Making The Best of Budget - Imaging

Discussion of using SharpCap for Deep Sky Imaging
Kopa
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Joined: Wed Jul 08, 2020 6:26 pm

Making The Best of Budget - Imaging

#1

Post by Kopa »

Hi All,
I am trying to get some DSO images on a budget with the following setup.

my Equipment:
Celestron 114 GTO Scope and Mount. 1000mm short tube reflector with inbuilt reducer in focuser tube.
Camera - SVbony SV305
Focuser - DIY Ardruino, UNL2003 28BYJ-48 stepper motor, Ascom myFocuserPro2 which works well for me.

Issues struggling to overcome:
Scope camera: very small FOV (0.32° x 0.18°) astronomy tools FOV calculator
Mount: maximum exposure 4 seconds. Anything beyond stars no longer round or sharp
Location: Bortle 8 and white LED street lights
Live Stacking: very hit and miss failing most of the time as very few stars picked up by small FOV.
Platesolve: Platesolve2 in ATP works ok but Platesolvers in Sharpcap hit and miss - ASTAP

I use Sharpcap Pro Brain function for gain, exposure and darks only so far with no flats.
I would like to use Software dithering but not sure if useful in my case.
can platesolve with Platesolve2 but not consistent with ASTAP.

Can platesolving be used as a guiding option? Any suggestions to help me get improved DSO image experience with my setup would be much appreciated

Thanks
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admin
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Re: Making The Best of Budget - Imaging

#2

Post by admin »

Hi,

I think that, as you've already pointed out, the bulk of your problems are coming from the very small field of view that you are trying to work with - this reduces the number of stars in view, making plate solving, live stacking, etc. less reliable.

As always with astro imaging, the important thing is to work out what is holding back your image quality the most at the moment and deal with that. For you I suspect that it is the small field of view and the indifferent tracking (max 4sec exposures). Fixing the tracking would require a more expensive mount. Fixing the FOV will require a change to camera or to telescope.

I'm guessing that you have the Celestron 115LCM telescope - this is unfortunately not a highly regarded model, particularly because of the 'Bird Jones' optical arrangement with a spherical mirror and corrector lens in the focuser.

I think if it was me, I would be seeing if the computerised mount could be used with a cheap short focal length refractor, setting aside the 114mm Bird/Jones tube. The challenge might be being sure that any replacement telescope would fit the mount in advance of purchasing, but (as an example) the Starwatcher SkyTravel 102 has a focal length of 500mm at f/4.9. This will double your field of view and the reduced f-ration means that images will be substantially brighter for the same exposure time. The scope is only an achromat, so will show colour halos around stars, but if you want a wider field of view then this is probably cheaper than buying a camera with a larger sensor. Using a shorter focal length should allow slightly longer exposures too (with less magnification you can allow a longer exposure before star shape problems become noticeable).

cheers,

Robin
Kopa
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Re: Making The Best of Budget - Imaging

#3

Post by Kopa »

Thank you Robin for your very prompt reply.

I shall definitely investigate the alternative scope option.

Given my limited mount tracking capability, how much would liveStacking influence my exposure roughly if at all?
for example if with the suggested (Starwatcher SkyTravel 102 with a focal length of 500mm at f/4.9). I can achieve 10s exposures with round stars. Can live stacking compensate for my tracking to let me use say 15s exposures?

Also more generally struggling to know how direct guiding dithering vs software dithering(without guide scope) compare.

Kind regards

Vince
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oopfan
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Re: Making The Best of Budget - Imaging

#4

Post by oopfan »

Hi Vince,

I just watched this video on your telescope's SkyAlign feature:
https://youtu.be/B4RvPnSC_Eg?t=2

You said that the longest exposure is 4 seconds before star trails appear. I am curious. If you center the field on a star, and then watch it over an extended period of time, does the star remain roughly in the center, or does it eventually drift out of view?

