vlaiv

9 minutes ago, spillage said:

I do not want to hijack the thread but would you say high gain lower exposure times or lower gain and longer exposure times? I understand that being able to take more exposures at a shorter time allows for tracking and being able to be more ruthless deleting poor lights. If I can manage another late night and the sky is clear I will have another play around tonight.

My preference is to have shorter exposure times for precisely the things you mentioned - any sort of interference with capture process will ruin less data with short exposures - meaning plane / satellite trails, gusts of wind, spells of very poor seeing - what ever makes you discard your subs - you'll discard less data with shorter subs.

To determine what is good exposure length you really need to do some calculations - as each case is different. I'll briefly explain what happens - because of read noise and how "strong" it is compared to other noise sources (LP noise, target shot noise, and thermal noise - which is generally low with cooled camera so in most cases we don't even need to consider it). Each time you stack a sub you add one "dose" of read noise.

If we for example do one hour of exposure in 60 x 1 minute - your result will end up with 60 "doses" of read noise (all others will be the same between the two), but if you use 10 minute subs - you'll end up with only 6 "doses" of read noise in final stack.

Now depending on how high single "dose" of read noise is in comparison to other noise sources - this can create either significant or minimal impact. There is a point of diminishing returns when you increase exposure length and in turn decrease total number of subs (to keep total imaging time the same) - and difference in noise between the two is imperceptible to human eye.

There is no general rule for this, as it will depend on bunch of things - target brightness, LP levels, what sort of scope you use (aperture at resolution) and such. You can do some complex calculations to approximate where this point of diminishing returns is for your setup and base decision on that, or you can just do what most people does - take some guideline values and adjust for your conditions.

Just for comparison - CCD rule is something like 5-10 minutes for RGB. But CCD cameras have 5-10e read noise. CMOS has read noise about 5 times less than that. Although dependence is not linear, you can see that recommended values for CMOS sensors are in 1-2 minute range for RGB (so as read noise is 5 times less, so will be exposure length - but I again stress - relationship is not linear and simple as that). Same goes for NB - most people use 20+ minutes for NB on CCD sensors. With CMOS you can use 4+ minutes

Some good advice given on gain settings. I'll just expand a bit on how to choose it depending on your conditions, and I'll give advice on offset.

With gain you are balancing two things really - read noise and well depth.

Read noise bit is important in two ways. Firstly, it is of course source of noise, and should be kept as low as possible. It is however probably the smallest contributing noise in most cases. Only case when read noise is significant is if you have incredibly dark skies or you use narrow band filters (like really narrow 3nm) and imaging very faint target. In this case it will become dominant noise source and you might want to reduce it as much as possible.

Second way read noise impact things is single long vs multiple short exposures. Difference between the two is read noise. If read noise were 0, there would be absolutely no difference between two approaches, but as read noise becomes higher there will be more and more difference between many short subs and a few long ones (all of this depends on other factors/noise sources as well).

Depth of well is inverse of gain, so larger the gain you use - less pixel well depth you will have (and saturate in less time) - these two are fighting each other, because for narrow band, for example you want to raise gain (to have less of read noise) and you want longer exposures - but higher gain will lower full well capacity of pixels and longer exposure will saturate in some cases - you need to find a balance.

There are no exact settings that will work for everyone because all of the above depends on sky brightness but also on target brightness and scope used - how much aperture and what is working resolution (sampling rate - or arc seconds per pixel).

For most targets and LRGB, you can use unity gain and 1-4 minutes (if you use longer exposure, you star color will suffer on brighter stars, so you'll want to use a few short ones as filler for color - with luminance you don't need to worry about that - just make sure target is not clipping). For NB you'll want to use 4-5 minutes and higher gain - look at gain/read noise chart and select one that will not eat too much into full well capacity but will still provide you with low read noise.

Above is balancing act - so try with "recommended" settings, but tweak to your particular conditions.

Offset on the other hand is something that you want to do right, and there is right and wrong setting. With offset you aim to avoid clipping "to the left" - or trying to avoid 0 values in the image (in both darks and lights). Don't be afraid to go with high offset - it's impact on full well will be minimal, but it is safer bet that you won't get that clipping that will mess up your calibration. I personally use offset of 64, but anything higher than 50 is a good starting point.

There is a way to determine good offset for chosen exposure length (and it will probably be valid for range of exposure lengths) - take dozen or so darks, stack them with minimum method (not average, and no sigma clip and such, just straight minimum), do stats on resulting stack and if you find that you have pixels that are 16 in value (or 1 if you bit shift your images to 12 bits from 16bits) - or stats on that stack give you minimum of 16 (meaning there is at least one pixel with that value) - raise offset.

