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Do I need longer subs ?


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Hi,

Finally started imaging with my new 2600MC Pro, my first dedicated astronomy camera, so there have been a few things to learn coming from a DSLR.

I'm currently imaging and have just opened a sub in Fits Liberator... Do I need longer subs? Looks like the Histogram is stuck on the left hand side even though I'm imaging at Gain 100 and 3 minute subs, optolong l-enhance filter, bortle 5, 78 percent Moon.

fits.jpg.171fec83e8a7582d00442779e7096cf9.jpg

 

This is the histogram ( I think ) in APT, basically I'm not really sure how to view the histogram in APT, with a DSLR it was fairly obvious. Presumably it's the window with the auto-stretch function, in which case it appears to be off the left hand side?

 

apt2.thumb.jpg.61256eeae9fae0fabe98745df1cdd707.jpg

Edited by smr
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A gain of 100 and 3 minute exposure with the L-enhance filter is maybe a little short, unless you get lots of them.

You could try increasing the gain or the exposure time, or both and see what the results are like. It will be a question of trial and error to see what suits the camera & your scope setup the best.

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Concur. The number of photons hitting the sensor is the same except for noise effects whether you do a few long exposures or lots of short ones. But longer exposures will help dig signal out of the noise. I run 10 minutes at gain 178 on  my IMX183 sensor for narrowband, at least when my guiding is behaving. Five works pretty well.

Is there a reason that you're not running at the highest gain (i.e., lowest read noise) available? You're not going get good star color with narrowband so there's little reason to avoid blowing out the stars, and you've got oodles of well depth and ADC resolution anyway. Nice thing about your one-shot camera is that you  can take a series of high-gain, narrowband images for nebulosity, remove the filter, and shoot much shorter exposures for stars.

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New to CMOS too, but I’m sure Gain 100 is the correct value for this particular camera.

What have you set offset to?  Maybe this value is too low and an increase will move the histogram right a bit.

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Yes gain at 100 is just what I ascertained from various threads on the 2600MC Pro as a good setting. 

My offset was 50.

Just had a look at last night's stack of 3 hours, I took some flats but not sure I took them correctly as they seem to have made the image worse. 

Haven't used any calibration files, I should build a darks library.

There's a big green cast in general on a lot of the images, presumably Moon glow.

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22 minutes ago, smr said:

Yes gain at 100 is just what I ascertained from various threads on the 2600MC Pro as a good setting. 

My offset was 50.

Just had a look at last night's stack of 3 hours, I took some flats but not sure I took them correctly as they seem to have made the image worse. 

Haven't used any calibration files, I should build a darks library.

There's a big green cast in general on a lot of the images, presumably Moon glow.

There are x2 as many green pixels as red of blue so maybe this is the reason. 
Offset 50 seems ok too 

Can you post your flats here?

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That is perfectly fine histogram for such restrictive filter.

Most of the pixels in the image are background or faint parts of target - these have very low value per exposure. This is why we have hundreds of exposures.

Correct exposure time is calculated based on noise in the image. With NB imaging - read noise becomes dominant factor - but with modern CMOS cameras that already have low read noise - that is questionable (LP can easily swamp read noise even with NB filters depending on FWHM of filters).

In any case - one should not obsess too much about exposure length as difference can be small (and often is when we approach point of diminishing returns).

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27 minutes ago, tooth_dr said:

There are x2 as many green pixels as red of blue so maybe this is the reason. 
Offset 50 seems ok too 

Can you post your flats here?

Yes although it was just a green cast I'm not used to seeing with my DSLR. Maybe it was the Moon brightness I don't know.

Here's 3 flats, I ran the flat wizard in APT and had to lower the brightness down on me LED panel all the way down for the wizard to accept the flats.

My flat panel has always been fine for my DSLR but it's one of those cheap LED panels from amazon... https://www.amazon.co.uk/Dimmable-Bi-color-3200-5600k-Battery-Camcorders/dp/B07QL8WHRV/ref=sr_1_5_mod_primary_lightning_deal?dchild=1&keywords=led+panel&qid=1614083961&sbo=Tc8eqSFhUl4VwMzbE4fw%2Fw%3D%3D&smid=A90RIP9U5D1UK&sr=8-5

So I probably need to look at buying a dedicated astronomy flat panel? Any recommendations ?

