vlaiv

17 minutes ago, sploo said:

Yea, that gives me some hope too. My 12" scope should be about 1.8x "slower", but that still means that 2s exposures may be acceptable.

Total integration time is related like aperture, provided that you match resolution. Actual SNR formula is rather complicated. You can get same SNR with smaller aperture as with larger if you change sampling rate - or arc seconds per pixel.

For example - you will get the same SNR from 100mm at 2"/px as 200mm at 1"/px in the same time.

You can still use 1s exposures - just means you need to get 3600 of them vs 2000 of them with smaller scope (that is just example number, actual number will depend on host of factors - QE of sensor, your sky background value, sampling rate, etc ...).

23 minutes ago, sploo said:

I experienced that (visually) for the first time last night (the first night since owning the 300P that I've seen clear skies). Being able to see Orion through the eyepiece is a pretty incredible experience. Unsurprisingly a 305mm aperture is collecting rather more light than the ~70mm of my longest camera lens.

Interestingly, Orion appeared as a dark cloud (probably with a greenish hue) to the eye, but shakily holding my phone against the eyepiece and taking a snap resulted in the more familiar pink-with-blue centre I've seen when using the DSLR.

I assume with a really wide aperture scope that colour might be visible visually?

That depends on several factors. It is surface brightness that is most important I think. It also depends if you are dark adapted or not. More dark adaption you have - more you will loose ability to see the color.

Prime candidate for seeing color in telescope is M57 for example, as it has high surface brightness. Don't magnify your target too much as you will spread the light over large area and the photo sensitive cells in your eye will receive small number of photons - and fail to trigger response. Don't get fully dark adapted (I know this sounds counter intuitive - but if you want to see color in planets and DSOs - you don't want to loose your color sensitivity and go into full dark adaptation).

And yes, it takes rather large scope to see some color in DSOs

Btw - you won't see red / blue color in Orion nebula like in images - you will start by sensing a sort of greenish / teal color. This is because eyes are most sensitive in green part of spectrum - so OIII and to some extent Hb wavelengths. Eyes are fairly insensitive to Ha wavelengths.

This shows two different regimes and sensitivity to light - note that Scotopic vision almost goes away for wavelengths above 600nm. Once you switch to night vision - you don't really see Ha wavelengths - Photopic vision is responsible for that. This means if you want to see any sign of red color - don't get dark adapted. Similarly in low light conditions when you start switching to Scotopic vision - you are much more sensitive to green/blue part at around 500nm - this is why you have hint of greenish hue in Orion nebula.

Best for viewing color / nebulosity would probably be Mesopic vision - this is crossover when eye uses both cones and rods to see - but neither at its best:

7 minutes ago, dan_adi said:

I have seen very short exposures on bright objects like the planets. I haven’t seen good pics of faint nebula and galaxies with such short exposures. Vlaiv can explain better than me about signal to noise ratio and optimum exposure length for faint fuzzies. I will look for dso images with very short exposures as you mention and see how they compare with “classic” long exposure photography. Thanks for the info!

Have a look here:

https://www.astrokraai.nl/viewimages.php?t=y&category=7

As far as I can tell - most of the images are taken with 16" dob and 1s exposures. Detail is incredible for such a short exposure and number of subs (most are less than half an hour of imaging).

Thing with short exposures vs long exposures is just read noise. If we had a camera that has no read noise - exposure length would not matter at all (as long as we can get stars to align subs to).

In fact as long as read noise is not dominant component of the noise - there is very little difference between short and long subs of equal exposure length. Large scope will such in photons making both target and sky background brighter in shorter amount of time (this really depends on sampling rate for particular case, but I just want to explain certain point) - higher values for target and background - higher the shot noise from target and LP noise and less important read noise becomes - less difference between shorter and longer subs for same total imaging time.

It looks like RA is acting pretty much the same. There is a small difference between the two - first one is 0.73", second one is 0.86" but second one is probably closer to truth since in pre meridian flip - you had saturated guide star which lowers centroid precision.

But DEC is acting funny post flip - and I think you have DEC backlash. I also think that you have disbalance in DEC that was working against PA error and hence minimized the backlash (similarly like you should make your mount east heavy to minimize RA backlash) but once you switched side of meridian - now disbalance started working with PA since it changed orientation and DEC backlash now started to show.