Not sure what the myth is all about - but I think that shooting at infinity is not quite a problem when discussing crop factor - if I understand it correctly.

In my view it is more related to angle than size on sensor? Angle also holds true for infinity shots - smaller sensor will have smaller TFOV, and you can define ratio of full frame sensor and any particular sensor using certain scope - and you will get a "crop factor" that way.

Of course there are other ways to think about image size in AP, but size of sensor is sometimes very useful (I tend to forget that from time to time) - especially with latest CMOS sensors where read noise is small and one can afford all sorts of binning in software.

3 minutes ago, StarlightHunter said:

In the images the distortion changes the position and they are taken with different camera rotations.

Thanks. I supposed the problem would be most probably due the attachment. I will order a threaded attachment because I'm using the most basic 2 thread rings that comes with the scope focuser.

 

Using threaded attachment certainly won't hurt, it can only improve stability. Do be careful about it - you will need rotator part as well, unless you feel comfortable rotating scope in the rings when you want to frame an object (some scopes have permanent attachment of dovetail bar so they require rotator to allow for camera orientation).

Distortions to one side of the image could mean sensor tilt.

It could be due to camera, but also because of way the camera is attached to the scope.

Best way to assert this is to take a test shot - note how much distortion there is (which corner / side) and rotate camera. If same side / corner is affected - it is sensor tilt. If aberration moves accordingly (you rotate camera by 90 degrees and aberration does the same - usually in opposite direction) - it can be because of attachment.

If it' due to attachment - you need to check if attachment is firm but tilted or if it is loose and camera sags under gravity. For this - take a test shot and then don't rotate camera but rotate scope - point to another part of the sky so that camera is oriented differently with respect to the ground (usually west and east is good direction to test this).

If you determine that you have connection problem and it is loose - for example because you are using standard clamping by either screws or compression rings - think about upgrading to threaded connection if your focuser supports that - or maybe upgrade of focuser itself - if you are using any sort of extension - check if it is causing sagging under gravity (too loose attachment).

If you have firm tilt - try to figure out what component is causing it - usually extension ring / tube and try replacing it.

if you have sensor tilt - you need tilt adjuster since you are using DSLR and it does not have integrated tilt adjustment.

Hope this helps.

I was about to quote that example as well.

I noticed that Rodd has issues even with RGB filters, R being particularly affected.

On the other hand I noticed on my ASI1600 that I don't get them with RGB imaging on a small scope (80mm F/6 reduced to F/4.8), for example this image:

Maybe effect is there but so subtle that it is masked by seeing blur (and my less than perfect processing).

I don't get them with large scope 8" F/8 in most filters, but I do get them in narrow band (very slight effect)

For example, examine these two very close wavelengths - Ha and SII

This is Ha and we can argue if effects is there at all (maybe, just maybe there is some of it).

This is SII sub - it is definitively there, but again subtle. OIII on the other hand shows it more clearly:

But pattern seems to be somewhat different.

How are they formed in the first place? Well light has to constructively interfere with itself to produce that pattern - it needs to bounce off micro lens and then bounce back of something in imaging train. Phase shift will depend on distance traveled to bring light back to sensor and wavelength of that light, and of course effect will depend on how much light bounced back towards sensor from some surface (filter or corrector). This distance changes with position but also with F/ratio of beam (angle light travels at). F/ratio also impacts spread of light and therefore intensity of it.

From above you can see that even small shift in wavelength is enough to throw the phase off (Ha vs SII) and decrease this effect. Changing position of elements for even small value will have similar effect - have an issue in Ha? shift filter or other element by as small distance as difference between Ha and SII wavelengths and you will reduce or eliminate effect. You can do this for all filters, but best of course is to avoid any reflective element (good AR coatings on other elements in optical train).

29 minutes ago, Adam J said:

I would say that the lack of micro lens diffraction effect on the 183 against its presence on the ASI1600mm pro is a significant difference to consider between the two chips too. Different people have different views on it based on personal preference. 

I think that this micro lens diffraction thing can be managed. I had several discussions and found that some people believe it is there regardless of actual optical setup - that is not what I've found. There is plenty of evidence that it depends on several factors - mainly speed of beam and other reflective surfaces in optical train (any sort of correctors and filters used). If one comes across it, I think it can be managed with some effort to rearrange spacing of these elements (usually moving filter to different position as moving correctors will introduce aberrations)

1 hour ago, fiestazetecmk2 said:

Yes agree .when I was looking for a dob I found that the 200p was easier for as I'm  retired and didn't want to be struggling to move the scope .I'm glad I did as I can manage it still.and hope to for many years to come.you will mot be disappointed with a 200p.