F_2021-02-22_23-08-18_Bin1x1_0.2125s__-10C.fit

F_2021-02-22_23-08-24_Bin1x1_0.2125s__-10C.fit

F_2021-02-22_23-08-31_Bin1x1_0.2125s__-10C.fit

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1 minute ago, smr said:

Yes although it was just a green cast I'm not used to seeing with my DSLR. Maybe it was the Moon brightness I don't know.

It is just because data is not color balanced / color transformed like in DSLR - which has this transform applied by default (although you can also get pure raw data from DSLR as well - and it will also look funny).

 

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13 minutes ago, vlaiv said:

It is just because data is not color balanced / color transformed like in DSLR - which has this transform applied by default (although you can also get pure raw data from DSLR as well - and it will also look funny).

 

Thanks vlaiv, do you recommend I calibrate with darks as well? I'm sure you've mentioned before that this is one of the advantages of having a cooled CMOS? 

If so I would need to take darks for 180 seconds and 320 seconds at -10. 

I started off last night imaging at 180 seconds, but then thought the histogram looked bunched up to the left too much so started 320 seconds, but on reading what you've said above, with the histo looking fine... well that was at 180 seconds, so I may as well go with that to reduce guiding errors... as my stars for whatever reason do not look great at all.

I don't think my back spacing is correct, although according to ZWO's diagram it should be at the desired 55mm....  it's either my back spacing or guiding anyway, not sure which.

L_2021-02-22_21-19-51_Bin1x1_320s__-10C.fitIn this sub you can see the stars in the middle look oblong, but then look really elongated at each corner..

However here's a sub taken on the 10th, where there isn't the same severe elongation, and same imaging setup.. not sure why my stars were so elongated last night - the only thing I did differently last night was rotate my camera 90 degrees with the camera rotator of the flat 73 field flattener.

L_2021-02-10_22-55-39_Bin1x1_180s__-10C.fit

Edited by smr
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54 minutes ago, smr said:

Thanks vlaiv, do you recommend I calibrate with darks as well? I'm sure you've mentioned before that this is one of the advantages of having a cooled CMOS? 

If so I would need to take darks for 180 seconds and 320 seconds at -10. 

Sure, I'm 100% for proper calibration. In this case - darks matching lights in every respect (temp, time, gain, offset, ....) and flat darks matching flats in the same way.

Do be careful to use appropriate algorithm when mixing long and short subs as they don't have equal SNR and simple average (or sigma reject) is not the best way to do it. PI offers weighted average - so go with that if you use PI.

58 minutes ago, smr said:

I don't think my back spacing is correct, although according to ZWO's diagram it should be at the desired 55mm....  it's either my back spacing or guiding anyway, not sure which.

From quick inspection - I'd say, check connection to the camera.

My guess is that you are not using threaded connection or there is some play in focuser.

Try to compare first and last sub of the session for star elongation in the corners. If they are the same - you have some sort of tilt issue - but if they are different - then you have some play in your connection (gravity will tilt stuff in different direction depending on where scope is pointing and it is pointing in different parts of the sky in the start and in the end of the session).

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I’ve started imaging with my new Asi  2600 with 3 minute subs at gain zero which is actually gain 0.778. Mainly to take advantage of the full well and dynamic range. Depends on your target of course. I can’t remember the exact figures, but I tested it when I first received it and at gain 100, which is 0.25 approximately the full well dropped quite significantly, 50,000 to 30,000 I think(can’t remember exactly) . I haven’t had it long and haven’t had a decent chance to get a lot of subs but this was just shy of 2 hours worth of 3 minute subs on bodes at gain zero. I have quite bad light pollution so I don’t want too long subs anyway and I want to try and minimise over exposed stars. Even with these subs I could still see a few stars in the linear subs albeit faintly. 

 

A5A5B424-E1D2-4ADF-92E8-A7785BA598DA.thumb.png.2904cf3a8855eefaf70a6d4adc5ddd50.png

 

This was the histogram as shown by the Asiair at time I’ve imaging, it’s a zoomed in view. I definitely cleared the left edge. Although this was a sub from an earlier night on the same target.

753F8F87-CE8F-47DF-B355-D689B964441A.thumb.png.feba386c6a93af4f5b69e3496030d4dc.png

Edited by Scooot
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As a side, also when calibrating I’ve decided to do it as follows as there’s zero amp glow with this camera:

I take Darks, Dark Flats and Flats. No bias.

I integrate the Darks & Dark Flats.
I subtract the Dark Flats from the Flats then integrate the flats 

I then subtract the darks from the lights, then divide the lights by the Integrated flats.