Thanks,
Brian
Kopa
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Re: Making The Best of Budget - Imaging

#5

Post by Kopa »

Hi Brian, thanks for your response.

Yes my star drifts gradually over time off my FOV. Hence my intention to use plates solving but only platesolve2 seems to work on my frames where ASTAP fails.

I use CPWI celestron version to align. i,e I Rough align scope North and roughly horizontal. CPWI software then suggests targets, on which I click and scope slews to. then I use the sharpcap reticule and and centre target in FOV and align. Four stars may be suggested to align on.

Robin is right I think as my very small FOV seems to magnifies the drift, Was hoping for Livestacking to compensate but low star numbers is a problem.
So I save all frames and try stacking via Folder Monitor like attached.
My FOV of Orion
My FOV of Orion
Stack_16bits_47frames_94s_.png (115.62 KiB) Viewed 1407 times
regards
Vince
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Re: Making The Best of Budget - Imaging

#6

Post by oopfan »

Hi Vince,

Thanks for the photo. That really helped.

The good news is that I have an Altair 290M which has the same sensor as your SV305, so I am able to relate to what your photo tells me. The bad news is the 1000mm focal length of your telescope. Not only does it cause a narrow field of view but it also spreads starlight over too many pixels, something called over-sampling; in other words, the image is dimmer than it needs to be. Also, over-sampling puts a huge demand on your mount's tracking. You really need excellent tracking in order to show round stars.

I agree with Robin. Time for a new telescope with a shorter focal length. My scope has a focal length of 418mm. It is well suited for my/your camera. I have a field of view of 0.75 x 0.28 degrees which is considering better than your 0.32 x 0.18. Next, you need to ask yourself how much are you willing to invest? I don't think that Celestron had "interchangeability" in mind when they designed your telescope. So, I am not hopeful that you can replace the telescope without inviting additional problems.

Did you purchase the telescope online or through a retailer? If you have a retailer nearby, pay them a visit and explain that you'd like a scope with a 400-500 millimeter focal length. Perhaps they can give you a good deal on a trade-in. Just be willing to walk away if you get the "hard sell". Personally, I don't have experience with alt-azimuth mounts, but I know that some people here at the forum have one. Most people have equatorial mounts including myself.

My best advice is to set an upper limit on what you are willing to spend, and hold to it. Don't rush into anything. Gather information, and come back to the forum and ask questions. We are happy to help.

EDIT: I should have thought of this sooner: Why not try lunar and planetary photography? Your scope is best suited for it since the Moon and major planets are bright, and therefore require very short exposures. Other people on the forum can speak authoritatively on this topic, but I think what they will tell you is that you can use SharpCap to capture images but you will need to process them offline.

Brian
Kopa
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Re: Making The Best of Budget - Imaging

#7

Post by Kopa »

Hi Brian,

I bought the scope/mount combination way back in 2014 so maybe it is time for an upgrade.

According to the sharpcap documentation "Live stacking enables the capture of deep sky images without the traditional requirements of high accuracy, guided, equatorial mount and long sub exposures". So maybe I should focus on upgrading the telescope first.

Thanks you for your very practical advice. I shall investigate the compatibility issue too, which maybe I would not have thought about.

Kind regards

Vince
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turfpit
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Re: Making The Best of Budget - Imaging

#8

Post by turfpit »

Vince

This link https://www.astrobin.com/search/?q=SVbony+SV305 is the results of a search on Astrobin for your camera. The first observation is that over half the images are lunar/planetary. By clicking on the individual images you will be able to see what equipment produced the image. This will give you a feel for how well combinations of mount and telescope work together with your camera.

To address the field of view issue, a 0.5x focal reducer will give an up to 40% increase in field of view. Typically these, new, cost in the region of £40 https://www.rothervalleyoptics.co.uk/an ... r-125.html. This would help with Live Stack and your number of stars issue when imaging deep sky objects. You may find such an item in web sites selling used astro-gear.

