Imaging with the 130pds

[alacant]

1 hour ago, RolandKol said:

differences in focus

You're making me feel guilty now! I focus once on a bright star at the start of the session and leave it there all night. Is expansion and contraction -that is what we're talking about I think (???)-  enough to make a difference? Sorry, now I am off topic....

Cheers and clear skies anyway.

[carastro]

Ditto.  Too little clear skies to be messing about checking focus every so often.

Carole 

[Stub Mandrel]

Not sure if this will be clear, I've trimmed the nylon button with a scalpel to tidy it up.

 

[Adam J]

1 hour ago, alacant said:

You're making me feel guilty now! I focus once on a bright star at the start of the session and leave it there all night. Is expansion and contraction -that is what we're talking about I think (???)-  enough to make a difference? Sorry, now I am off topic....

Cheers and clear skies anyway.

Do you focus with a mask? I can certainly see the change from the start of a session to the end if I start at 7pm and end at 4am..a refocus every 2 -3 hours is certainly worth while to me. 

The second thing is that the place you see it is not so much in the FWHM but the correction of coma in the corners of the image. I note that as my focus drifts the corner stars become more egg shaped as the coma corrector only works in the far corners with perfect focus.  Some people actually focus on a star offset from the center of the image to mitigate against this.

Finally its worth while mentioning that such fine changes in focus are much more apparent on my small pixel mono ASI1600mm pro than they will be if your working with say a Canon 6D at the other end of of the pixel size scale. 

Adam 

Edited November 23, 2018 by Adam J

[Stub Mandrel]

A 1-degree change in temperature will change the length of a steel 600mm f/l scope by about 0.004mm.

For a f4.5 scope like the 130P-DS the 'critical focus zone' where the focused image of a star is smaller than its airy disc is 50 microns or 0.05mm

http://www.wilmslowastro.com/software/formulae.htm#CFZ

So assuming you focus perfectly in the middle of that zone, a temperature change of just  over 6 degrees will put you out (just) of focus.

The 130P-DS when used with a DSLR pretty much matches the pixel size to the size of the airy disc of a perfectly focused star. I suspect the extent to which any out of focus caused by thermal expansion will be detectable given the added effects of seeing on star sizes (I have never got stars smaller than about 2 pixels across).

[Stub Mandrel]

Just seen adam's comment - I think his point about ASI1600 (with 3.8um pixels) and a Canon 450D with 5.7um pixels making focus more critical is valid.

[Adam J]

13 minutes ago, Stub Mandrel said:

A 1-degree change in temperature will change the length of a steel 600mm f/l scope by about 0.004mm.

For a f4.5 scope like the 130P-DS the 'critical focus zone' where the focused image of a star is smaller than its airy disc is 50 microns or 0.05mm

http://www.wilmslowastro.com/software/formulae.htm#CFZ

So assuming you focus perfectly in the middle of that zone, a temperature change of just  over 6 degrees will put you out (just) of focus.

The 130P-DS when used with a DSLR pretty much matches the pixel size to the size of the airy disc of a perfectly focused star. I suspect the extent to which any out of focus caused by thermal expansion will be detectable given the added effects of seeing on star sizes (I have never got stars smaller than about 2 pixels across).

Problem is that the perfect focus position in the center of the image is not the same as that on the edge of the image. So assuming that you focus on the center perfectly the edge may well still be within your critical focus zone.....but its much close to the edge of that zone than the center, hence the coma issue in the corners. As such I would argue that its possible that a smaller temperature change than 6c will show in the corners of the image, but as I noted above maybe not in the center star FWHM. 

I guess the way I understand it is that the Airy disk is not the limiting factor in resolution. For example the Daws limit for a 130PDS is something like 0.8 arcseconds from memory.  My pixel scale with the 130PDS and the ASI1600mm pro is about 1.25 arcseconds per pixel. 

Adam

 

 

 

Edited November 23, 2018 by Adam J

[Whistlin Bob]

1 hour ago, Stub Mandrel said:

So assuming you focus perfectly in the middle of that zone, a temperature change of just  over 6 degrees will put you out (just) of focus.

I suspect the number of nights where this occurs is quite small. It's a good thing to be aware of, but unless there's a change approaching that, then I'm going to carry on hanging out in the single focus club! ?

[Stub Mandrel]

47 minutes ago, Adam J said:

I guess the way I understand it is that the Airy disk is not the limiting factor in resolution. For example the Daws limit for a 130PDS is something like 0.8 arcseconds from memory.  My pixel scale with the 130PDS and the ASI1600mm pro is about 1.25 arcseconds per pixel. 

I would say that, in the UK at least, perhaps not on top of a volcano in Tenerife, seeing is the limiting factor. Imaging planets or the moon with live video shows how much things get spread, even on apparently 'perfect' evenings!

[rotatux]

2 hours ago, Adam J said:

as my focus drifts the corner stars become more egg shaped as the coma corrector only works in the far corners with perfect focus

Correct but YMMV depending on CC model and sensor size: I've seen the SkyWatcher's one being very tolerant, though with a smaller 4/3 sensor. e.g. I have used it at 2 distances separated by 19mm (factors 0.9x and 0.94x) and seen fair enough corner stars in both cases. I recognize however I don't image at the same level as most of you, I'm still a beginner in number of nights ?

