aleixandrus

Sorry for the late reply. Thank you for your insights. I suppose it is what it is, including sensor vignetting, flat panel unevenness and (probably) some user error. I'll skip this 'issue' for a while unless it becomes a problem during processing. As I said, thank you so much!

I tested different light sources. One, an old LCD tablet, it is almost for sure. Other, a LED flat panel, I supposed it is not but to be fair, I didn't check it.

Well... I 'reopen' this thread as I'm taking a second try to solve this flat issue...

I remove the whole optical train and try from flats, piece by piece. I've finally illuminate the naked sensor (just the camera resting over the flat panel) and what I found is that the illumination varies as the camera rotates (forget about the horizontal stripes, I was using a very low exposure length as the sensor is completely unfiltered and I can't lower more the illumination). Please, check how corner illumination varies and the gif above.

Is this an expected result, that the camera orientation affect illumination evenness? Has this ever happened to you? Maybe I'm doing something wrong but I am out of ideas right now...

Not a single clear night yet... and rain expected for the next 10 days 😭

Is it just me or is the weather being specially bad in many-many places in the world?

I'll check that info, kind of new to me. Thank you!

PS: I've received the 90 APO yesterday but, as expected, rain is announced for the next two weeks. I think I have some time to get the oculars for the first test 😅

Well, @Louis D, Spain is very different from north to south. Here in the north, in my town, there is an average of 190 rainy days yearly. Add a few tens of "normal" cloudy days and... yes, astro is hard here. In the south you can find a lot of places with less than 25 rainy days yearly. Quite a difference! I've been born in the wrong place 😅 Well, not really, I wouldn't change my land for the south, it is too brown, almost a desert and summers are getting more and more horrible. It is ok for a holidays trip but nothing else (to me). Tabernas Desert, in Almeria, is probably one of the more arid lands in Spain, it is not fair to compare to!

About your comments, thanks a lot. I'm "in a rush" to observe Saturn before it turns sideways. I'll take note of the exit pupil and eye relief explanations. I started wearing glasses two years ago (myopia, 2 diopters) and I've never done visual wearing them. As I'm not very familiar with visual observation in general, I suppose I need to try first and check if I feel comfortable.

Sorry for the late reply, busy week 😅 Wow, thank you so much for all reply and comments, I appreciate it so much. Very valuable info there. Some thoughts:

• I've never done visual except for some very-very casual observations using a very old 1000/100 eq1 manual newton and a 12" dobson from a friend, time ago. I know that the dobson is in another league, I get my 'wow' moment with Saturn using it and I do know the 90 APO will no provide anything similar. But it is fine. In addition and to be fair, I don't know any of those features of the target objects: bands, transits, divisions, etc. In the past I've just enjoy the object itself so my expectations are quite low, probably more in the "I want bigger" than "I want details". Of course I'm probably wrong thinking this way.
• Never try sun observation, it's not high on my priority list but I'll take a note on this for the future.
• Based on your comments, I'll discard the Mak 90 for know, give first the APO 90 a try.
• I don't have the scope yet so I have time to rob borrow some oculars from friends. I'll try to get something such 4-6-10mm ocular plus x2 barlow just to test magnification in the range of x90 to x180. Scope specs mention "up to x300", this may be just a marketing scam but I'll try to push the "aperture x 2" magnification limit if I find the right oculars. Here in north of Spain, close to the Ocean Atlantic, I think seeing is similar to UK (clouds and rain most of the year) so I don't probably could go that far with magnification very often. Time will tell.
• That Svbony 3-8mm zoom seems quite appealing at least for casual use. The Baader Hyperion zoom is simple out of my budged for now. I'll probably need to open a thread in the future before purchase anything as I said, never did visual and I'll probably screw it up. Can't figure out yet how the take into consideration things such as 'exit pupil', 'field of view' or 'eye relief'.
• Do I need a diagonal (or prism?) or it will simply prevent me from doing the kamasutra with my scope? The focuser as almost 150mm of travel so I should do focus without extenders. Isn't it?

As I said, thank you so much for all your insights.

Hi! I've just purchased a 90mm refractor, a TS-Optics CF-APO 90mm f6 FPL55 Triplet APO (540mm focal length). It will be used just for imaging (mono camera, eq6r mount) but I'm asking myself if I also could use this scope to show planets (Jupiter, Saturn) or the Moon to my friends. This would be a one/two times in a year 'event'. I live in a Bortle 4-5 village and I don't do visual. I know it would be a very sub-optimal experience at best case but it is what I have. I don't have any barlow or ocular, I'll need to purchase all accessories. So...

