[RCC] M101 focus and star size - Forums

Topic starter

Mar 15, 2021 · Edited: Mar 16, 2021

currently in the process of collecting data on M101 and it has not been without issues! Dealing with learning how to focus my new GT81 and weather its been challenging to add hours on this galaxy!

I had setup last saturday but i think my focus was slightly off so i'm forgetting those and starting over. setup last night but only got about 1.75 hrs before i pulled the plug due to wind and clouds. this image is from last night. at least i know this setup will handle 24mph wind gusts! but i had to throw out several subs due to clouds.

I'm wondering if this image looks focused properly, stars etc.? I'm using the built in bahtinov mask to focus by along with BYE. I've read that with BYE to use the least FWHM value as the point of critical focus. but doing that still seemed the center diffraction spike was a bit off. so i had to take pics and look at them in PS to figure out which way to turn the focuser to get it centered. took some trial and error but i got it to where it looked centered. at least i hope it looks focused!

Also, do the stars look too hot, or blown out or bloated? I used a mask in PS to bring the values down a bit a that reduced the size some. still think they could be a bit smaller but not sure if reducing the highlights is the right way to go. still new to astro processing! Thanks!

mount: cgx, unguided
scope: WO Gt81 with 0.8 field flattener
cam: canon 6d unmoded, no filters
exposure: bulb, iso 800 80s
Lights: 61/79 subs
Darks: none
Flats: none
initial processing in APP
touch up and noise reduction in PS
heavy crop from the full frame

Andrea Alessandrelli

Mar 15, 2021

I think it's a quite nice image especially taking into consideration that it's on a GCX unguided with an unmodded camera. The stars look fine to me but this is a delicate matter, very subjective. What I can suggest is to have a look at any star reduction tutorial if you haven't tried it yet.. I think astrobackyard covers the stars reduction procedure in Photoshop. By the way I am not a fan of star reduction.

About the focus, the image is slightly off and also the bahtinov diffraction shows it, the center spike goes a bit to the right side. A suggestion that I can give you here (I have GT-81 too by the way) is to avoid to overexpose the star you are focusing on , you need to look at the center of the bahtinov diffraction if you want to reach perfect focus. I usually use 1 sec exposure for the first rough adjustment the I drop to 0.2 0.3 seconds for fine adjustment, at this point seeing might trick you but a trained eye can focus without any particular problem. I suggest to look at a few short exposures as they come before starting the fine focusing adjustment.

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Cfeastside

Topic starter

Mar 15, 2021

Andrea Alessandrelli:
About the focus, the image is slightly off and also the bahtinov diffraction shows it, the center spike goes a bit to the right side. A suggestion that I can give you here (I have GT-81 too by the way) is to avoid to overexpose the star you are focusing on , you need to look at the center of the bahtinov diffraction if you want to reach perfect focus. I usually use 1 sec exposure for the first rough adjustment the I drop to 0.2 0.3 seconds for fine adjustment, at this point seeing might trick you but a trained eye can focus without any particular problem. I suggest to look at a few short exposures as they come before starting the fine focusing adjustment.

thanks that's very helpful! i did try shorter exposures but seemed harder to see. i'll try what you suggested. definitely seeing is an issue as its usually poor to average.

Björn Arnold

Mar 16, 2021

@Cfeastside Very nice image! I've nothing to add to it.
Regarding the faint Bahtinov pattern with reduced exposure time: I would expect, if you raise the ISO to 12k or above, you should see the Bahtinov pattern even for short exposure times in the live view?

@Andrea Alessandrelli, I join you in your opinion about star reduction. Stars are there and part of the universe. Reducing star intensity, hmmm, ok, removing stars: No go!
I also agree that the focus is not perfect from the Bahtinov. What's also a bit tricky/mean is when you realize that the star is a double star. Sometimes on first glance one could miss that.

Cfeastside

Topic starter

Mar 16, 2021

@Björn Arnold thanks for commenting. The iso is automatically set to 25000 in backyard eos. The bahtinov image, bigger one above is mizar. Which is a double apparently and so are many in the area around m101. I suspect due to the small image scale and poor to average seeing its hard to determine focus off live view. I think BYE also doent really support that zoomed in resolution well. I get better res when looking at it in PS. You guys must have much better eyes than i do as even when i zoom in just above pixel level its hard to tell its not quit centered. I wonder if my back focus is a little off? I have it set at WO recommended 7.2 on the adjustable field flattener. Hoping to get calmer conditions tonight though the seeing forcast is meh.... thanks for the help!

Björn Arnold

Mar 16, 2021

@Cfeastside,
I wouldn't say that I've better eyes but it depends a bit on how you look at it. If you turn the image such that the center diffraction pattern is horizontal, it's not that obvious. If you turn it by 90degree so that the center pattern is vertical, you'll see it. It's the way how we see symmetry.

Tim Hutchison

Mar 16, 2021

I certainly agree with @Andrea Alessandrelli regarding focus, but I would also suggest that you give strong consideration to guiding. The CGX mount is simply not capable of unguided imaging at that focal length. I know...I have one. The slight inaccuracies in the mount are causing a softness in your image that looks like a focus problem. Really it's just an issue with tracking.

