symmetal

2 hours ago, Adam J said:

I think your assuming allot about how its deriving its data, I doubt that its actually referencing voltage ratio all the way back to derive a received power ratio, to do that it would need to understand the pixel value in terms of e/ADU. I would be shocked if the algorithm has that information avaliable, more likely its just looking at the pixel values and using the deviation of those values from the average referenced to the offset (signal). Hence 10 Log could well be more appropriate. 

Adam 

My reply was nothing to do with how PI derives its SNR figures, but how SNR values relate to voltage which was what StuartT was confused about.

The e/ADU figure isn't relevant here as the signal and the noise go through the same voltage amplification process in the ADC. The camera offset which is really just a fixed voltage added to the signal (with noise) could be subtracted if it's preserved in the fits header for PI calculations, but it maybe just looking at the SD in the sky background as a noise figure. 

My point was that ADU is directly related to voltage and not power, so 20 log (V1/V2) would be appropriate if using ADU values to calculate a SNR figure.

Alan

4 hours ago, StuartT said:

not sure what you mean here. The dB scale is logarithmic. A Bel is a power of ten. So a decibel is a tenth of that. Hence, 40 dB is 10^4 (or 10,000)

Signal to noise ratio  = signal power / noise power. As the ratio can be large numbers, the dB scale is used where SNR(dB) = 10 log ( signal power / noise power)

Note the use of the term power. In electrical circuits power(W) = volts(V) x amps(I). Using ohms law where V = I x R or voltage = current x resistance you can rewrite it as

power = V² / R.

The signal and noise levels we talk about in our images are actually voltages expressed in ADUs. The resistance is the same in all cases and so isn't relevant and can be considered as having a value of 1 so it can be rewritten as

power = V²  and  therefore

SNR(dB) = 10 log ( signal voltage² / noise voltage² )

Working in squares of voltages is awkward so it's rewritten as 

SNR(dB) = 20 log ( signal voltage / noise voltage )

A 40dB SNR therefore corresponds to a power ratio of 10,000 : 1 or a voltage ratio of 100 : 1

As mentioned above we are only concerned with voltage ratios and not power ratios.

If we are referring to AC signals then the RMS values of the signal and noise voltages are used.

Still confused? 🙂

Alan 

Yes, this is usually solved with either using curved spider vanes or fitting curved masks over existing straight vanes to spread the diffraction spikes throughout the image.  This topic on the PI forum gives some examples.

Alan

Impressive colourful image Rodd, though PGC14030 is more muted in this palette. 😊

Alan

It's at the position of PGC14030 which is classed as a spiral galaxy. Here's an HST image of it confirming the bright star in front of the spiral arms and the two bright stars either side. 🙂

Alan

Thanks Wim. Another great image.  🙂 Is that dust, just becoming visible, extending from the right side of Leo A?

Alan

Yes, your file has extra entries compared to ones I checked. Maybe you had the option to keep the planet centred while capturing checked. I assume this requires FC to measure how many pixels width the planet occupies and from this the image scale. If it knew the current angular size of the planet, it could work out the focal length. Maybe it takes an average angular size over a complete orbit to use which would throw the calculations off somewhat.

Alan

Glad to see it's working now. 🙂 I think the three header files are actually the same, and you could have copied it from another folder but better to be safe.

It's surprising that having so many extra stars displayed it doesn't seem to slow the program down to any significant degree.

Alan 

My capture txt files didn't have any details on the scope though I downloaded the latest version and the text files now report 'Unknown Scope'.

Under 'Settings/Misc/Observer' you can set the scope name and aperture but not its focal length. The capture text files still don't mention any focal length or focal ratio though, just the scope name. Are these the same txt files you're referring to?

Alan

Did you use the torrent file to download them from here. Your three gaia folders should each have an gaia.hdr file in them which is needed to work with CdC. If you downloaded them from elsewhere they will just be the Gaia data without the CdC specific header file. I can upload the header files if you wish, to add them to your gaia folders, to save you having to do it all again.

