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festoon

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Posts posted by festoon

    Advice on choice of Camera

    in Imaging - Discussion

    Posted

    Thanks @vlaiv. I guess I never appreciated how sensitive the setup is. If I went for another sensor like the ones I put above would the sensitivity be reduced compared to what I have now?

    I'm not sure about the physics behind fixed pattern walking noise. I guess the effect is more prominant at lower exposure times as the SNR is lower. If I decrease the gain setting on my setup, will walking noise be the improved, get worse, or be the same for a given exposure. My thoughts are it would get worse. I think overall I'm limited by the dymanic range of the sensor to detect faint and strong signals at the same time. If I decrease the gain to zero, the dynamic range will increase slightly, the full well increases to 19k (x4 improvement) and the noise only increases by x2.5. So I'd have an overall dynamic range increase of x1.7.

    Advice on choice of Camera

    in Imaging - Discussion

    Posted

    Just wanted to illustrate the point about wanting to do exposures greater than 3s, I guess preferentially 10s or above.

    Image 1 30mins of 3 sec exposures. Stacked in DSS, processed in Siril, levels adjusted in GIMP. I haven't applied a flat to this image yet. Whilst detail is good, I have not applied any noise removal, but clearly walking noise is present.

    M42_G_3s.thumb.png.46648962ea0f5f848f56a50f76ef16a6.png

    28 x 10 second subs (only 4 mins and 40 seconds...all I had before the trees obscured the image). I've applied a flat to this image. For sure this could do with more data, but at this exposure the walking noise is much less. However the raw subs show many stars that are overexposed (as attached)

    M42_G_10s.thumb.png.56fd71ebb47ad821ba9e2db7c8869228.png

    2021-01-26_00-18-17__-15.00_10.00s_0022.fits

    Advice on choice of Camera

    in Imaging - Discussion

    Posted

    3 minutes ago, vlaiv said:

    Did you add UV/IR cut filter between camera and the lens? Your stars are indeed bloated but that is due to IR and UV part of the spectrum and lens not being corrected for it.

    There is no filters added for these images. Are you saying the image would benefit from having a luminance filter and this would prevent a lot of the star bloating?

     

    5 minutes ago, vlaiv said:

    Stars do look bloated and this is because of above issue. Another thing to consider - this Samyang lens is good - but you want to use it at F/2.8 if you shoot with OSC camera. It will be sharper that way. F/2 is good for narrowband data and similar, but with small pixels and OSC camera - F/2.8 is better / sharper.

    I have not given f/2.8 a go yet. I guess that would also mean a little less light gathering, and possibly reduce over exposure? Is it sharper as the lens is more forgiving to aberrations at f/2.8?

    Advice on choice of Camera

    in Imaging - Discussion

    Posted

    Also a bit off topic...but has anyone tried with NINA (taken from the NINA documentation)

    Built-in Dithering

    There are cases where there is no guiding equipment in use, and thus no PHD2, but where dithering is still desirable. Examples of configurations like this usually include small, portable setups that have a main camera and telescope or lens, but no guiding. In cases like this, N.I.N.A. can still effect dithering operations, but on its own through its Direct Guider facility. Once activated, dithering operations become available in the Sequence and are effected by N.I.N.A. directly.

    • Like 1

    Advice on choice of Camera

    in Imaging - Discussion

    Posted

    Here is an example image for 10s exposures. Yes the galaxy core is not overexposed (about 50,000 on a 16 bit scale) but the quite a few of the stars are overexposed and when stretching this leads to very bloated stars.

    Also is an example of M42 with 3s exposures, and even then you can see stars that are over exposed

    I guess if I exposed at 1s maybe I would have all the stars below the full well capacity. But I guess then I do have to worry about walking noise.

    This is why I thought maybe the better option was to go for a deeper well :)

    I'd really be interested in trying to do as you suggest of combining images. In general I'm using Siril and GIMP as software. I'm not sure how I would do the pixel math as you suggest.

    2021-01-22_18-47-56__-15.00_10.00s_0013.fits 2021-01-22_00-15-12__-15.00_3.00s_0292.fits

    Advice on choice of Camera

    in Imaging - Discussion

    Posted · Edited by festoon

    I'm looking to upgrade the sensor on my grab and go imaging rig and wanted to seek some advice on here if posssible.

    My rig is a Samyang 135mm f/2 lens mounted on an AZGti in Eq mode. Currently I'm using a ASI224MC-Cool CMOS sensor, but am quickly seeing the limitations of this 12 bit sensor and limited full well capacity. At unity gain the FWC is 4096 electrons. My imaging rig uses an intel M3 compute stick which I remote desktop to when imaging. I use sharpcap for polar alignment and NINA for my imaging runs to acquire data.

    I'd like to stick to a sensor which has small pixels. I've tried 6.45um pixels on an ATIK414EX colour and was not happy with the undersampling, and much prefer the image with pixel size from the ASI224MC-cool at 3.75um.

