M33 with the RASA8

[gorann]

My third image with my new RASA and ASI2600MC (I am still processing the second one) and the first one (M31) is here: 

Obviously this 400 mm FL scope is not really a galaxy hunter so I have gone for the biggest, M31 and M33, as seen from Earth. From now I will aim at nebulae. Stars look better than with M31. The spikes are gone after I organized the camera cables in a circle. A lot of dew but no dew heater seems to be needed since the camera releases enough heat inside the dew shield.

88 x 2 min at gain 100 (offset 30).

 

EDIT: Added a new version benefitting from deconvolution and a bit more distant dust.

Edited August 23, 2020 by gorann

[AbsolutelyN]

Fantastic image. How have you got the stars so pinpoint at the edges on APS-C? I can't get them that good even with my 1600MM. 

[gorann]

Thanks! Yes, my stars were also elongated in some corners probably due to minor tilt (not chip distance since one corner looks good). I simply fixed them with an old trick in PS, took 20 minutes. The way I did it was to make a duplicate layer and then in blend mode Darken move that top layer with the arrow keys until I get round stars and use the brush tool on the layer to apply only in the affected area. Have to do it separately for each side or corner as the elongation was in slightly different directions.

Edited August 18, 2020 by gorann

[AbsolutelyN]

15 minutes ago, gorann said:

Thanks! Yes, my stars were also elongated in some corners probably due to minor tilt (not chip distance since one corner looks good). I simply fixed them with an old trick in PS, took 20 minutes. The way I did it was to make a duplicate layer and then in blend mode Darken move that top layer with the arrow keys until I get round stars and use the brush tool on the layer to apply only in the affected area. Have to do it separately for each side or corner as the elongation was in slightly different directions.

Thanks Gorann, that's a nifty little trick. Not come across that one before - just tried it and work really well. Love the power cable trick too.

[MarkAR]

Lovely image Gorann, well done. Like the cable trick.

Edited August 18, 2020 by MarkAR

[gorann]

23 minutes ago, AbsolutelyN said:

Thanks Gorann, that's a nifty little trick. Not come across that one before - just tried it and work really well. Love the power cable trick too.

Yes, probably easier for me than trying to fix the tilt. Very small margins at f/2. Are your elongated stars in all corners? Maybe your chip distance is a bit off?

[gorann]

9 minutes ago, MarkAR said:

Lovely image Gorann, well done. Like the cable trick.

Thanks a lot Mark!

[Laurin Dave]

Very nice Goran... 

[AbsolutelyN]

17 minutes ago, gorann said:

Yes, probably easier for me than trying to fix the tilt. Very small margins at f/2. Are your elongated stars in all corners? Maybe your chip distance is a bit off?

I think the distance is very close, possibly just a bit of tilt in one corner. Two images attached of each corner, just quick crops from raw stacks.  Happy with the HA but LRGB filters are never as good. All Baadar 2" filters.  

RGB


HA

Edited August 18, 2020 by AbsolutelyN

[gorann]

I would guess we both have a slight tilt problem. There is tilt adjustment on the ASI2600 but I am not sure I want to try to fix it. When the moon is back I will start collecting Ha with an ASI1600 - it will be interesting to see if I am more or less lucky than you.

[gorann]

18 minutes ago, Laurin Dave said:

Very nice Goran... 

Thanks Dave! The RASA8 and the ASI2600 appear to be a nice combination.

[gorann]

Realized that the image sharpness could benefit from some deconvoluton and more distant dust could also be brought out:

 

Edited August 23, 2020 by gorann

[Datalord]

50 minutes ago, gorann said:

Realized that the image sharpness could benefit from some deconvoluton and more distant dust could also be brought out:

I like it. Well done!

[gorann]

1 minute ago, Datalord said:

I like it. Well done!

Thanks a lot!

[x6gas]

Excellent rendition Göran - looks like you already have the set up working very well.

[MarkAR]

That's come out even better, definitely picked up more of the faint outer arms.

[gorann]

4 hours ago, x6gas said:

Excellent rendition Göran - looks like you already have the set up working very well.

Thanks a lot! Yes, the setup has been quite productive so far. F/2 helps when astrodarkness is still only about 3-4 hours up here. One problem is that stacking these 50 Mb files, which for some reason become 300 Mb after debayering, takes longer than the actual data collection. And at f/2 the exposures have to be short - have maily gone for 60 s, which meant 180 subs in 3 hours last night, so I am still stacking those.... Fortunately external harddrives are cheap nowadays.

[gorann]

1 hour ago, MarkAR said:

That's come out even better, definitely picked up more of the faint outer arms.

Thanks! Yes, there always seems to be something that remains to be done to improve an image.