34 minutes ago, michael.h.f.wilkinson said:

I have used the ASI178MM for DSO imaging with decent results, using my APM 80mm F/6 on an EQ3-2 mount, and likewise with the bigger ASI183MC. Using even a chea EQ mount you do not have to limit yourself to sub second exposure times, and that makes life a lot easier

That is just regular DSO imaging with shorter exposures. You won't achieve any additional sharpness over regular DSO imaging, except for some issues with mount. Seeing related blur will be the same.

Lucky imaging differs in its goal - to remove as much as possible seeing induced blur.

Depending on your budget - there might be something else that is very interesting / tempting to try out with C6.

EEVA with C6 and ASI178 and this:

https://www.teleskop-express.de/shop/product_info.php/info/p11425_Starizona-Night-Owl-2--0-4x-Focal-Reducer---Corrector-for-SC-Telescopes.html

This will make your C6 F/4 scope. It works with only smaller sensors, but ASI178 is rather small so should not have any problems.

C6 with that reducer will give you 600mm of focal length and with ASI178, if you bin x2 you will get 1.65"/px. That is very fine resolution to be working at for EEVA.

4 minutes ago, sploo said:

In 35mm (full frame) DSLR photography terms, the "500 rule" is often used; that is, an exposure time no longer than 500s divided by your focal length. I.e. for very wideangle shots (16mm lens) you can expose for 500/16=31 seconds before star trailing becomes an issue. A 1500mm telescope used for prime focus would, I assume, only allow 500/1500=1/3s exposures.

If using a Canon APS-C body then it's 1.6x shorter (as you get a field of view on the crop sensor that's approximately the same as a lens with a 1.6x longer focal length).

No, that 500 rule is gross approximation. If you want to do proper calculation for this case - use sampling rate and sidereal rate and see what you get from the two.

Let's say that you are using modern DSLR sensor that has pixel size of about 4um or so. You are using 1500mm focal length. This gives you 0.55"/px, or each pixel is 0.55 arc seconds "long". Sidereal rate is about 15"/s. This means that in a single second - star will streak across 30 or so pixels. If you have FWHM of about 3" - that is 6 pixels, and you can start to see star elongation at 20% larger major radius. This means that elongation can be at most about 1.2px (6 pixels * 20% = 1.2px). That is 0.55 * 1.2 = 0.66 arc seconds.

With sidereal rate of 15"/s - this will give you 44ms exposure not to exceed 0.66 arc seconds elongation.

As you see - this figure is about x7.5 less than you estimated using 500 rule.

Point of lucky imaging is that you discard some of your subs, and only keep some of lucky subs that are sharp enough.

Otherwise you can consider lucky DSO imaging to be just a regular form of DSO imaging. Therefore you should treat it like regular dso images - calibrate them properly and so on. Issue is that you will have a lots of data and another issue is that you don't limit your self with sampling rate that is adequate for long exposure, but you rather aim for critical sampling rate of your telescope.

In case of C6 and ASI178 - you just use native pixel size, that is close critical sampling rate (f/ratio for critical sampling rate with 2.4um pixels is about F/11 - F/10 of C6 is close enough).

If you use DSS (which I would discourage in this case) - simply stack subs that have star FWHM below certain threshold. Otherwise use AS2/3! and do "scientific" stack to produce 32bit image. In the end, continue with planetary type workflow and apply wavelet sharpening in your processing. This will give you best sharpness.

Don't expect that you will capture much, but what you capture - should be sharp / much sharper than you can get with regular long exposure imaging (if you do lucky imaging properly). In order for this type of imaging to work good and provide "deep" images with good SNR, you really need to have very large telescope that gathers a lot of light - like 16" or above. 6" is just going to give you a "taste" of this.

It is for use with 2" eyepieces.

In order to reach focus, you must place eyepiece at certain distance to primary mirror. This is what the focuser is for, but it might not have enough travel, depending on eyepiece design used. This extension let's you use 2" eyepieces and puts them in good position for you to reach focus with your focuser.

You can put 2" eyepiece directly in focuser, but odds are that you will need to rack out focuser very much and you might not even reach focus in that configuration.