    The options I have been thinking about are

    • QHY268C currently at £1950 in the UK. Advantages that I can see are still small pixels of 3.76um giving a resolution of 5.74 arcsec/pixel. 16 bit ADC so vastly superior full well capacity of 65,000 electrons at unity gain, and even at 0.4 e/ADU the FWC is 26,000 electrons. And a huge field of view due to the APS-C sensor size at 10.01° x 6.71°. Being a colour sensor its much easier to create full colour images, but one question I was wondering is would I get significant image resolution improvement moving to a mono and filter set up. The other question I had was file size and download over usb. The sensor is a whopping 26MPixel image. What do people think about the chances of the compute stick not coping with the usb file size or memory issues? Also the weight is 980g, so a significant weight addition onto my set up (compared to the 410g of the ASI224MC cool)
    • QHY268M currently at £1800 in the UK. All of the above except I would also buy a filter slider (not a filter wheel - to keep costs and weight down) and filters. As mentioned above  - would I see significant resolution improvements over the colour sensor? Would the sensitivity (quantum efficiency) be significantly better without the bayer matrix? Disadvantage, to create colour images I'd have to change filters.
    • Starlight Xpress Camera Trius PRO-814 colour currently £2000 in the UK. Pixel size is again 3.69um, so resolution is 5.63 arcsec/pixel. 16 bit ADC with full well capacity >15,000 with a gain of 0.3 e/ADU. Read noise will be a little worse than the CMOS sensors above at 3 electrons. Sensor size would give a field of view pretty large at 5.29° x 4.23°. No amp glow at all (CCD) - I imagine would probably get away with no darks. Weight is only 450g so similar to the camera I have now. I'd hope at 9MPixels my compute stick would still be able to cope.
    • Starlight Xpress Camera Trius PRO-814 mono currently £2000 in the UK. As with the SX colour but needing a filter slider and filters into the equation - would I see significant resolution and sensitivity improvements over the colour sensor?
    • ASI533MC currently at £900 in the UK. Pixel size is 3.76um giving a resolution of 5.74 arcsec/pixel. 14 bit ADC giving a full well capacity of 16,000 electrons at unity gain. Decent FOV at 4.79° x 4.79°. Weight only 470g so similar to my current set up. I'd hope at 9MPixels my compute stick would still be able to cope.

    I'd really appreciate some insights, advice, and help on this. Many thanks in advance.

    A new set up, a few firsts...and plenty of time to process data

    in Imaging - Deep Sky

    Posted · Edited by festoon

    Well the weather has been awful since the new year here in Cambridgeshire, UK.

    I've been building up my portable imaging rig over the xmas holiday. First arrived the AZ-GTi mount which has been modified for Eq mode. Second, my xmas present to myself of the Samyang 135mm f2 lens. For now I'm just using an ZWO ASI224MC Cool camera. I chose this over my other cameras as the pixel size is the smallest at 3.75um. Connected using the Geoptik cannon T2 adaptor. The setup is using an Intel M3 Compute stick and to control I'm using remote desktop.

    6555F060-245A-4196-86B3-370B1AA8410B.thumb.jpeg.a276f1fd8ce443a2e2c249dcb1d00930.jpeg

    I still have to tidy the cables up, but for weeks now have been desperate to try it out. Also during the lockdown period, I've taken the time to take a look at NINA. It looks to have all the capability needed, so installed it on the compute stick ready for first light.

    On Sunday night I had about an hour of clear sky, so the the first time managed to get some data. Did a manual focus of the lens....I need to buy some extension rings to get the sensor the right distance from the lens flange, but focus looked sharp enough. Rushed polar alignment, and set NINA off for my first target....I wanted an easy target so M42 was chosen. Goto on the mount worked well. Plate solving got the target bang in the middle of the FOV. Managed to only get 74 x 10s, and 300 x 1s lights...so only 17 mins of data. I also used the time to get some darks, but didn't manage to get any flats before it got too late in the night for being up the next morning for work.

    Spent the last 2 evening playing with the data, stacked DSS, Colour calibrated and gradients removed in Siril, and curve/levels in GIMP followed by merging two layers containing the 10s and 1s data. I'm not bothered about the quality of the image as it was more of a try out for the first time to make sure all was OK. But actually pleased with the result given how little time I spent actually gathering data for such a small aperture lens (albeit at f2) and that no flats were taken. Couple of different final images shown here. Personally I prefer the second image.


    M42_170121_final_2.png.bbc896517cdbbad0e89560827140450a.pngM42_170121_final.png.e4e125a9ea5d6e27257935af9730e1d0.png

    Looking forward to getting plenty of use of this little set up, and hopefully the weather will improve soon :) 

    I am considering a mono camera and filters for this set up, but TBH, OSC is nice and quick! That's something I'm sure will be a discussion going forwards. I think the ASI533MC will be a great match for this set up....but with the new ASI2600MM coming out, I'm tempted! The obvious improvements for any of these will be higher ADC bits for higher dynamic range, no amp glow, and a much bigger FOV without compromising on resolution. Of course mono should give a better resolution than the OSC and a higher sensitivity, but as I say I'm still in two minds.