[Datalord]

33 minutes ago, gorann said:

And at f/2 the exposures have to be short - have maily gone for 60 s

You may want to have a look at the version I posted this morning from the RASA11. I ended up using 3 min exposures in a pretty light polluted area without blowing out the stars. I think the extra time gives me more colour data, if I'm comparing our images. I only boosted saturation in mine, I didn't alter hue. 

[gorann]

9 hours ago, Datalord said:

You may want to have a look at the version I posted this morning from the RASA11. I ended up using 3 min exposures in a pretty light polluted area without blowing out the stars. I think the extra time gives me more colour data, if I'm comparing our images. I only boosted saturation in mine, I didn't alter hue. 

You caught a nice version of M33 there!

It depends a lot upon the gain or ISO setting. The ASI2600 has a sweet spot at gain 100 (some extraordinary electronic thing happens to the S/N ratio), but at that gain it is quite sensitive. My M33 was with 2 min subs since there are no very bright stars there and tonight I am shooting Tulip nebula and surroundings with 2 min subs, but last night Sadr was in the frame so I had to go down to 1 min, and stacking took 5 hours.

[Datalord]

12 minutes ago, gorann said:

but last night Sadr was in the frame so I had to go down to 1 min

Yeah, a mag 2 star will do Bad Things to any image with the RASA. I can't remember my gain settings without powering up the rig, but I seem to recall 100 as well.

If I read your chart right, you have a full well capacity of 18000 at gain 100? That seems low. I'm trading FWC for noise with my settings because of the light pollution and the photon buckets we have.

Here's the QHY247C charts.

[gorann]

Yes, you are right, 50 k full well at gain 0 and around 20k at gain 100. But I think I read somewhere that the S/N will be lower if shoting at gain 100 with more subs, than at gain 0 with fewer subs. For M31 I tried both gain 0 (60 seconds) and gain 100 (30 seconds) and did not see much difference. I do not understand the gain-scale for the QHY in your graphs and how that translates to ASI gains. We should get Vlaiv @vlaivto explain this. One good thing with the ASI2600 is that it is 16 bit and I love that it has no amp glow whatsoever, so I see no reason for taking darks.

[Datalord]

10 minutes ago, gorann said:

I do not understand the gain-scale for the QHY in your graphs and how that translates to ASI gains

Me neither. I put my trust into a short article made by the chief engineer at QHY which basically boils down to: Try it out with your own gear and light pollution, tune the settings. I tried 2800 with 10s and 30s subs, but the result simply isn't as good as low gain with 180s subs.

[gorann]

The ASI2600MC has the Sony IMX571 sensor and the QHY247C has the Sony IMX193 and I do not know the difference between these.

By the way, have you ever had to collimate your RASA and does it hold focus over the night? My collimation appeared to be perfect when it arrived and focus is spot on all night. These are usually problematic thing with fast telescopes but maybe not with RASA. The probelm I do have is star shapes that probably are related to minor tilt.

Edited August 23, 2020 by gorann

[vlaiv]

Why are you trying to compare these two cameras? They don't have same sensor.

QHY247C has IMX193 sensor while ASI2600 has IMX571 sensor.

QHY has 36K full well and 14bit ADC while ASI one has 50K and 16bit ADC.

ASI has rather simple gain scheme - gain is 0.1db units. This means that for 60 gain increase (or closer to 61) - e/ADU value doubles (or rather halves). This makes it easy to calculate any e/ADU value if you know unity gain or any other gain value.

In this particular case, since ADC is 16 bit and FW is 50K - there will be no unity gain - initial gain is close to 50K/64K = 50000 / 65535 = ~0.763e/ADU.

On the other hand QHY seems to have linear gain - or gain translates into e/ADU linearly (look at first graph - it is line).

It starts somewhere around 3.4 at above graph - which is a bit strange since 36K /16K = ~2.13. Initial gain should be somewhere around 2.13e/ADU.  There is error with above charts.

Indeed, here is chart from QHY website:

Other charts can be found here:

https://www.qhyccd.com/index.php?m=content&c=index&a=show&catid=94&id=14&cut=1

In any case - you can't just compare gain 0 and gain 100 with doubling exposure and expect it to be the same.

Only thing that you get by changing gain is read noise decrease and smaller full well capacity. If you want to compare two gains you need to compare these two values.

Let's compare gain 0 and gain 100 for ASI2600 from this perspective:

Gain 0 has e/ADU of 0.763e/ADU and gain 100 has e/ADU of about 0.25. Difference in FW will be about 3 times. You need x3 times shorter exposure with gain 100 to match full well capacity when you sum those three exposures.

Read noise at 0 gain is about 3.4e while at 100 gain it is about 1.48e (very close to 1.5 but a bit less). When you stack 3 of these smaller read noises - you get 1.48 * sqrt(3) = ~2.5634352e

This is less than 3.4e.

There fore it is better to use 1/3 exposure with gain 100 than whole exposure at gain 0 - for ASI2600.