I have mine located on a separate drive  at F:\Astro Catalogues\CdC Catalogues\Gaia_EDR3 and it works fine with CdC along with the others

Alan

You need to create a folder called something like 'Gaia DR3' and put the three Gaia1, 2 and 3 folders inside it. Then point CdC to the 'Gaia DR3' folder or whatever you've called it and it should work. 🙂

Alan

According to Meade UK ETX models before 2014, as yours almost certainly is, use a 2.5mm power jack and according to the FAQ on Weasner's ETX site it's centre positive. The  Tech Tips power supplies section confirms it's 2.5mm and may also contain some tips for you.

If you can solder It's probably easiest to buy an extension cable or similar like these and replace one end with what your power tank uses. As 2.1mm connectors and cables are more common an adapter like this plugged into your scope may help too.

Alan

Edit: since starting this I see others have beaten me to it. 😊

In Celestron's RASA White Paper 2020, Rowe stated that a more powerful corrector plate was required compared to the SCTs but Celestron wanted it based on the 11" SCT corrector plate to keep costs down, so a longer radius of curvature mirror is used instead compared to the SCT, (using the same blanks) with a standard 11" SCT corrector plate followed by the 4 element lens group to create a flat field and to maintain focus over the full visual spectrum. Whether this includes further SA correction it doesn't say.

Alan

42 minutes ago, davidc135 said:

Below are images of graph paper taken with an 8'' sct, with and without the corrector plate in place. The effects of 16ish waves of spherical aberration can be seen.

Thanks David. To my untrained eye it just looks out of focus. What features of the image indicates it's SA? 🤔

Alan

15 hours ago, Rusted said:

Does household contents insurance cover the accidental dropping of an OTA?

I believe mine covers accidental damage to any personal items in your possession at or away from home up to around I think £5000 in value.

6 hours ago, Malpi12 said:

When I first had the thought my concern was how to support/attach the vanes without resort to drilling holes for bolts !!  If 3d circles could be printed that is good - could the vanes be made of stiff material (conventional metal) and be included in the 3d plastic ? Perhaps like rebar.

No one (I imagine !) would want to take apart a rasa just for this, but here we have an interesting possibility, I am sure the rasa will get over the insult to its dignity when the new corrector arrives !

Good suggestion. I'll see how easy it is to fabricate something, and depending how long it takes for the replacement plate to arrive. 🙂

Alan

4 hours ago, Malpi12 said:

Oh dear ! or similar words.

Would it be considered a heresy if I were to suggest making some vanes for it, to support the lens assembly pro tem ? Only as a scientific curiosity of course just to see how much correcting it /is/was doing  ?

An interesting thought. I could 3D print an outer and inner circle with vanes linking them but it would likely sag with the weight, introducing tilt and be a nightmare to maintain any semblance of focus. 😬

1 hour ago, Louis D said:

It would have the spherical aberration of an uncorrected f/2.2 spherical primary, which is to say a lot.  I've looked through an f/4 spherical primary Newtonian (Celestron FirstScope 76), and only the central region of a 20mm Plossl was barely usable.  f/2.2 is going to be way worse.  I'm not sure how much the camera corrector lenses would help with the SA.

My initial thought in posting was whether the lens assembly could be made to correct the SA instead of the corrector plate, but it may be simpler having a separate large corrector plate doing it as it's a tried and tested solution. 🙂

Alan

Used a 400mm aluminium bar with 2 holes drilled for protruding M3 screws and it did the trick without having to heat it. Cleaning all the old threadlock out the threads with toothpicks is taking a while though.🙂 I marked an edge where the two parts met before unscrewing it so that assuming the replacement corrector is the same thickness I can apply the same torque as the original had, with fresh threadlock. The last thing you want is the central assembly to start rotating when you tighten or unscrew the camera.

Alan

1 hour ago, Paul M said:

Very intriguing. You'd think that such a thin plate of that size would distort more than that amount of figuring under the weight of a camera hanging off it at a right angle.

It's a conspiracy!!!

