First Effort of M101 and M51 and questions!

[JCIdaho]

Hello All!

I'm completely new to AP and here to post my first pic! I followed Adam Blocks' excellent fast track tutorial on PixInsight. I have the follow equipment:

• StellaMira 90mm ED Triplet f/6
• StellaMira 2" 0.8x Reducer / Field Flattener
• ZWO ASI 2600MC-PRO
• CGEM II
• ZWO Mini Guide Scope
• ZWO ASI 120MM Mini
• AisAir Plus

I was able to take these images of M51 and M101, which I'm very happy with as a first attempt!

 

 

I would love any feedback on those! Things I could improve on.

I do have the following newbie questions (please direct me elsewhere if this part of the forum isn't adequate):

1) Gradients in images:

I have quite a big gradient - you can see the blueish band near the bottom, then a green-ish band. I'd like to ask, is this normal? I understand I can (and have to a degree) process it away using DynamicBackgroundExtraction, but it feels quite pronounced.... is there something else going on that I'm doing wrong? For example see this pic, that I have just auto-stretched:

2) Image not flattened.

I'm using a Field Flattener, but if you see in the image above, the stars nearer the sides of the image are warped / stretched. Again, should I expect this? If I were to take an image of a FoV filling object, like M31, then presumably this would have a large negative impact on the overall image?

3) Flats

I'm using AsiAir to generate my flats. The generated preview JPGs look pretty scary, with the ring below. But then the actual downloaded files look much better.

Preview

 

Is there anything to worry about here? Testing around today, I made the following flat that looks awful from the AsiAir preview image, but I was able to get a spec of something off the sensor, to make it go away. Again though, the actual resulting flat image looks OK.

 

 

Probably nothing to worry about, but thought I'd check. It's all new to me!

Thanks.

 

 

[barbulo]

First of all, welcome to SLG.

Congratulations for your first lights: In my opinion, those are a good entry to the dark art of AP.

I don't know what can cause that banding in your images, but posting the exposure time, gain and offset might help to determine. There are many IMX571 users around that surely will help you. 😜

Since you are using a field flattener, the coma is caused by a wrong backfocus: the distance between the sensor and the FF. That radial pattern means that that distance is too short: try using some spacers. 

To help you with the flats, you could explain how you took them and attach a FITS file. Did you use other calibration frames such as darks or bias? Did you correct the flats with its darks?

Finally, a very personal opinion: cropping that M51 image the way you presented it, you missed a few other smaller galaxies that would have fit in a bit wider FoV:

[Clarkey]

Just as an additional piece of info the 0.8x reducer can be put on the wrong way round as it has the same thread at each end. If your spacing is correct at 55mm try reversing the field flattener.

[alacant]

On 31/03/2022 at 17:11, Jon Pither said:

images of M51 and M101

Very nice.

One small tip. Maybe ease off the black? You may have lost some of the fainter fuzzy stuff. I'd put good money on there being a lot more to see.

21 hours ago, barbulo said:

try using some spacers

+1. You need 55mm sensor to shoulder of ff.

Cheers

Edited April 1, 2022 by alacant

[JCIdaho]

Thanks. I don't think I have the 55mm backfocus sorted, so will try that! Thanks!

[AstroNebulee]

22 hours ago, Jon Pither said:

3) Flats

I'm using AsiAir to generate my flats. The generated preview JPGs look pretty scary, with the ring below. But then the actual downloaded files look much better.

Hi and welcome to sgl 

Very nice images indeed, you should see my first ones and they are truly awful indeed, yours are great. 

You already know the issue with your elongated stars so will bypass that. 

I'm not sure either on the banding in your images

I use an asiair pro too and when taking my flats I use a led light panel with several sheets of blank a4 paper in a homemade sleeve to pop over my scope opening. I try to get the histogram hump near to halfway along the graph and use 2 second manual exposures and mine look not good but work fine (the jpeg version always looks bad) . There nothing wrong with your flats to my eyes. 

Good luck your doing great. 

[JCIdaho]

Thank you for the feedback. Onwards and upwards!

@alacant @barbulo @Clarkey - adding 55mm of backfocus properly has solved my curvature issues! See my new image below.

@AstroNebulee - thanks for reassuring me on the flats!

When the clear skies return, I'm going for shorter (3 min) exposures and lots more integration time (only managed 90mins thus far). Thinking of getting a LP fitler, and a Bahtinov mask!

Jon.

Looks flatter:

 

And here on M44.

 

[AstroNebulee]

53 minutes ago, Jon Pither said:

Thank you for the feedback. Onwards and upwards!

@alacant @barbulo @Clarkey - adding 55mm of backfocus properly has solved my curvature issues! See my new image below.

@AstroNebulee - thanks for reassuring me on the flats!

When the clear skies return, I'm going for shorter (3 min) exposures and lots more integration time (only managed 90mins thus far). Thinking of getting a LP fitler, and a Bahtinov mask!

Jon.

Looks flatter:

 

And here on M44.

 

No problem, looks much better for the curvature. I always shoot 3 min subs, never dared to go longer yet. Definitely get the batinov mask, makes it a breeze for focusing 👍

Cheers 

Lee 

[ollypenrice]

As Alacant said earlier, I'd leave your black point higher. When you bring it in too far you get an unnaturally jet black sky - which is also distractingly glossy - and you clip out faint data. It's tempting to use the black point to reduce gradients, but don't!!!

Your level of gradient struck me as quite low.  Even though I live at an exceptionally dark site I get that level of gradient sometimes, though the fact that you have a horizontal band is odd. (That's if your LP is at the level shown in the M52 unflattened image. If so I wouldn't use an LP filter.)

Olly

[iantaylor2uk]

It would be worth, in my view, trying shorter subs. 3 mins sounds like a long time to me. Try 30-60 seconds if you are using a UV/IR filter only. Each sub may look less impressive, but if you image for the same total time, you will have a lot more subs to stack. I only go to 2 mins if I'm using the L-enhance filter. If your telescope is f/6 and you are using a 0.8x reducer you are effectively at f4.8 which is quite a fast system, so shorter subs should be fine. Shorter subs mean you can also put the gain up on your CMOS camera (with the advantage of lower noise) without filling up the well of the sensor.