    Clear Sky's!

    • Like 4

    Filter size and exit aperture Samyang 135mm

    in Imaging - Discussion

    Posted · Edited by festoon

    I've been investigating what filter size is required if I go for a bigger sensor for my imaging rig using a Samyang 135mm f/2 lens to avoid vignetting.

    If I use the calculator https://astronomy.tools/calculators/ccd_filter_size then it follows the basic logic that if the filter distance from the sensor is zero then the filter needs to match the diagonal of the sensor. Whilst if the filter is placed at a distance equal to the focal length of the lens then the diameter of the filter needs to be the diameter of the lens. Basically this formula

    Filter size=(Diagonal x (Focal length -filter sensor distance)/Focal length)-(Filter sensor distance/f number)

    If I use a APS-C (e.g. QHY268) sensor then the diagonal is 28.43mm. The flange sensor distance for the samyang/canon lens is 44mm. Thus if I was to hypothetically place a filter 44mm from the sensor, I would expect the filter size requirement to be 41.16mm (using diagonal=28.43mm, focal length=135mm, filter sensor distance=44mm, f number=2)

    However if I measure the exit aperture of the Samyang lens it is pretty close to 36mm.

    That being the case I am at a loss to explain how I would ever need a filter larger than 36mm as we have this arrangement, if we look at the image below. It looks like if we follow the filter size logic that the exit aperture of the samyang lens (36mm) is too small and would cause vignetting for an APS-C sensor i.e. the lens exit aperture would need to be 41.6mm to not cause vignetting. Also if we work backwards the maximum sensor size before vignetting occurs due to the 36mm exit aperture should be a sensor diagonal of 20.75mm i.e. just shy of a 4/3" sensor.

    image.png.1107a9333b0c7d3463e9e31027b4926a.png

    So two possible discussion points here...1) does the Samyang f/2 have vignetting due to the exit aperture of the lens only being 36mm, and if so this means there is no point with filters larger than 36mm in diameter and 2) if we work backwards the maximum sensor size before vignetting occurs due to the 36mm exit aperture should be a sensor diagonal of 20.75mm.

    I guess this review backs up these calculations https://www.lenstip.com/442.8-Lens_review-Samyang_135_mm_f_2.0_ED_UMC_Vignetting.html

    In this review the 50d is an APS-C sensor and vignetting is indeed observed at f/2.

    It would be great to have some opinions or comments on this.

    Self Guiding

    in Imaging - Discussion

    Posted

    I think I’ve answered my own question - the purpose of the guide cam is to keep the guide star not moving during the exposure, not to keep the star at the same position between frames.

    So the guide cam needs to be exposing for short times and updating the position to enable longer exposures. Which it can’t do if it’s doing long exposures. 

    Forget my comment earlier - I was being thick :) 

    • Haha 1

    Self Guiding

    in Imaging - Discussion

    Posted · Edited by festoon

    I was wondering if with any software it might be possible to self guide during my image acquisition.

    I have thought about this before and had good feedback here on SGL that some natural image drift is good and would naturally dither the image....so maybe if the software would every say 20 frames re-align the chosen guide star - wouldn’t that be a useful feature?

    The set up I’m using is a Samyang 135mm f/2 lens with an Atik 414ex CCD on a HEQ5 - so quite wide field. At the moment I’m running unguided.

    I understand normally I could auto guide with a separate guide scope and camera.
    However, as my imaging set up is widefield I should be able to have a decent guide star within my FOV. So my question is why wouldn’t I be able to choose a star from my live main image FOV and guide from that? If so what software would be able to do this? Maybe I’m missing something, but just wondering why a separate guidescope is needed.

     

    Setting CCD rotation for imaging

    in Imaging - Tips, Tricks and Techniques

    Posted

    2 minutes ago, callump said:

    I was just hunting for a link to this - yes, I have one, and it works really well... It's nice low profile, useful if your back focus or working distance is short.

    Callum

    Thanks @callump

    Good to hear it works well.

    For me I'm very limited in space. I've a Samyang 135mm (44 mm flange focal distance). Geoptik EOS to T2 adaptor which takes up 19mm, and 17.5mm distance from front of CCD to sensor. So that only leaves me 7.5mm spacing for any extra pieces e.g. a rotator and filter wheel/changer. If I use 2mm filters this adds 0.66mm to the flange focal distance, so I have 8.16mm available for a rotator and filter wheel. Which if I use 5.5mm for the rotator leaves me with a filter problem :)

    Only other option would be ro replace the Geoptic EOS adaptor but I find this works very well.

    Setting CCD rotation for imaging

    in Imaging - Tips, Tricks and Techniques

    Posted

    Superb @Stuart1971. Searched camera angle adjusters with T2 and found this. Thank you :)

    https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5547_TS-Optics-T2-Thread-360--Rotation-and-Quick-Changer---5-5mm-short.html

    Looks like this might be useful to have in my T2 extenders.

    I wonder does anyone have any experience of this particular adaptor...is it worth getting.....and if it works well?

    • Like 1
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