In my view, software is worth paying for in two cases:

1. It earns you money

2. It saves you time in which you earn money

In other cases it is really ok to use free software (it is also ok to use free software in first two points as well).

For most of us, AP is hobby and as such does not qualify for point 1.

Sometimes it qualifies for number two - if you end up frustrated by your hobby rather than happy it will impact your work ability the next day therefore sometimes it is ok to spend money on piece of software if alternative is not available or it would take too much time to learn it or whatever.

Back focus is distance of focus plane to telescope - or how far back from the tube is your focus plane.

It has nothing to do with moving focuser in or out. It does however have meaning in context of using certain accessory and it's optical path.

In order to use accessory, total optical path must be shorter than back focus.

+1 Gimp

2 hours ago, Davey-T said:

How large ? I tried 10 minutes O3 with 110 f/7 refractor and saw no sign of it.

Dave

I think OP will use 12" reflector and above image of M51 was done with 16" scope. In any case, if you have detail on surface brightness of that object - we can calculate needed exposure. Both scopes will capture more than 7-8 times the light 110mm aperture captures (in case of 16" aperture, difference is about x12 or so once you account for reflectivity of the mirrors and central obstruction)

For example, above mag 26 source is captured in total of 2000 seconds, or just a bit more than half an hour. And that at resolution of about 0.9-1"/px (just a guess, I did not measure).

3 hours ago, Davey-T said:

As long as you don't plan on imaging the Squid Nebula 😂

Dave

With a large scope - why not? Granted, it is very large object so mosaic will be necessary, but I think it can be done given enough imaging time.

13 minutes ago, Dash1st said:

Thanks again for all the help, but my wife, after watching & reading things, is already talking about wanting a new Barlow lens & possibly replacing the stock lenses. 

So I am again looking for some more help/guidance/advice from your good selves, as I know I will get nothing but good advice here.

The first lens she is talking about is a 5x Barlow lens, so I can't afford an expensive one, but I don't want something that's cheap & nasty. So can you possibly help again?

Many thanks

Budget will be helpful, but in general you don't need to use barlow with that scope.

For most purposes you want to limit yourself to about x200 maybe x300 on a good night. That is magnification. With 1200mm focal length this means 6mm or maybe 4mm at extremes.

If you really want a barlow then get either GSO x2.5 barlow (sold under Revelation brand in UK, I believe), or maybe go for ES x2 focal extender - that is telecentric lens.

I would recommend 32mm eyepiece - like GSO 32mm Plossl as wide field eyepiece - cheap option (but good option), or ES68 / 28 as a bit better and more expensive alternative.

25mm Eyepiece that you have with your scope is very usable so keep it and use it.

Next you want an eyepiece in 16-18mm range. This will probably be your main DSO eyepiece for galaxies. Simple plossl will do there as well, but there are other wider field eyepieces that are as good.

I would also recommend something like 6mm as your planetary eyepiece.

I have two in that focal length or there about ES62 5.5mm and ES82 6.7mm.

Explore Scientific are good but a bit more expensive lines of eyepieces (68 series is good, so is 82 degrees). People often recommend BST Starguiders as very good cheap eyepieces that are better than stock EPs and better than regular plossls - so that is viable option as well.

1 minute ago, Davey-T said:

If you're using one of those "modern" CMOS cameras a .5 second file will be the same size as a 15 minute one, something to bear in mind depending how much disc space you have.

Dave

I remember when I did a bit of planetary imaging - same approach but instead of using 0.5s exposures - I used x100 shorter exposure at 5ms (or there about), and yes, single recording of 3-4 minutes did eat up couple of gigabytes of memory.

SSDs tend to be cheap these days, you can get decent half a terabyte ssd for price of decent eyepiece

Depends on what you mean by detail.

If you mean signal or what we call depth - that is rather controversial topic - I'll will put forward my view on it:

- short exposure has as much signal as long exposure, even single short exposure - this is controversial part as many people will disagree with me on this one

- short exposure has much more noise.

- Signal to noise ratio is important for what we call depth of the image. Difference between many short exposures and few long exposures - all adding up to same total time is in read noise of the camera - everything else is the same. If one had 0 read noise camera - there would be no difference between the two (many short vs few long).