1. Should I forget this path or give it a try?
2. If yes, what diagonal/ocular/barlow would you purchase?
3. If no, I can purchase a SkyWatcher Mak 90 with diagonal and 10/25mm oculars for ~£130. I know this is a cheapo alternative but, is this a better approach?

Any advice is welcome

I've considered adding a tilter but the good ones (with easy access to adjust) are expensive and after many many struggles with backfocus, I get reasonable stars without adjusting tilt (specially as my ASI183MM has a very small sensor). I have finally decided to live just this way, specially after discovering BlurXTerminator. Maybe in the future I would get a camera with integrated tilter as those PlayerOne. But please, check my previous post where I added more info. I have a very similar setup, so I think you could find it useful.

I also suggest purchase some 0.1-0.2mm spacers for fine tuning. My ASI183MM plus the Samyang focuses a little bit away than suggested 44mm+1/3 filter. Adding that small spacers makes a huge difference in focus position at f2-f2.8 . I also suggests to 'surgery' the Samyang to move the infinity hard stop to have some extra room past infinity, specially in the case you are adding auto focus. Is a quite straightforward operation and it worth it.

Oh, I also have that adapter to replace the Canon EF mount. I measure the backfocus distance from the front face of the adapter one put in place (not the screw, not the back face).

1 hour ago, alacant said:

Luis at str-astrum will do it for you:

Oh, thank you so much, didn't know about them. I'll consider get in touch once I could install my mount in my backyard observatory, which I should start building in the very near future, so this way the mount could stay untouched and adjustments should be easier.

I'm not ready yet to perform that surgery to my mount 😅 I need get some experience before and try adjusting the backlash using the external screws or the software. In addition, I don't have Plan B if something goes wrong, there are not many opportunities here to repair the mount if I mess things up. You are probably right... but not yet 😁

Holly molly! My 12 panel mosaic is just a nightmare, I can't imagine what something so big will be like 🫣

Thank you for your comments, really appreciate it. Some additional info:

• A dither to perform drizzle x2, as I undersampled.
• I don't see fixed pattern noise in my ASI183 when processing non drizzled master lights. I see some hot pixels, but darks seem to keep them off.
• My EQ6R is 2nd hand, about 4 years old or so. No maintenance was done as far I know. Although the mount has not many (real) work hours, maybe is time.
• USB cables are supposed to be "good" and they are less 1 year old. Yes, I know this means nothing but everything works except in that rare moments after dithering. No equipment disconnections at all, never ever.
• I take some time to route them carefully to avoid snags. I have a PegasusAstro Powerbox Advanced attached to the OTA and all cabling is tied and static except the one with power from the ground to the powerbox and both power and USB from the powerbox to the mount. To me, no cable failure is happening.
• The OTA is a bit unbalanced, down towards camera, as my filter wheel crashes with the mount head if I put the scope higher. The Samyang is so small it is not easy to handle or balance. Maybe I should add risers or additional weight to the front (not easy).
• Those spikes *only* occurs after dithering, randomly, without apparent reason.

As I said, given my pixel scale, I'm not worried by the current performance but for the future, when I go for a longer scope. If I can learn and fix things now, I prefer doing it before I'm having more problems. Based on your comments, I'll try:

1. Check cables again.
2. Increase dithering.
3. Re-run PHD2 Guiding Assistant to double check backlash values.
4. Study how to better balance the OTA (not that easy).
5. Check EQ6R mechanically (this will take longer, have to study how this goes, never done it to any mount).

Hi! I'm having some issues with my guiding. From time to time, I have very large spikes after dithering and the mount takes a while to recover. My pixel scale is quite permissive but I'm planning go for a longer scope and I want to address this before. Here is the thing:

• I have an EQ6R, my OTA is a Samyang 135mm with an ASI183MM Pro and I guide with an 32mm/f4 guidescope with an ASI120M.
• Guiding goes well, stable at 0.6-1.2" depending on sky conditions. I dither every frame, usually 300s (narrowband monocrome sensor). But, suddenly and with no apparent reason, I get a very large spike in DEC just after dithering. This happens from time to time, usually few times in the night. Other nights, this get worse and they are more frequent. The screenshots below shows a worst case scenario.