Bias will be deducted from the lights when the darks are deducted.

It seems to work quite well.

 

Edited by Scooot
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7 minutes ago, Scooot said:

As a side, also when calibrating I’ve decided to do it as follows as there’s zero amp glow with this camera:

I take Darks, Dark Flats and Flats. No bias.

I integrate the Darks & Dark Flats.
I subtract the Dark Flats from the Flats then integrate the flats 

I then subtract the darks from the lights, then divide the lights by the Integrated flats.

Bias will be deducted from the lights when the darks are deducted.

It seems to work quite well.

 

That routine is proper calibration and works well even if amp glow is present with CMOS sensors.

I would advocate everyone use it (only drawback is no dark scaling or dark optimization - but those are not needed if temperature and exposure lengths are matched).

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1 minute ago, vlaiv said:

That routine is proper calibration and works well even if amp glow is present with CMOS sensors.

I would advocate everyone use it (only drawback is no dark scaling or dark optimization - but those are not needed if temperature and exposure lengths are matched).

Good to know, I used to take bias with my 450D and no dark flats but temperature of the darks would have been inconsistent. 

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8 hours ago, vlaiv said:

Sure, I'm 100% for proper calibration. In this case - darks matching lights in every respect (temp, time, gain, offset, ....) and flat darks matching flats in the same way.

Do be careful to use appropriate algorithm when mixing long and short subs as they don't have equal SNR and simple average (or sigma reject) is not the best way to do it. PI offers weighted average - so go with that if you use PI.

From quick inspection - I'd say, check connection to the camera.

My guess is that you are not using threaded connection or there is some play in focuser.

Try to compare first and last sub of the session for star elongation in the corners. If they are the same - you have some sort of tilt issue - but if they are different - then you have some play in your connection (gravity will tilt stuff in different direction depending on where scope is pointing and it is pointing in different parts of the sky in the start and in the end of the session).

Thanks for the advice vlaiv. 

I've attached an image of actual first light a couple of weeks ago, spent most of the night adjusting to the Camera, coming from a dslr there were a few things to learn. 

This is an image of the Elephant's Trunk Nebula at the end of the night, about a half hour integration, conditions weren't great during this imaging as I was shooting through high clouds but I just wanted to see what my stars looked like.

In this image the stars aren't anywhere near as elongated as the previous image of the Soul Nebula above, which would align with your theory about possible tilt? The Elephant Trunk was a lot lower in the sky whereas with the Soul Nebula last night the scope was almost vertical.

Having said that the stars in the Elephant Trun aren't great either, but I'm trying to diagnose where the problem is, with some stars towards the corner in the Trunk being more round, but certainly not tight, I'm wondering if it is something to do with tilt as opposed to back spacing ? 

Curiously ( I don't know why ) the stars in the top left of this image are very elongated, but in the bottom left, whilst not round, don't have nearly as much elongation ? 

Apologies if this is getting rather long winded, probably deserves it's own thread as I really want to be able to take images with tight stars across the field now!

So this is the image, a very quick stretch.

Elephant Trunk with severe elongation top left but bottom left not half as elongated:

test2-jpg.thumb.jpg.ee3e987c5e2329dd28b717386ebecac1.jpg


Soul Nebula (with severe elongation in each corner)

SN.thumb.jpg.a18db3aa1918611ce20f37f644ffff31.jpg

Edited by smr
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I had a look at my scope last night and realised that I could easily turn the whole camera (with all spacers attached, and the flat 73 field flattener quite easily, even though the thumb screw for the flattener was tight. 

Not sure why this is, I think I may have a faulty flattener, anyway the fact that it felt loose probably had something to do with the elongated stars when imaging the Soul Nebula.

I'm going to buy the updated Flat 73A (Adjustable) Flattener so I can do away with the spacers and get exact back spacing much more easily, by just adjusting the drawtube on the flattener.

I recently bought the Flat 73R, which is the 0.8X Reducer/Flattener so I'm going to test out the stars with this combination over the next few days, looks like we've got a clear spell from Friday - Sunday! Only problem is, as typical it coincides with full moon.

But I'm grateful anyway because I can try and dial in my back focus and hopefully get better stars, and sort out / recalibrate my guiding.

Will keep updating this thread if anyone else is having similar problems so that it might help. Should probably change the thread title now though as it's gone off on a proper tangent :D

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