dph1nm

DSS needs 8 stars in common (at least) to stack frames. If you can't get it to detect 8 due to trailing etc then try selecting superpixel mode for the debayering. In crude terms this reduces the resolution by 2x so makes it easier for DSS to detect stars.

NigelM

The Skywatcher Aplanatic corrector is very good in my f4 Quattro (for which they are designed). Not sure how well they work at f5?

NigelM

Try

http://www.astro-baby.com/EQ6 rebuild guide/EQ6 Development and Software Releases.htm

NigelM

22 hours ago, geoflewis said:

I’m interested in your observation, so what would you recommend please?

I think vlaiv's picture answered that! I cannot see the difference between the two noise fields, so I personally would be quite happy with a sky 5x the square of the read noise (when measured in electrons of course). After all, what matters for amateur imaging is what the final image looks like - if you were doing precision photometry you might make a different choice. But it is all a matter of personal taste I suppose - I prefer my exposures to be as short as possible.

NigelM

On 23/09/2019 at 19:47, Phillyo said:

Can you get around it with LP filters and/or more exposures to increase SNR?

Yes, but my advice is not to do the maths on how long it would take or you will just get depressed! Enjoy what you  have ...

NIgelM

On 27/09/2019 at 22:47, vlaiv said:

However, like I said, factor of x5 is arbitrary - which means that above calculated exposures are not "optimal" or something like that - they are just good guide line

Seems very over cautious to me! 8.7e- RN is equivalent to 75e- signal. A sky 5x this signal (not noise) - i.e. 375e- will only see the overall noise increase by 10% or so due to the RN contribution and I am pretty certain most people could not spot such an effect

NigelM

22 hours ago, geoflewis said:

Simple maths suggests that since I'm using 4 physical camera pixels in the 1 large (binned 2x2) pixel, that I'm capturing at x4, but I read that practical experience puts the ratio as more like x1.6 to x2.

Although you get 4x the signal (per binned pixel) the noise also goes up by the sqrt of this. So your S/N per binned pixel improves by 2x not 4x. I suspect this is where some of the confusion comes from. Of course, the total number  photons you collect from an given object has not changed - only exposure time can alter that.

NigelM

On 24/09/2019 at 23:17, Calzune said:

What does that do?

Cleans the sensor of dust - and very effective it is too.

NigelM

I think the EQ5 has a 10min worm period, so it should be easy to tell if this is your issue as the stars will move back and forth in RA on this timescale. You will have periodic error, it is inevitable. It can be pretty large - e.g. several tens of arcseconds.

NIgelM

Beware of DSLR histograms! I did some tests with my Canon 1000D and discovered that it only shows 8-bits of data and the right hand end of the display corresponds to 1024. But saturation in RAW on this camera is 4096. So setting flats to 1/3 would result in a very underexposed flat. In fact, even if you hit the right hand end you are only using 1/4 of the available well-depth!

NigelM

Probably the secondary dewing up. I have had that problem (and those exact symptoms) with both Newtonians I have owned. Solution - a hairdryer, but I do have mains power available.

NIgelM

On 26/07/2019 at 15:22, vlaiv said:

Would it depend on difference between stellar magnitude and sky surface brightness? (something tells me that it should, but I did not give it much thought).

Yes - obviously if there was no background noise at all then you would do best with an infinite aperture (i.e. collecting all possible photons).  If background noise is the dominant source of noise then it comes out at about 0.67*FWHM (for a Gaussian PSF)

http://wise2.ipac.caltech.edu/staff/fmasci/GaussApRadius.pdf

NIgelM

1 hour ago, vlaiv said:

Best thing to do when having this requirement of capturing single star and doing measurements is to make sure star is covered by single pixel.

Strictly not quite true as if you did this you would get more sky than necessary. There is an optimum sampling area for best S/N which is some fixed fraction of the FWHM of the star, but I can never remember exactly what it is.

NIgelM

23 hours ago, DrRobin said:

How faint you can see depends on how many arc-seconds per pixel and the signal to noise ratio of the chip.

No - within reason, it is not dependent on the pixel size unless you have a significant contribution from read-noise. If your exposures are dominated by sky noise then all that matters is the ratio of the number photons you receive from an object to the noise in the area on the sky the object covers. These will be the same for the same aperture, irrespective of f-ratio.

NigelM

18 hours ago, Jessun said:

f you built this and put it in orbit, or indeed kept it in the back yard, it would fill the wells of a given CCD or CMOS at the same rate as the Hubble

Sadly you wouldn't see many objects (in the same exposure time as a typical Hubble shot), as how faint you can see depends on the aperture not the focal ratio.

NigelM

I presume it is possible to attach it - I have done so via some Skywatcher tube rings, but that did require an OTA to be present (just to hold the tube rings!).

Being an alt-az mount your are limited in how long you can expose by field rotation. This varies over the sky. Straight up is not possible (infinite rotation rate), but E or W towards the horizon you can take expose for several minutes. It is not dependent on the focal length of the lens. Generally I stick to 30 sec exposures on the SLT mount. This picture was taken using a Canon1000D with Canon 55-250 zoom (at 250) by stacking a couple of hundred 30sec subs.