This is my configuration for guiding in NINA and PHD2:

• Dithering: 5px with a scale of 6.45"/px (guiding), ie., 32"/px. My scope and main camera have an scale of 3.67"/px, so I'm dithering 8-9px. I saw many people moving 15-30px in the main camera but I think mine is just fine to fix de fixed pattern noise of the ASI183, which is  quite low.
• Guiding: I run the PHD2 Guiding Assistant and I accept all the recommendations. Each time I try to 'fix' it, results get worse. So I ended up trusting PHD2 and its recommendations.

My OTA is just a bit unbalanced in DEC as the Samyang is so small that the camera, filterweel, etc. almost touch the mount head, so it is a bit heavy towards ground (if pointing north). Not a lot, but noticeable.

What could be the reason of this spikes? Unbalanced OTA? Wrong configuration? Too much dithering? Backlash? I accept any ideas as I run out of them.

Happy new year to everyone!

This is a "quick" processing of the Question Mark Nebula. I've stacked a SHO master light with no flats to perform a 'quick and dirty' editing just to test if I can ignore them, but somehow I managed to do the processing to the final stage. There are some issues there (specially in the background and nebulosity colors) but... hey! I think I can get some nice pics although I didn't resolve my issues with the flats 😅

For those interested in the details: EQ6R + ASI183MM Pro + Samyang 135mm @ f2.8 300s each with S2 (~7h) + Ha (~5h) + O3 (~7h) Baader 6.5nm filters

Thank you for all you comments, they really help. I didn't have good weather yet and next days seem even worse. I run son 'garage light flats' instead of 'sky flats' and the uneven flat pattern shows similar than before. It is true it is still LED lighting so I need to run sky flats anyway.

What I also found is a very strange grid pattern in Ha flats, which only shows with very-very strong stretching and strong flat illumination. None of that shows in the lights and I didn't find any of this when processing data. Even ZWO suggest this is normal with the IMX183 as it is a sensor-related issue, but it calibrates well. Oh, keep in mind this is not related with debayering as it is a mono sensor.

I read some data in forums that indicates this unevenness in the field may be 'normal' in some sensors, specially the first with BSI technology, such as the IMX183 (ASI183) or IMX492 (ASI294). This is not a brand-related issue but a sensor-related issue. I still have not a definitive clue on what it is happening but it seems feasible that the combination of sensor and glass window coating and filter glass coatings and band-pass size *may* be interfering in an strange way. I saw similar cases with OSC cameras and dual/quad band filters, where a large 'X' pattern shows even in the light subs. What people suggest with this 'lemon' sensors or equipment combinations is to take flats in the very same conditions than the lights (gain, offset, temperature...), with a proper illumination (30-40% histogram peak) and large exposures (>3s very minimum, even 50-60s) and stack >30 flats with no optimization at all. They also suggest use a even illuminated flat pattern, both in intensity and color hue as these sensors are quite sensitive. Contrary to what I thought, an iPad seems not to be a good flat panel, although my sensor is mono, so not sure if this applies. Finally, they also suggest "don't judge flats for their appearance but for master light final result".

This is what I've being doing since day one, so I don't know, maybe everything is going the way it is supposed to go. In any case, the flat unevenness and the varying flat illumination doesn't seem related issues, which is what worries me. I can understand the first, but not the last. Maybe I should just keep shooting narrowband and process nebulae to taste and forget all of this if the images keep going well to my eyes. I still want to know what is happening as this is usually my way of doing things (fully going down the rabbit hole) so I'll post an update if I get more evidences.

49 minutes ago, Elp said:

1 degree works well if the lights clearly show a linear only light gradient [...] DBE manually is much more reliable

Yeah, I agree. This is just a quick&dirty test to show the data. To me, the light sub is reasonably flat even without master flat applied. This is a more stretched version of the previous screenshots

Master vs Master+Flat

Master+Flat with 1-degree ABE vs. Master+Flat with 4-degree ABE (default values). Extracted background in the small bottom images.

On 20/12/2023 at 21:54, Elp said:

What does the calibrated light stack look like

Well, maybe this is not the most correct approach, but I did a 'manual' calibration of a master light (Ha) of my current project. All images are linear (just STF applied).

• Left: master light with only dark calibration (no flats)
• Right: with master flat calibration with PixelMath (shown in small at the right)

• Left: Master light with dark and flat calibration
• Right: ABE applied with default values (extracted background shown in small at the bottom right) > maybe the ABE is messing things up.

