[RCC] Andromeda - Forums

Topic starter

Dec 6, 2020

Hi folks:

This is my first opportunity at more than 45 minutes of usable data (by a significant margin - I managed nearly 90 minutes before the streetlamp glow started to creep into the corner of the frame). I am using SiriL and Photoshop. I'm pleased with the progress since my last attempt, but unhappy with the amount of noise still present.

https://www.astrobin.com/fkof88/

The camera is un-moddified, and I'm working with a pretty bright moon and nearby streetlamps, and a cheap achromat.

I'd appreciate any input on acquisition, processing, and even recommendations for equipment upgrades. I'm looking at maybe adding guiding, modifying my camera, or adding a filter in the near future and would appreciate advice on which direction to go. I can't afford a new OTA/lens anytime soon, sadly, but anything else is fair game.

Thanks in advance!

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Marc Agostini

Dec 6, 2020

Hi Mark,

First of all congrats, you have really handled your existing equipment well, getting 90s subs out of an unguided star tracker is no mean feat! And you really hit that focus point very well, whis isn't easy with the halos on an achromat.

I would suggest the following improvements:

Aquisition: Try ISO 800 for the T3i. I always use this site as a resource for the best DSLR ISO. This should improve your signal/noise ration by cutting down the read noise, especially for shorter exposure times. Of course, if you are going to invest in an guidescope+cam you can get longer exposure times which will also help. I used autoguiding with my Star Adventurer and it really worked well. Also (local Corona regulations permitting) use the fact that your equipment is light and go somewhere with little light pollution, if I remember correctly there are some beautiful mountains right next to Vancouver

Modifying your camera will mainly help with Ha-heavy targets like the rosette nebula. I also did that with my Nikon D3200, I used the tutorial from this site. The link will lead you to a T3i conversion tutorial. Its also a great DIY improvement if you 're not afraid to open your camera.

Filter-wise, I would recommend the Baader Semi-Apo filter, which should get rid of some of the blue halos as well as being a soft light pollution filter effective against Sodium Vapor Streetlight glow.

Otherwise, a field flattener would make the stars in the corners of your frame rounder, but I don't know if there is one for an 1,25" focuser. I only know of field flatteners with 2" barrels.

Clear skies, Marc

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Mark Germani

Topic starter

Dec 7, 2020

Hi Marc:

Thanks for your reply, and advice! Lots to think about there. I have a few questions:

Marc Agostini:
Aquisition: Try ISO 800 for the T3i. I always use this site as a resource for the best DSLR ISO. This should improve your signal/noise ration by cutting down the read noise, especially for shorter exposure times.

My strategy so far has been to find the longest exposure time I can get without misshapen stars and then adjust the ISO until the histogram lies on the lower third. I live in a Bortle 8 zone, so often this means a rather low ISO. Should I instead be shortening my exposure time and sticking to ISO800? It was my understanding that fewer longer-exposure subs were better than more shorter-exposure subs, but if the noise difference between ISO200 and ISO800 is negligable, I could see how more subs at a higher ISO = more light gathering. Is that what I should be taking away here?

Marc Agostini:
Filter-wise, I would recommend the Baader Semi-Apo filter, which should get rid of some of the blue halos as well as being a soft light pollution filter effective against Sodium Vapor Streetlight glow.

I've been eyeing the Baader Semi-apo. Would it help much with CLS? I'm hesitant to spend much money on filters & flatteners to try and improve the performance of my achromat, as I figured it would be better to save up for a proper apochromat (eyeing the Redcat) instead. That said, the Semi-Apo is not terribly expensive (about the same price as the OTA itself) especially if it helps with CLS. I have the Svbony CLS and I'm getting terrible gradients from it, and it's making the fringing around the stars worse. Could I even use both filters together?

Thanks for all the advice - I think I'm going to have to modify the camera at some point in 2021. And yes, a dark-sky site would be a great idea, but I have a young daughter with an early bedtime, and I don't drive, so I can't really go anywhere at night at the moment. But maybe next summer we'll go camping

Thanks again!

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pcyvr

Dec 8, 2020

Hi Mark,

I'm certainly no expert but from my experience so far, without a light pollution filter, you will end up having to take very short exposures at ISO 800 under Bortle 8 skies. You mentioned that you've had really terrible gradients from the CLS filter - if the pictures were taken at close to full moon or with very strong artificial light nearby (you mentioned a street light outside the house), I think this is to be expected. After you stretch the image, these gradients get really exaggerated.

I haven't used Siril, so not sure what it offers in terms of gradient reduction, but AstroPixelProcessor does an amazing job. I've had some really terrible looking results after initial alignment/stacking that were dramatically improved after the gradient was reduced.

Do you have the subs that you took with the CLS filter online anywhere (e.g. in DropBox) - I could run them through APP to see what you end up with, if that would help.

I know that for me the light pollution filter was a game changer. I don't think I could get far without it from a Bortle 7 zone.

Paul

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Mark Germani

Topic starter

Dec 8, 2020

Hi Paul:

Thanks for your thoughts! I will give the shorter exposures a try next time. I hadn't even thought of how the streetlamp might have contributed to the gradients. Even the flats look pretty bizarre, chromatically, with the CLS filter in so I assumed it was a bad filter. I've had some good luck recently doing background extraction on the linear data, so that it doesn't get exaggerated while stretching, and this has helped a fair bit. Maybe I'll save the CLS filter for moonless nights, and try to get away from streetlamps.

I would be curious to see how AstroPixelProcessor works. My strategy has been to acquire as much imaging as I can and then start trying the time-limited demos so that I have plenty of examples to work with when evaluating the various options, but I'd gladly take you up on the offer to try running some of my subs through. I'd have to capture a bit more first, but I'll message you when I have something

Thanks again for the advice. Clear skies!

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pcyvr

Dec 8, 2020

Good strategy to gather a bunch of data before starting the trials. I made that mistake with Pixinsight - the trial ran out before I really got the chance to work with it.

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Marc Agostini

Dec 8, 2020 · Edited: Dec 8, 2020

Mark Germani:
Would it help much with CLS?

It should help quite a bit, as long as your local Light pollution is mainly Sodium vapour lights. I own the Baader Neodynium filter, which is the same substrate (which has the CLS-effect). On my moderately light-polluted southern horizon the difference is like this (same camera, different scope) Baader UV/IR:

Baader Neodynium:

Both are single exposures, the UV/IR was on a 432mm f/6 and the Neodynium on a 400mm f/5. Note the Histogram Peak is roughly the same area, even though the exposure time is much longer on a faster scope. So you can see it works well against the orange glow of Sodium vapour lights.

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Mark Germani

Topic starter

Dec 8, 2020

Thanks Marc, I've read good things about the Baader Semi-APO. The screenshots you posted definitely show good CLS, and my light pollution is mostly sodium-vapour I believe. My biggest hesitation is that it would cost more than my OTA, and I wouldn't be able to use it with a future APO scope or lens purchase. Would I get similar or better results with an L-Pro filter, or do you think I might run into more issues with gradients & natural colour balance?

Cheers,
Mark

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