NIgelM

16 minutes ago, jeffwjz said:

I tried single star alignment before and noticed from PHD tracking log that the RA axis is deviating more than DEC, so using two star alignment will adjust the performance of RA tracking. 

The star alignment (which is just a software model for the GOTOs) does not really affect the tracking - this is just a mechanical property of the mount (and polar alignment).

If you don't have much cone error (optical axis of telescope not aligned to the mount's rotation axis) then two star alignment should produce reasonable GOTOs.

NIgelM

On 01/07/2019 at 16:57, Ships and Stars said:

If that's the case, I'd be limited to taking very short 2-3 sec exposures

Over most of the sky you can happily do 30 sec exposures in alt-az before rotation becomes an issue. Just don't point straight up! The challenge is going to be the tracking accuracy of the dob mount at 2000mm FL with 4.8um pixels ...

NIgelM

... I reckon that if it fails the 1 star test then you either have a mechanical or software problem with the mount.

NIgelM

On 16/06/2019 at 12:02, msacco said:

So after finishing with the 3-star alignment, it says that the alignment is successful, but it's simply completely off.

Some simple tests: after the 'successful' 3-star align get the mount to GOTO to the same stars you have used for alignment (using the named star option in the hand-controller). Does this work? If not, just do a 1-star align and  repeat the test (i.e. GOTO the named star you have just used for the alignment). In this case mount should (hardly) move and the star should remain in the eyepiece. If it does, then randomly move the mount off to some other part of the sky (with the hand-controller buttons) and repeat. Does it go back to the alignment star?

NIgelM

On 04/06/2019 at 13:20, vlaiv said:

I'm also having trouble distinguishing between UBVRI and UBVRcIc

Sigh - astronomers are very sloppy these days! Strictly, RI should refer to the Johnson system and RcIc the (Kron-)Cousins system. They are (quite) different filters sets and numerically produce colours which may differ by several tenths of a magnitude. However, I suspect many people now use RI to mean the Cousins system. Anything based on Landolt stars (e.g. Jordi et al) is RcIc.

On 04/06/2019 at 13:20, vlaiv said:

two different conditions are listed, but Rc-Ic < 1.15 is redundant. There is no need for it to be listed if transforms cover all star types - U-B condition is enough. If on the other hand Rc-Ic is genuine condition - transform set is not complete and does not cover all stellar classes

I think this is just a of what subset of stars were used to derive the fits. So if you add the words "Fitted to" in front of the descriptions it makes more sense e.g. "Fitted to all stars with Rc-Ic<1.15". You will probably find the differences for stars with U-B<0 are very small (at the level of 0.01-0.02 mags).

On 04/06/2019 at 13:20, vlaiv said:

- two different transforms are given for same expression (which one to use)

- There is mixture of letters which confuses me (UBVRI and ugriz seem to be mixed on lhs - is this correct)? 

Again I think this is a question of subsamples of stars used for the fits. I would stick to the general one rather than the populations specific ones. The mixture of upper an lower case is correct - just done for convenience I think - some of the transforms are for colours (e.g. V-I) and some for the difference in magnitudes between similar bands (e.g. r-R).  Hopefully the equations are all consistent!

Behind all this is the fact that these transforms are not really linear, and ideally you should also be using second order terms or higher. But mostly people just rely on linear transforms, with maybe a change of slope somewhere.

NIgelM

This might be useful for you

https://archive.stsci.edu/prepds/atlas-refcat2/

It is probably the most accurate stellar catalogue around. You will still need to convert to B-V though!  https://www.sdss.org/dr12/algorithms/sdssubvritransform/ should do the trick - I would be pretty confident in these transforms.

NIgelM

On 27/05/2019 at 11:18, geordie85 said:

I have such lines approximately in every 10min.

In which case it looks like classic periodic error to me. The image scale is sub-arcsecond per pixel - getting untrailed images in 120sec at that scale with an unguided EQ6 would be a pretty impressive feat.

NigelM

So I have a 12" F4 Newt permanently mounted on an EQ8. It will *usually* do 45 sec unguided exposures at 1"/pix at any dec IF you train with PPEC. Without PPEC forget unguided on mine, as it has +/-6.5" periodic error and the EQ8 has a really fast (~3min) worm cycle. I have even had it do 2min unguided exposures at at 0.6"/pix at high declinations. However it can be temperamental, and it has an annoying habit that the  PPEC eventually stops working and has to be retrained (so much for "permanent") - no idea why it does this.

I think if I were buying now I would be very tempted by the CEM120, but it is more expensive than the EQ8 - and of course I have never used a CEM120 so it might have as many quirks as the EQ8!

NigelM

On 02/05/2019 at 21:22, Star101 said:

SPAD appears to be the latest name. I could be wrong.

No, this is not SPAD, it is QIS! Which claims to be better ...

As far as I can see you have very small, very shallow 'pixels', which can only hold a few electrons, but have read-noise of only ~0.2 electrons. You then read out at video rates and build up a picture that way.  You can also have single photon detection if you want. The improvement over SPAD seems to be that you don't have to do any fancy photon avalanching to get the low read noise, amongst other things. I guess one issue will be what happens if you have a bright source in the image which gives too many photons to count one by one!

NigelM