Actually the weight of the lens assembly hanging off the back of the corrector does balance the camera weight to some degree when horizontal but then doubles when the scope is pointing straight up so it must sag to some degree. Maybe not enough to cause a problem if it's thick enough, but then its own weight would add to the sagging effect. 🤔

Alan

17 minutes ago, Franklin said:

Heat worked for me, I used a hair dryer to warm and soften the thread-lock.

That's a good idea. 🙂

Alan

@DaveS Thanks for clarifying that. I'd always assumed they were quite curvy. 🙂

Alan

Trying to separate the two components clamping the RASA 11 corrector plate but the lens removal tool isn't strong enough. It looks like threadlock has been applied at three spots on the thread. There is about 5mm deep of meshed thread. I've removed the lens tilt adapter which are what the screw holes/marks are on the lower picture.

I can make a longer bar with two M3 screws poking through to give more leverage, and perhaps put the plate in the oven on low heat to possibly soften the threadlock. Googling suggests using WD40 to dissolve the lock compound.

Anyone have any other suggestions? 🙂

Alan

Looking at the broken corrector plate of my RASA 11, I was surprised that it appears to just be a flat piece of glass. Laying a steel rule across it there is no visible curvature on either side and calipers measure 6.16mm thickness at all places from the outside to the centre.

There are no shims to adjust its orientation to the mirror, as it just lies directly against a fixed metal flange on the scope front housing and is pushed against it by the front mounting ring screws. Four nylon tipped grub screws push against the plate edges from the side, to centre the front quad lens assembly which does the actual correction I imagine. I'd have thought there would be a ring of softer material betwen the glass and the metal though. No wonder the plate broke when the scope was dropped. There is some grey fibrous material each side of the plate where it touches the lens holder. The ring below the plate screws into the central ring and has two holes on the bottom to fit an adjustable lens assembly tool which arrived today from Amazon. 🙂

I may as well try fitting a new 'corrector' plate myself, when Celestron can supply one, as there are several Youtube videos on hyperstar corrector plate replacement and adjustment. It just looks like I need to get it central over the mirror as there are separate tilt adjustments on the lens assembly used for collimation. Here's the broken plate edge on, with the lens assembly removed. The lens assembly comes in from the bottom and engages with threads on the inside of the top ring on the picture. The push/pull tilt screws are just visible below the top ring.

Alan

Excellent image Adam. You're right, at 100% it still looks good. Glad to see you left some noise, to stop it having a too 'processed' look. 😉

Alan

7 hours ago, dark knight said:

How about an engine hoist, they lift up to 1 ton and can be purchased for less than £200, not sure on how high they lift though.

I actually have one that I bought for another purpose, not lifting engines. They don't travel well over grass though, and the scope is about 60 yds from the garage. The lifting height is possibly OK but one person manoeuvring it into position to engage with the saddle would be rather awkward.

I bought an electric height adjustable table and fitted a plywood top, and 100mm castor wheels in place of the feet. 200mm castors would be better over rough terrain but they would require a separate mounting plate as they are too large for the existing base. 100mm castors just very neatly. A manual adjustable height would have been fine but electric ones seem very prolific. The scope will sit in a mounting frame fitted to the plywood top, with small fixed rubber castor wheels holding up the scope so it can easily be rotated into position for offering up to the saddle with the counterweight bar horizontal as shown in the video. If it's successful I'll post some pictures. 😉 Fixed castor wheels seems to be an oxymoron, but that's what they're called. 😀

Alan

A new RASA 8 will most likely have the mirror edge flare issue if you get one that's still in stock anywhere. I had to return three of them as they all had the issue, and got a RASA 11 instead. Celestron returned their stock of RASA 8 scopes to China for the mirrors to be replaced or re-worked. FLO have stated 90-120 days delivery which effectively means when ever they start shipping again.

The RASA 11 is recommended if weight and cost isn't a problem, and you don't drop it like me. 😭 It doesn't have the limited backfocus issue of the RASA 8 so you can use mono cameras with a filter drawer, and gives very good stars out to the corners on full frame, although there is significant vignetting with the stock 48mm adapter.

Back focus spacing is critical to get good stars, which can be misinterpreted as tilt, if it's out by more than 0.2mm.

Alan