Since there is no such thing as camera without read noise - fewer long subs always win in terms of SNR over many short subs - but it does not always win by same "amount". If read noise term is small compared to other noise sources - and that happens when light pollution is very strong (LP noise) or target is very bright (target shot noise) or camera does not have cooling and is running hot - thermal noise. In all of these circumstances there will be very small difference between many short subs and few long subs.

If you have cooled camera and dark skies and going for faint target, then read noise is not negligible and there will be quite a bit of a difference between short subs and long subs.

But there is another meaning to detail - actual resolution or detail captured and here we can argue that more short subs have better detail than few long subs.

Again things are not as simple as that but we have two important factors to consider when we talk about level of detail - atmosphere and mount precision. Both reduce level of detail (in terms of sharpness) of the image, and here shorter subs have advantage because they don't accumulate as much of a blur as long sub. Of course, that depends on atmosphere and mount and respective exposures, but yes - there is a technique called lucky DSO imaging that exploits this and uses many, many very short images - like 0.5s images and tens of thousands of such to produce very good sharp images of DSO objects.

For example this:

This image is made out of 2000 one second exposures with dobsonian telescope mounted on EQ platform

Have a look at more images from Emil here:

http://www.astrokraai.nl/viewimages.php?t=y&category=7

(btw, that is author of planetary stacking software AS2/3!).

1 minute ago, Adam J said:

Could be astigmatism could be caused by pinched optics. Worth doing a star test but I am not optimistic. 

If it is, and it is very possible that it is either pinched optics or maybe collimation issue, it will show in star test.

I would personally feel better sending back the scope together with image to confirm my finding of issue (not saying that one should not be able to return faulty scope without image - it just better to include evidence of scope's poor performance in my opinion).

1.5" - 2" RMS - poor performance suitable only for short focal length wide field shots with resolution 3"/px or lower.

1"-1.5" RMS - suitable for resolution 2-3"/px - wide field work

1" - most of stock mounts can achieve this as good result (on a good night) - suitable for work 2"/px

0.7" - 1" - good guiding, suitable for work at 1.5"/px

0.5-0.7 - very good guiding, suitable for work at 1.2"-1.5"/px

<0.5 - top tier mounts, suitable for high resolution work at 1" - 1.2"/px

Otherwise, I would recommend to deal with DEC backlash. You have very high DEC rms and this is because mount is "loose" in DEC. That level of slack in DEC axis can be felt by hand. If you reduced that with mount tuning, and bring DEC RMS to be equal to that of RA - about 0.55 - total RMS will then be ~0.78 and that is good guiding, on a better side of stock EQ6.

28 minutes ago, sploo said:

Ignoring the question of coma, is it possible then to take a long focal length telescope (e.g. this 1500mm scope) and project a wide field of view onto a relatively small area? E.g. project about 2 degrees (I think) for M42, but inside an image circle of about 15mm diameter; such that you could capture it with a ~11mmx11mm sensor like the ZWO ASI533MC?

I remember discussion about making mount for this scope and my recommendation to go for equatorial mount.

Now if you want to project certain surface angle onto certain sensor size - you are limited by focal length.

Either that or really doing a mosaic. Thing with mosaics is that you won't waste time (people often thing it is slower to do mosaic because you need to shoot multiple panels) - if you bin your image accordingly.

Let's say that you want to use 533 sensor and put 2 degrees on it. 533 sensor has 3000 pixels and 2 degrees has 60*60*2 = 7200 arc seconds. We end up with 7200 / 3000 = 2.4"/px. Pixel size is 3.75um and required focal length in that case is about 322mm. This is about x4 or so times less than 1500mm, so you would need something like 0.25 reducer - again not going to work.

But you can use APS-C sensor and do mosaic. Do 3x3 mosaic and you will have something like 2.5 degrees x 1.5 degrees of FOV with your scope and APS-C sensor.

That is really complicated topic.

Let's start by saying this:

if you are interested in proper astrophotography - what sort of mount are you using with 12" F/5 scope? That is a lot of telescope and you need really big mount for that.

This is actually not what I wanted to say - I wanted to say - use prime focus and forget about focal reducers. Forget about eyepiece projections and all of that.