These images require an explanation

How much of it is after market add on? I can identify couple of things, but slow motion controls and maybe "suspension" system are quite intriguing.

I've also fitted RACI (50mm one, not 60), replaced EP receptacle (compression ring 2"/1.25" low profile) and added Lacerta microfocuser. I find other mods quite interesting, how did you do that?

2 minutes ago, Space Oddities said:

I do have a 1x flattener for the TS 60mm, however I haven't heard of any reducer compatible with that scope. Could be interesting indeed, f/6 is quite a slow aperture.

Hm, compatibility is an odd thing - sometimes its there when you don't expect it to be, and sometimes it's not quite there even if manufacturers claim it

This one might work?

https://www.teleskop-express.de/shop/product_info.php/info/p5965_TS-Optics-REFRACTOR-0-79x-Reducer-Corrector---APO---ED---2--connection.html

But I would advise you to browse the internet and see if anyone actually tried this FF/FR with the scope you have and what results they obtained.

(TS page for this product - that is no longer in stock but can be ordered? under reviews section has example of it being used with TS photoline 60mm F/5.5 - not your exact scope, but it does suggest that it might work).

Both these cameras (1600 and 183) will suit you well for the gear that you have and plan to use.

1600 has a larger sensor and therefore it will record more of the sky in a single go, but also has larger pixels, which is beneficial for your case. You work with small apertures and mount that you are using is very basic. This means that you won't be able to resolve high detail even if atmosphere is playing ball.

Although you have short focal lengths and 183 with smaller pixels is better suited for picking up detail on shorter focal lengths - your gear will not provide that detail. Maximum sampling rate that I would think using in your case would be somewhere in range of 2-3"/px. We say that those are medium resolutions and you should not worry about not getting close up with some targets. For that, you would need some serious kit. What you have will be plenty of resolution to enjoy.

Both 183 and 1600 will need specific approach depending on scope / lens you will be using it with, so non is at distinct advantage there.

Take for example 60mm f/6.

it has 360mm focal length, so 183 will have sampling rate of 1.38"/px - this will in my view be oversampling for your setup. 1600 will have sampling rate of 2.18"/px - which is excellent sampling rate for this scope, however, 1600 sensor is large enough that you will want to use flattener - and you can use one that is reducer as well. In that case sampling rates change. If you put x0.8 FF/FR on that scope, you will have 288mm FL and respective sampling rates will be 1.72"/px and 2.72/px.

183 you can bin x2 to get different resolution (you can do that with 1600, but for your setups there probably won't be a need for that).

Similar goes for 200mm lens - both cameras will provide you with good resolutions. Both of these cameras will work with 1.25" filters.

In the end only difference is in size of the sensor - and if you are willing to pay extra to have more sky recorded in each session (1600) or you want to go with cheaper (but only because it is smaller sensor) 183.

Longer the FL - odds are that you will need to bin x2 183 as resolution provided by it will be too much - keep in mind this if you plan on using 130PDS for imaging in the future (will require more serious mount).

9 minutes ago, cletrac1922 said:

I came up with a bortle of 4

Mag 20.8 is pretty decent for AP. A bit less so for visual - for visual you benefit from very very dark skies.

With AP, at some point sky noise will be approaching other noise sources, so there will not be much difference. Anything above 20.5 is a good place to do AP.

I'm currently at mag 18.5, but plan to move to ~20.8 skies by the end of the year. I did some calculations - this will reduce my total exposure time by factor of about 6 to 6.5.

So whatever signal I now capture in six hours with my scope - I'll capture in one hour in those skies - so that is quite a bit of improvement.

Actually you can but it is not easy ...

For a good image you will need fast lens and a lot of total exposure, but it can be done even in very severe LP.

Processing will be a challenge as well.

Here is example:

This was taken from my balcony and I'm in bortle 7 skies (you can see evil street lights shining from below). This image is quite limited since I used short exposure (and not many of them, I think it was less than 20) because camera was mounted on alt-az mount without tracking (actually piggy backed on ST102 - it has appropriate thread to mount camera on one of its mounting rings).

Camera was Canon 760d and I used kit lens (18-55mm), so speed was F/4.5 max (I don't remember zoom and F/speed I actually used for this image).

