. Here is a simple explanation: https://rasc-vancouver.com/2020/08/23/surface-brightness-vs-magnitude/

Clear skies!

Thank you so much for this! Great help! 

I will et you know how it goes

Clear skies 

3 hours ago, vlaiv said:

I just ran some basic calculations and it will actually take about an hour of exposure in those conditions to swamp the read noise x3

If you really want shorter exposures - look at CMOS sensors with very low read noise.

I also have a QHY 268M which I will use with my Esprit 80 for nebula. That has a very low read noise. What other CMOS cams can you recommend. One without  amp glow and and large enough pixels for a 1300mm ? 

Kind regards 

Roger 

Indeed, 

I have very dark skies and allot of potential to get some good photos. I noticed that my 60 and 120sec exposure of M13 was perfect for stars at least. I also did a another with M92 where I exposed for 180sec where the stars are sharp but not as tight as M13. This leads me to believe that my guiding could be better its but good for my mount (around RMS 0.4 which is great). Its a Skywatcher EQ8r-Pro with a 40kg load. See examples in these links https://www.astrobin.com/53lqze/B/. https://www.astrobin.com/fbphrz/

The reason for the 40kg load is because I have a dual setup which I have now dismantled in favour of single telescope configuration because it gives me more freedom to experiment with different telescopes without having to unload a 40kg rig every time. And hopefully my guiding will improve too if only slightly. 

Not sure I gave you full well but it is 31268 from my measure in pixinsight. I believe this also has to be in your calculations ? 

Thanks for your help! Really appreciate it

Kind regards

Roger 

My goal is to take as short exposures as possible without adding too much noise. I like the stars to be very tight. But I didn't want to miss out on faint details due to noise from my CCD. 

Here are some parameters if you dont mind. 

Read noise 10,78

Dark current 0.018 

Pixel Size 5,3

QE 57% 

A 120mm FL 630mm (f5.25) 

SQM (Sky brightness) 21.78mag (Bortle 1.4) 

What would this look like ? 

Kind regards and clear skies

Roger 

6 hours ago, dan_adi said:

I had to write my own software for this. Install python, synphot and astropy, then run ETC.py. All files should be in the same folder.

Didn't get a chance to make a standalone app. I'll get to it some time. Tested on IOS and Ubuntu, windows not yet. If you can't compile and run it, PM me and I'll calculate the total exposure time for your target. 

Clear skies!

 

CCD.csv 6.05 kB · 0 downloads compute.py 7.47 kB · 0 downloads device.py 1.86 kB · 0 downloads ETC.py 21.19 kB · 0 downloads utility.py 16.33 kB · 0 downloads

Oh my, How amazing! And thank you for sharing. This is what I am after! 

The season is over for me here in Norway due to the sky brightness but ill write down my measurements and send you as soon as the skies get dark again. . Is the calculator available for Windows ? im not familiar with any of the programs you mention. 

Kind regards

Roger 

41 minutes ago, vlaiv said:

Offset is not important for sub exposure length. Use gain setting that you will be using for imaging.

If you want to determine what is best tradeoff for sub length - here are guidelines:

1. How much data you want to stack and process? Shorter subs mean more data. Some algorithms like more data, others like good SNR per sub

2. How likely is it that you'll get ruined sub (for whatever reason - wind, earthquake, airplane flying thru the FOV - whatever makes you discard the whole sub - satellite trails can be easily dealt with in stacking if you use some sort sigma reject). Longer discarded subs mean more imaging time wasted

3. Differences in setup - in general, you'll have different sub length for each filter, but sometimes you will want to keep single exposure length over range of filters (like same exposure for LRGB and same for NB filters) as this simplifies calibration - only one set of darks instead of darks for each filter

4. What is the increase in noise that you are prepared to tolerate?

 

Only difference between many short subs and few long subs (including one long sub lasting whole imaging time) - totaling to same total imaging time - is in read noise. More specifically, difference comes down to how small read noise is compared to other noise sources in the system.

When using cooled cameras and shooting faint targets - LP noise is by far the most dominant noise source, that is why we decide based on it, but it does not  have to be (another thing to consider when calculating). If you have very dark skies and use NB filters - it can turn out that thermal noise is highest component, so this calculation should be carried out against it instead.

In fact - you want "sum" of all time dependent noise sources (which are target shot noise, LP noise and dark current or thermal noise - all depend on exposure length) and compare that to read noise.

Read noise is only time independent type.

Noises add like linearly independent vectors - square root of sum of squares. This is important bit, because this means that total increase is small if you have components that are significantly different in magnitude. Here is example:

Let's calculate percentage of increase if we have LP noise that is same, twice as large, 3 times as large and 5 times as large as read noise.