If you are new to imaging and guiding seems complicated, it's really not. I'm happy to help if you need some assistance.

Keep at it! You're off to a great start!

Tim

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Cfeastside

Topic starter

Mar 16, 2021

@Tim Hutchison thanks for the advice and words of encouragement! Yep new to this and trying to keep it as simple as possible and keep the learning curve just this side of vertical! I know i will be giuding soon though. Need to read up on a few things first.

this forum has been great and very helpful!

Honeycakehorse

Mar 16, 2021

I agree as well. You did a great job for these difficult conditions (no guiding, filters, focus etc).
The learning curve in AP is no joke and i understand you 100%, but i also encourage you to use guiding. It is a game changer.
When you feel comfortable with it, consider to use autofocus as well.
For many month i used a bathinov mask as well and checked the FHWM and eccentricity values 100 times per night to be sure, im in acceptable focus range.
Spend the money, buy a decent autofocus and never look back. Beside not doing it manually, the biggest deal is the automatic temperature compensation. I did not know, what a big deal it is for every degree celsius.
Now it is one click using N.I.N.A. and let it do its FHR thing - totaly love it! You will save so much imaging time, nerves and you will have crispy images every time.

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framoro

IOTD Staff

Mar 16, 2021 · Edited: Mar 16, 2021

Hi, just to participate in the discussion, I have two comments:
- When you use the bahtinov mask, try to look at the spikes in smaller stars in the background. I was once told that smaller stars spikes are better to tell you if your focus is OK.
- You have some "rain noise" that dithering may improve. In this aspect, guiding may help you.
- It is not exactely in the theme of the discussion, but your image needs some flats as you have a big bubble on the left side of the galaxy.
Anyway, IMO it is a nice picture.
CS

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Cfeastside

Topic starter

Mar 16, 2021

Hi, just to participate in the discussion, I have two comments:
- When you use the bahtinov mask, try to look at the spikes in smaller stars in the background. I was once told that smaller stars spikes are better to tell you if your focus is OK.
- You have some "rain noise" that dithering may improve. In this aspect, guiding may help you.
- It is not exactely in the theme of the discussion, but your image needs some flats as you have a big bubble on the left side of the galaxy.
Anyway, IMO it is a nice picture.
CS

Thanks for your comments CS
good point about using smaller stars in bg. I had picked up on that on my m45 image. I tried that with this one but most were too blurry to tell. I did see one on the edge of fov that showed off center. But wasnt sure it was representative of center. Ya i usually do darks flats but wasnt planning to use these subs so didnt this time. At least i know my darks and flats are working as i dont usually see that blob. Haha

Carlos D. Hernandez

Mar 17, 2021

It looks really nice! I got the same scope and new at this. I did Whirlpool last weekend and did not come out as cool as this one. I'll see if I can capture the some next.

Cfeastside

Topic starter

Mar 19, 2021 · Edited: Mar 19, 2021

as a follow up to this post i wanted to share some insight based on comments received here and my last session which was 4.5 hrs of data collection. regarding focus, it looks like i have several contributing factors for the soft image. but after going through all my subs and looking at the exif data it appears temperature change is probably the biggest factor in my soft image. i only have set focus at the beginning and didn't touch it after that. apparently i need to be adjusting focus at least every degree in temperature change. I started imaging at about 47F and when i stopped it was about 27F. looking at the unprocessed images from the beginning to the end you see the stars start small and get bigger by the end. over that 4.5 hr duration FWHM value started at 2.07 and ended above 5.0. for sure i have some tracking polar alignment issues but i think the change in focus due to temp is the biggest reason for the soft image. at some point i guess i will get a auto focuser and connect it to software that can adjust focus due to temp change. for now i guess i will have to manually adjust every hour or degree of temp change. ugh

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Tim Hutchison

Mar 19, 2021

@Cfeastside , It certainly seems like you have some focus change due to temperature going on here. I do agree with your analysis. However, if you look at the star shapes in your image in this post you will see that the stars are not round, but rather are elliptical. As you say, this is an issue with either polar alignment or tracking. So I would keep in mind that, until you solve all three issues you likely will not solve 100% of your problems. I only mention this because I don't want you to be disappointed after acquiring a focuser if your issue isn't completely solved.

As far as focusers go, I use an Optec focuser on my Takahashi refractor and I am VERY satisfied with it. There are certainly lots of choices out there for quality focuser solutions, but give these a look. I have been using it for around 1 year and it is fantastic. It was very easy to install and hasn't given me an ounce of trouble. Because they attach directly to the focus knob on the telescope, there aren't a bunch of fidley brackets or anything to deal with. It is a very clean, compact, and reliable solution.

Optec Focus Motor for GT71

Please feel free to reach out if I can be of any further assistance.

Tim.

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Cfeastside

Topic starter

Mar 19, 2021

@Tim Hutchison i'm definitely going to start guiding as well. hurts going through 180 subs and tossing out more than i'd like to admit. ;). thanks for the suggestion on focuser i'll look into that. thanks for the offer of help. i'm sure i'll be back with more issues to solve!