I also wondered if I can capture it - so I gave it a go. At very best / very transparent nights you can actually start to distinguish MW near the zenith (you can tell that there is something there and you can even guess direction of it). I also managed M31 with averted vision on more than one occasion - but only when it is highest in the sky.

I think that these are very interesting filters, but I also think that most people don't use them "properly". Best way to utilize them in my view is in conjunction with regular narrow band filters to form sort of "LRGB" set for narrow band imaging (meaning mono camera).

Such tri-band or quad-band filter would act as luminance - having higher total SNR as it would combine signal from all sources of interest in single frame (reducing impact of read noise which is significant in NB imaging). Single narrow band filters can then be used for much shorter as they provide only "color" information or rather separation of signal types. Here one can exploit the fact that targets have dominant emission(s) - usually it is Ha and one other (either SII or OIII) - this means that you can end up with shooting only those stronger lines and calculate remaining one (each strong component will have better SNR and there is a subtracting stacks is in fact additional stacking - odds are that third component will end up being better SNR then if shot directly with that filter).

1 hour ago, ollypenrice said:

 Larger scope means more aperture? Damn, I hadn't thought of that!!! 

Seriously, I think everything I have to say is included in my first post which is strictly devoted to comparing 12 inch F4 and 16 inch F4. Where, in your view, does the 16 inch win in seeing which is limited to, say, an arcsec per pixel?

(I always enjoy our disagreements!)

Olly

No, it's not about resolution / resolving power (yes, 16 inch will have small theoretical advantage even in poor seeing, but that is besides the point).

Your post that I first answered to, stated that you can't see any advantage in going with larger scope. I offered distinct advantage of larger scope - it will be faster for given target sampling rate. If one is willing to sacrifice FOV (has reasonably large sensor for intended targets), will bin data anyway, then why not go for biggest aperture one can handle?

21 minutes ago, ollypenrice said:

Isn't that exactly what I said???

lly

 

I was just pointing out that larger scope means more aperture, and as such has benefits for target resolution - it will collect more photons and will be faster.

1 minute ago, ollypenrice said:

That's not what I'm saying. Note that I'm comparing a 12 inch F4 with a 16 inch F4, not a 16 inch with the same FL as the 12 inch. The question is not whether a small scope and a large of the same focal length will be equivalent. They won't, as you say. The question is, what will the OP gain by moving from a 12 inch F4 to a 16 inch F4 bearing in mind the FL will increase? In my opinion both will be seeing-limited and risk resolving detail at the same level but with a loss of FOV in the large one. 

Olly

Same difference Although "speed" of the scope stays the same, "speed" is not really the speed of gathering photons. Best to think of it as "aperture" at certain pixel scale. As you pointed out, both scopes will over sample on most cameras (pixel sizes) and there will definitively be some binning involved to get to proper scale. If we consider that, then both scopes can be matched with a camera and bin factor to produce same target sampling rate.

16" vs 12" at same target sampling rate - more photons in first case.

Another important fact to consider with this type of reasoning is available FOV - longer FL will reduce FOV, so that should be taken into account when considering scope.

8 minutes ago, Louis D said:

I just bought a set of Paradigms to compare to the HD-60s.  It's your turn to buy some equipment to check out for the rest of us. 😁

Indeed! I'm in the market for ~ 5mm-7mm eyepieces for planetary and simply love my ES82 11mm, but at present time, given that ES82 LER would cost me total of about 185e (shipping, import duties, tax, you know, all the nice things government makes you pay ) - that is a bit steep for me.

Looking into ES62 5.5mm though - that one I might be able to afford at this time, so if I pull the trigger - will certainly post a review

17 minutes ago, ollypenrice said:

What do you expect to gain from the 16 inch for deep sky imaging? The 12 inch has a FL of  1.2 metres. The 16 has a FL of 1.6 metres. A FL of 1.2 metres with modern cameras will easily take you to an image scale of below, or even well below, an arcsecond per pixel. What resolution will your sky support?  If it will support scales below an arcsecond, at least on any kind of regular basis, I'd be amazed and would be similarly amazed if the theoretical gain in optical resolution would translate into new details resolved on the image. We are seeing-limited.  I'm struggling to find much improvement in resolution when comparing data from a 5.5 inch refractor at 0.9"PP with data from a 14 inch reflector working at 0.6"PP. (Different cameras.)

I ask this because the big scope is part of your mount game plan. Personally I'd go for the bigger aperture for visual but not for imaging. I can't see the point. I think you'll end up with the same final resolution and a smaller FOV.