Let's say that you want to use x0.5 focal reducer. It will try to take field that is twice as large as sensor and put it onto sensor. DSLR sensor is probably APS-C type sensor (worse if you actually have full frame sensor). That is about 27mm diagonal sensor. You want to put twice as much of field onto sensor so you need 54mm of clear aperture. 2" Accessories have 48mm of clear aperture at maximum (usually 47mm).

Your telescope needs to have flat and corrected field that is 54mm wide for that to work, and reducer needs to be matched to the telescope optics. This simply won't happen in real life. Telescope that you are using has Coma, and a lot of it. So you'll need coma corrector. Best you can get is x0.95 coma corrector in terms of field reduction. You can spend a lot of money and get x0.73 Coma corrector like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p9779_TS-Optics-NEWTON-Coma-Corrector-0-73x-Reducer---2--Connection.html

or this one:

https://www.teleskop-express.de/shop/product_info.php/info/p4685_ASA-2-inch-Newton-Coma-Corrector-and-0-73x-Reducer-for-Astrophotography.html

But pay attention to specs of such coma correctors:

They correct and illuminate less than APS-C sized sensor - up to about 11mm of radius or about 22mm diagonal. Last few mm will be vignetted and stars bloated.

If you want to record more of the sky at prime focus - do mosaic, that is best way to go about it with such scope.

Did I ask about the mount? Such tube is 27Kg in weight, so you need something like EQ8 class mount to hold it. With such scope, weight and focal length, I think you have other things to worry more than FOV of sensor at prime focus.

Just now, alexbb said:

It was ~10m away. But shouldn't it be good enough to verify the colimation at least?

I don't really know. There are two things that you need to pay attention to when doing star testing with artificial star.

1. You should not resolve your artificial star

2. You should account for spherical aberration introduced by source that is close

Above we did first criteria - how not to resolve artificial star - by making it smaller than fraction of airy disk.

We can also do second criteria - we need to express difference in distance from artificial star to center of aperture and distance of artificial star to edge of aperture - in wavelengths of light and make that less than a fraction - let's say less than 1/8 or /10 waves of spherical in order to test telescope for correction.

I think that for collimation you need to satisfy first point and second is not important. For star testing you need to satisfy both.

Artificial stars are made with apertures in microns - like 20um. That is about x50 smaller aperture than one you used. This of course means that you could do collimation test at only 4-5 meters away with such device. Spherical would be very pronounced at that distance, so you still need to do something like 50m if you want to eliminate that.

2 minutes ago, alexbb said:

I did a pseudo star test this morning. The star hole was a bit large (~1mm) to asses more optical defects, however, the out of focus disks appeared round with concentric circles.

I'm not sure you can use 1mm hole. I mean, you can - but you need to calculate distance so that image of a hole is smaller than let's say half or third of airy disk diameter.

For 72mm aperture Airy disk diameter is about 3.5", so you want your hole to be about 1" or less. This gives about 206.26 meters of distance. Anything closer to that and you will impact shape of star with shape of aperture producing a star (1mm hole).

How close did you do it?

If you want to try DIY artificial star - maybe look into metal ball from ball bearing - you need something that is very round and very reflective - so any sort of very round metallic ball with small diameter will be good. It also needs to be placed far away from telescope.

I would personally do couple of things prior to sending the scope back - just to eliminate any possibility of it not being the optics.

1. Do some test shots without x0.79 FF/FR, to be sure that nothing else except the telescope is at fault

2. Do couple of shots of single star with in / out focus image - to check collimation. Maybe check collimation with high power eyepiece if you have one? It is best to have it in images.

Just use a bright star, defocus star a bit in steps (small defocus, larger defocus, and a bit larger defocus) - in both directions - in and out focus and take some shots to see what pattern looks like. Make sure you don't use any other piece of optics - remove any filters, or use good narrowband filters you know are working - like Ha or OIII. Make sure star is in the center of the field.

I know this is waste of imaging night, but if you have artificial star - you can do it when it's cloudy and you can't image otherwise - just place artificial star far away enough.

1 hour ago, Sidecontrol said:

Only beneifit I could see is the Samyang 135mm has a faster aperture

and there you go ... For someone not owning sharp lens at 135mm - Samyang F/2 is rather nice solution. If you are happy with your Fuji 55-200mm then just use that.