festoon

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

So here is a 3 second exposure at f/2.8 with the UV/IR filter in place. Still you can see many stars that are overexposed (including the M42 core). 2021-01-25_23-05-02__-15.10_3.00s_0104.fits

Thanks @vlaiv...I'll give f/2.8 a go. Also looking through my kit I do have a 1.25" UV/IR filter that came with my old Revolution imager. I'll also give that a go and compare.

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? 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?

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.

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

Also wanted to say @happy-kat..yes dithering would help. Trying to avoid if possible, but a guide camera may be a route I head down if it proves an advantage

Thanks again @vlaiv. I really appreciate that you are asking relevant questions. in general I am happy with the FOV, so not massively fussed about going to a bigger sensor area. How are you combining exposures after stacking?

My set up is unguided...have managed 90s with no issues

Thanks for the analysis @vlaiv I’ve tried exposures at 1s and found walking noise to be an issue, but have not tried 2.5s for instance. I can see the argument holding against the 533mc. But for the QHY268 surely there is an advantage there with a full well capacity of 65,000 at unity gain?

At unity gain the fwc is 4096 electrons. For a given exposure I believe this is leading to bloated stars being over exposed in a moderate sky pollution even for exposures of 10s at f2

Not sure if this should be or is best in the quipment - cameras section. If so mods please feel free to move.

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.

Managed to get 1 hour of data of M31 tonight between dusk and the rain. Imaged at f2 with an ASI224MC Cool OSC camera, and mounted on an AZ-Gti with no guiding. 352 x 10s exposures at unity gain. Stacked in DSS, Colour correction and gradient removal in Siril, Curves adjusted in Gimp, and final touch ups in Startools.

Thanks I'd be very happy to get ideas as to good targets for Jan/Feb with this set up. As much as I love imaging the Orion nebula, it requires a bigger dynamic range sensor than my ASI224MC Cool and I'm limited to exposures sub 10s as I dont have any narrowband/multiband filters in the setup.

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. 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. 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!

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. 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.

Thank you @discardedastro. This makes really good sense. Is there any software that will automate dithering without guiding?

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

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.

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.

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?

Thanks @Stuart1971 Glad I'm not the only one who's come across this issue

Any tips as to how you rotate your camera such that is orthogonal with RA/DEC in e.g. portrait - but keep the back focal lenth the same. When I attach my CCD with T2 extensions the camera is at the correct back focal distance from the lens (Samyang 135mm) but the camera is rotated at an arbritary angle dependent on where the T2 thread mates to the ccd.