In any event, even the 12 inch can take you into imaging territory where you'll want a guide RMS of 0.4" and that's a high level of precision. For me the EQ8 is a big hefty mount more than it is a high precision one and for that reason I'd go for the iOptron. (That is if I couldn't find a second hand Mesu. Both mine cost less than the new price of the iOptron.)

Olly

There is a lot to be gained by using 16" F/4 scope (I'm guessing it is F/4 scope because you mentioned 1600mm FL).

Take for example difference to my 8" RC. I use it with ASI1600 - it's oversampling at 0.5"/px, but binning sorts that out, and I usually consider my images to be between 1.5"/px and 1.0"/px (x3 or x2 bin).

16" F/4 scope will work with the same sampling rate as my 8" RC, but will have x4 light gathering capacity. I would not mind having a scope that will gather x4 more light and keep similar "properties" as my current imaging scope.

Bump up ...

Anyone has any news on these now?

I see them stocked again at TS - they were in stock at some point at the beginning and were pulled out of stock - in all likelihood related to first faulty batch. They are back again, but I don't seem to see any further mention online of this "line" (or rather addition to existing line of EPs?).

6 hours ago, alan potts said:

must say that seem rather a waste of 26 Televues, in fact I was thinking of getting rid of some now

I'll be happy to help if you considered donating them to a good cause

Yes, Starguiders at FLO are best priced as far as I know. They are quite cheap for the set if one counts in discount for multiple items.

I've seen the same line at TS marketed as TS ED 60 Flat Field. I wonder if there is any difference. I suspect that it is the same design, but wonder about QC and coatings since they are 50% more expensive.

Anyone ever came across comparison between different brands of these EPs?

I love the ruler test.

Paradigms look a bit sharper in lower focal lengths? Is it just me or is the effect real? Field stop looks better defined as well.

Goto feature is what accounts for bulk of additional cost.

It allows you to both find objects and track them. Such scope is not well suited for serious AP, but can be used for EEVA/EAA quite successfully. It can also be used for planetary imaging.

Not sure if flex version is useful for anything else except storage and transport (which is not huge issue with 8" scope anyway).

For me, goto would be useful for tracking when observing at higher magnifications - like Moon and the planets. However, for that purpose there is a cheaper solution that works almost as good (and in some respects even better) - EQ platform. It is "base" that you can put your dobsonian telescope on and it will track object for up to one hour before it needs "rewinding". It does not use continuous motion but rather tracks for certain segment of "circle" (because it is low lying platform and not full fledged EQ mount), after it reaches end of its motion, it needs to be "reset", or "rewinded" to start position (this is fairly easy to do and lasts about a minute). It is cheaper alternative even if buying it ready made, but can also be DIY project if you fancy that sort of thing - there are plenty of blueprints online for it. It suffers form one drawback - it is built for particular latitude (+/- few degrees) so it is useful for particular place but can be used on trips to local dark site.

If you after all decide to get proper EQ mount, then SW version of dob is not the best to get now. Look at Bresser 8" dob. This is because SW version has integrated parts for mounting to dob base - these are not easily removed and fitted back. Bresser dob has different solution - it uses tube rings, and it is much easier to transfer to EQ mount and back.

If you think of using such a large newtonian on EQ mount - please consider the fact that EP/focuser can end up in really strange positions and this is handled by rotating the OTA in its mounting rings. 8" newtonian can be as much as 10Kg in weight and rotating the scope in its rings is not as easy for such a large scope. If you want just to observe and/or do some planetary imaging, possibly EAA - either Goto or EQ platform is better solution.

Haven't read the whole topic (will do tomorrow), but in my view both unity gain and gain in dB how most manufacturers use it is quite right way to do it.

Gain in dB is relative measure - it needs a reference point, in same way that magnitudes (stellar) need reference point. Choosing unity gain to be reference point might make sense, but it is likely to confuse most people as we would end up with both positive and negative gain (in the same way we have positive and negative mags when using Vega as reference point). It makes more sense to keep gain positive and choose reference point to be in terms of other characteristics of sensor - quite reasonably manufacturers choose gain value at which ADC matches full well capacity. This is a good baseline value - 0dB gain in this case will let you gather as much light per pixel as sensor allows (is designed to do) - no "amplification", hence 0 value.

Unity gain as term then makes complete sense - it is gain in dB units at which there is 1:1 correspondence (unity conversion factor)  between e and ADU.

@FLO

Any plans for quark combo?