If you go to the Astronomy Tools website http://astronomy.tools/calculators/field_of_view/ and select the Imaging button at the top of the page, you will be able to select Messier objects and enter your scope and camera details to see what the combinations are likely to achieve. A reducer can be added to your existing equipment. This is now my first port of call before purchasing any new scope or camera.

With your existing equipment, a good place to start would be the moon. It constantly changes and thus provides lots of material to work at. There would be a learning curve both for image acquisition and post processing of data. To deal with brightness and atmospheric instability a Wratten #25 red filter would help https://www.365astronomy.com/Castell-25 ... -1.25.html. One of these can be purchased for under £15.

Some things to consider:
  • Astro-imaging is a long haul hobby.
  • Astro-imaging is a money pit.
  • Spending £5000 on equipment does not get you instant access to great images.
  • Expect to spend many hours learning how to set-up and get the best out of any equipment you have.
  • Be prepared for good nights and not so good nights.
  • The used equipment market (where people have upgraded to newer equipment) can save a lot of money. You need to understand what you are buying.
If you look in the Tutorials sub forum there are some HOWTOS I have written which may help you in getting started.

My battles with M42 are documented here viewtopic.php?f=16&t=596.

Dave
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oopfan
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Re: Making The Best of Budget - Imaging

#9

Post by oopfan »

Hi Vince,

You mentioned:
"Live stacking enables the capture of deep sky images without the traditional requirements of high accuracy, guided, equatorial mount and long sub exposures"
The without...guided part deserves further explanation. I don't guide and yet I am able to achieve round stars with 30-second exposures using a simple set up. My short focal length helps, but mainly it is due to the quality of the polar alignment that I get with SharpCap. If my polar alignment is not accurate then I suffer from something called "declination drift". Your telescope doesn't have a physical polar axis like mine, but you do have a mathematical polar axis. I think that your polar axis is misaligned, and therefore you suffer from a good amount of declination drift. That explains why stars drift off field over time. There are two possible remedies: (1) choose a different set of stars in your alignment procedure, (2) be careful not to bump the tripod or "man-handle" the telescope; avoid applying any undue force that might alter your mathematical polar axis.

The without...long sub exposures part is complicated. What you call long is short for me. With my Altair 290M and suburban Bortle 5 skies the Brain tells me that 30-second exposures are just right. The closer you are to a city, the recommended exposure goes down. What kind of skies do you have? Also remember that you are "over-sampling". My telescope's focal length is half of yours, so my camera's pixels receive four times the number of photons per second. So, what I can achieve in 4 seconds will take you 16 seconds. Dave's advice about looking into a good quality 0.5x focal reducer is great advice.

There are a couple other things to look into:
1. Are you applying calibration frames during Live Stacking? Even though you are using 4-second exposures, you should capture 30 "dark frames" beforehand. Each dark frame will have information about the bias (or offset) that SharpCap will subtract from each frame before adding it to the stack.
2. Check with Robin if SvBONY cameras offer you control of the bias (or offset) setting. If you have the bias set too low then it will affect the detection of faint nebula. My Altair 290M offers such a control in the SharpCap panel. I set mine at the midpoint of the slider control.

Brian
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Re: Making The Best of Budget - Imaging

#10

Post by oopfan »

Vince,

Let me show you my very first DSO, captured May 10, 2017. I can't begin to tell you how thrilled I was with this. 100x 10-second exposures for a total integration time of 16 minutes:
M13_2017-05-10.png
M13_2017-05-10.png (659.93 KiB) Viewed 1366 times
Compare that to the following. I did this a year ago, February 26, 2020. Same mount, different telescope and camera. I bought a cheap, used refractor for $500, and a 10-year old, used CCD camera for $400. But the equipment is secondary to the most important thing: experience and a willingness to experiment:

https://astrotuna.com/m81-bodes-galaxy-in-11-6-hours/

Astrophotography is an exercise in humiliation and self-flagellation.

Brian
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