• Left: Master light with dark and flat calibration
• Right: ABE applied with 1 degree interpolation function, which I feel more exact given the gradient a see in the image (extracted background shown in small at the bottom right)

4 hours ago, Elp said:

You still haven't mentioned if this issue is causing you any problems....

I'm going to be honest: I don't know. As far as my little experience says, it seems they are just fine. However, I always feel the background extraction is not completely right but I'm not sure if there are my (poor) skills with DBE or my untrained eye. I also find some squared patters if I over-over-stretch the stars image after using StarXTerminator. Not a problem with normal processing but it may be a symptom of something going wrong. What really makes me uncomfortable is having different flat patters depending on the orientation of the light source, given *I do not disassembly my OTA*, I carry everything from the garage to the garden in a single piece, mount included.

6 hours ago, Oddsocks said:

Below is an example flat frame (un-stretched, from a 100mm f5/6 refractor equipped with a rotor, image distributor, photometer and spectrometer), and its corresponding flat profile, as measured in PixInsight, showing a collimation issue where the heavy (~5Kg) image distributor and spectrometer/photometer mounted on this system is pulling the rotator out of alignment. The important thing to note is that the flat image shows a variation of ~8.82% in ADU from the central beam out to the edges and each contour line represents 0.5% of the total ADU range in this image, but overall there is minimum non-linearity across the whole frame. The light source for this flat was an electroluminescent cover/calibrator panel, not LED. 

I won't be able to try sky flats for a few days... you know, bad weather 😢 For now, I run some analysis using Pixinsight. What's your opinion?

• Top>Down: Ha - O3 - S2
• Left>Right: master flat - stretched master flat (STF) - FlatContourPlot

I can provide more samples or the flats/masters if this help.

Flats with the 10" LCD old tablet + difusser:

The values of the contour plot ranges for the LCD Tablet are:

• Ha: 0.423 to 0.442
• O3: 0.442 to 0.467
• S2; 0.349 to 0.372

Flats with the homemade LED Flat Panel + difusser:

The values of the contour plot ranges for the LED Panel are:

• Ha: 0.335 to 0.383
• O3: 0.348 to 0.385
• S2; 0.324 to 0.370

   

12 minutes ago, Oddsocks said:

One issue that crops up with old TFT tablets used as a flats source is that the output light is strongly polarised, which can in itself cause gradients in flats and random movement/rotation of the tablet between flats subs is therefore necessary.

This is a very good point I was aware of. This is other reason why I want to test with other tablet.

13 minutes ago, Oddsocks said:

In Aleixandrus case he states that he sees the same gradient in flats created with his new led panel, and an old tablet, but we don't know if he is calibrating the flats before examining them for linearity, which is important since any fixed bias gradient in the sensor will show in the flats when stretched.

The screenshots are a flat sub at it goes from the camera. Zero processing (just virtual histogram stretching). I think I'll need to shoot some sky flats... never done that before.

Could you please share some tips to check for flat linearity? I stick with the same parameters as my usual light subs (gain 111, offset 8, temperature -5º) while keeping the histogram close to recommended ADU value by ZWO (adjusting brightness for 4-10s exposures depending on the filter).

Just now, Elp said:

And every session the flats can differ in pattern, especially at F2 if you compare them to each other, and also depending on what filters are being used. I see black areas at the edge of my flats a lot (on histogram stretch preview) but when calibrated with the images it works fine.

I really appreciate your thoughts as I already know you have a similar setup 😇 So, can you please confirm the light pattern is different each time you shoot flats? Do you think this is normal with all telescopes or may be this a Samyang135 related "issue"?

I currently use a conventional 10" tablet with a diffuse layer and a 3D printed flat panel with LED lighting and a diffuse layer. I also test different exposures from 0.1s to 15s and different histogram average values. I'll try again with a different tablet and I'll add more diffuse layers.

But the key is not the quality of the flat itself but the different illumination patterns according to the panel orientation.

I do not move the panel during the flat sequence. The issue is that every time I put the panel to take flats (ie. different night sessions), the light pattern changes so also the flats. What I understand is the flats cannot be dependent on how I put the flat panel.

I tried with the flat panel and an old 10" tablet. I'll try with a new tablet (iPad) as soon as I can borrow it and do some sky flats. I can understand the flat panel is wrong... but that tablet? It has no sense to me.

Hi. To not make this thread even longer I've just created a new thread about some issues I'm having with flats and the Samyang 135. I can update this post with the conclusions (for future references) but now I ran out of ideas. Any help is welcome.