"sum" of noises will be sqrt( read_noise^2 + lp_noise^2) so we have following:

1. sqrt(read_noise^2 + (1 x read_noise)^2) = sqrt( 2 * read_noise^2) = read_noise * sqrt(2) = read_noise * 1.4142 ... or 41.42% increase in total noise due to read noise

2. sqrt(read_noise^2 + (2 x read_noise)^2) = sqrt(5 * read_noise^2) = read_noise * sqrt(5) = read_noise * 2.23607 = (2 * read_noise) * (2.23607/2) = (2*read_noise) * 1.118 or 11.8% increase (over LP noise which is 2*read_noise in this case)

3. sqrt(read_noise^2 + (3 x read_noise)^2) = sqrt(10 * read_noise^2) = read_noise * sqrt(10) = read_noise * 3.162278 = (3 * read_noise) * 1.054093 = 5.4% increase over LP noise alone (which is 3*read_noise here)

4. sqrt(read_noise^2 + (5 x read_noise^2) = sqrt(26 * read_noise^2) = read_noise * 5.09902 = (5* read_noise) * 1.0198 = 1.98% increase over LP noise alone

From this you can see that if you opt for read noise to be x3 smaller than LP noise - it will be the same as having only 5.4% larger LP noise and no read noise, and if you select x5 smaller read noise - it will be like you increased LP noise by only 1.98% (and no read noise).

Most people choose either x3 or x5 - but you can choose any multiplier you want - depending how much you want to impact final result. Thing is - as you start increasing multipliers - gains get progressively smaller, so there is really not much point going above ~ x5

Ok, but how to measure it?

That is fairly easy - take any of your calibrated subs and convert to electrons using e/ADU for your camera. CCD will have fixed system gain, while gain on CMOS will depend on selected gain. Pay attention when using CMOS cameras if your camera has lower bit count than 16 bits. In that case you need to additionally divide with 2^(16-number_of_bits) - or divide with 4 for 14 bit camera, with 16 for 12bit camera and 64 for 10bit camera.

When you prepare your sub - just select empty background and measure mean, or even better median electron value on it (median is better if you select odd star or very faint object that you don't notice). This will give you background value in electrons.

Square root of this value is your LP noise. You need to increase exposure until this LP noise value is your factor times larger than read noise of your camera.

Alternatively, if you want to get exposure from single frame - take your read noise, multiply with selected factor, square it and this will give you "target" LP level. You need to expose for "target" / "measured" longer (or shorter - depending on number you get).

Makes sense?

 

Thank you so much for this. This makes sense and I see what you mean when you say a calculator might not work for all these factors

Cheers 

Roger 

48 minutes ago, vlaiv said:

Any such software tool is bound to be very imprecise.

This is because you can't model every little bit of in order to accurately calculate optimum exposure length - and even then - there is no such thing as optimum exposure length.

Optimum implies best possible value - and in reality best possible value for any setup is single exposure as long as total imaging time. As long as we use cameras that have read noise - optimum / ideal solution is that one - single very long exposure.

What you can do instead is accept some level of tradeoff. What level of tradeoff you accept has much more impact on calculated sub length than other parameters.

Some parameters that you can select also have very large range of "implied" values to be of any use.

For example - you say to take Bortle 4 or 5 sky. That is roughly 19.1 to 21.3 SQM - more than 2 magnitudes of difference. That is more than x6.3 in sky brightness between two extreme points. Consequently - that is more than x2.5 in LP noise levels.

This translates into sub length difference for all other conditions of about x2.5

On one side of spectrum we have for example 1 minute and on other 2.5 minutes exposure - this is just by being indecisive about what exactly is our sky brightness.

Best way to tackle this problem is to actually measure values - measure you background sky flux and your read noise and based on these two parameters alone you can select tradeoff that you are ready to accept.

You seem to know what you are talking about so Ill ask some newbie questions if you dont mind.

Lets take my setup for instance. 

QSI 638 - Esprit 120 (This one was at full well max setting when I did the measurement off the sensor in pixinsight) (f5,3 , pixel 5,3 , QE 57% , read noice 10,78e , full well 31800e) 

QHY 268 - Esprit 80 (At what setting should I do measuring for instance ? At what Gain and Offset?) 

SQM reading is 21.70 so Bortle 1,3

Skywatcher EQ8r-Pro mount (stationary) 

How would you go about finding the best exposure? 

Cheers 

Roger 

Hello, 

Been through a couple of sites now and watched a few YouTube videos on this topic as it seems to be quite useful. Looks to me as it can save allot of people some head ache and money. 

When I am after is a tool that can calculate the optimal exposure for your setup under your particular sky. 

I know Sharpcap has a sensor analysis tool which is used to calculate the specifics of your camera, but you also need something to calculate your sky background. Then we can go populate the values in http://www.gibastrosoc.org/sections/astrophotography/optimum-exposures-calculator. This calculator does not talk about CMOS cameras which has Gain and offset etc.. So I believe its a bit outdated (only relevant for CCD). Am I wrong is assuming this ? 

A tool that can do all this for you so you dont need to jump back and forth to different calculators and softwares to determine your exposure would be amazing. So if anyone has any idea, please let me know. 

The benefits of knowing this could actually save you allot of money. Say you live under Bortle 4 or 5 sky and invest in a have duty mount for better long exposures which is apparently completely useless with a CMOS camera. In fact (if you use LRGB only you MAY not even need to guide as your ideal exposure times can be reduced dramatically with the same result). 

Any thoughts and help to understand this better would be welcomed 

Cheers 

Roger

There are a few galaxies in there but not sure it qualifies as a cluster

Clear skies