[Susaron]

Latest tunning. The scope cover was broken, so a friend made me one with his 3d printer. 

[Uranium235]

On Friday, November 23, 2018 at 12:59, Whistlin Bob said:

I've not had too many problems with the focuser and mine seems to hold collimation pretty well- I check it most sessions and it's usually fine.

The thing I've always thought would be good was a shorter focal length f5 newt to go into the pds range, for a wider angle, so you can really go after some of those giant nebulas like the veil and the California. Wouldn't need to think so much about full frame then. I'd be more than happy to pay a similar price for that.

Sorry if this is getting off topic...

The problems I can see with that are:

1) Using an f5 scope of a shorter focal length would mean less aperture - and therefore would require a smaller central obstruction and secondary mirror size - not a satisfactory result.

Or

2) A faster focal ratio - which brings up a whole other bag of (perhaps worse) problems. 

Take a look at the Tak epsilon 130, and you will see the price of a fast widefield newt is rather high  

However, there is one cheap way around the issue... mosaics! 

[Whistlin Bob]

1 hour ago, Uranium235 said:

would require a smaller central obstruction and secondary mirror size

So presumably a smaller illuminated field? I wondered if it might be something like that. Perhaps one day I'll get a short f/l frac instead- but I'm sure the 130pds will keep me going for a long time to come, before further investment is needed!

[Whistlin Bob]

Finally got a finished version of last Saturdays subs. I managed to collect 16 ten minute Ha subs (it was a bright moon), along with another 10 of the shed wall (not included here!). I used them for the B&W image below, then combined them into the red channel of an RGB image from 9th October on the same target. My framing was a bit different this time (I definitely need to improve my framing), so the colour image below is the intersection between the 2.

130pds, Canon 600d, HEQ5, Baader Ha Filter (Ha), IDAS D2 filter for RGB from October.

[Susaron]

A reprocess around M8 taken last summer, bad weather has one con, to read and test a lot around Pix.

 

[JohnSadlerAstro]

Hi,

I feel slightly embarrassed posting yet another reprocess of this year-old data, but with the lunar illuminations this week and the bad weather I haven't been able to get and quality data with the new mount yet. I feel this is a big improvement over the old version of this image, the noise is still bad but the galaxies have a far more true-to-life colour balance and the stars are waaay more colourful.

Unfortunately the short-ish integration time refuses to give the image the "pop" which I'd like, so I'm planning to do a re-run on this pair as soon as they get a bit higher.

The stars have got coloured rings around them, this seems to be an issue with most scopes but especially when imaging with a DSLR. I have reduced the effect by applying a gaussain to a star mask--it works quite well but the stars do tend to swell a tiny.

2.5 hours at ISO800 with 1000d (unmodded), roughly 4 min per sub. 130PDS on EQ5. 

This was the old version:

John

Edited November 27, 2018 by JohnSadlerAstro

[rotatux]

Hello fellow 130PDS imagers

I've processed these subs of Pleiades taken during last country holidays.
It turns out much better than my previous successful try at it, more smooth, still good colors and much more structure.

My problem is I've developped two versions of it:
* One with moderate stretching, which IMO preserves colors better (especially that Pleiades' electric blue I like so much), and still shows structure on good monitors
* Another where I pushed streching more, to ease showing on less good monitors, but it find it faded colors a bit and eventually I have mixed feeling about it.
I'm looking for others' opinion/preference/advice about which one to keep, or is more generally acceptable, as myself having good monitors at home I prefer the first one.

 

Thanks for looking.

[Susaron]

Masked stretch preserves colors better, but it creates donuts and artifacts around big stars. I have found it is best to mix with Pixel math (In Pixinsight) a 70% fro Histogram transformation and a 30% from Masked Strech.

After playing with masks you can increase the saturation on the nebula and dark the background.

Cheers.

[Stub Mandrel]

Last night's M33. 250 minutes of 5-minute subs.

[Susaron]

M34 from last weekend very close to Madrid outskirts, I could save only 25 subs of 60s taken between the clouds.

Nothing brilliant indeed but another Messier to the count.

[Pid]

Hi all made the most of the clear skies monday night  decided to shoot what I thought was a bit more of a difficult target of ngc6888 the creasant nebula in the clear dark skies of Northumberland so hear it is I do feel I need more data over the winter on this one. 

24x5 min lights 

10x5 darks 

20 flats 

10 bias 

Skywatcher 130pds

Canon 450d modded

Heq5 mount 

Guide camera zwo asi120 

9x50 guide scope 

[sagramore]

On 03/12/2018 at 13:08, rotatux said:

Hello fellow 130PDS imagers

I've processed these subs of Pleiades taken during last country holidays.
It turns out much better than my previous successful try at it, more smooth, still good colors and much more structure.

My problem is I've developped two versions of it:
* One with moderate stretching, which IMO preserves colors better (especially that Pleiades' electric blue I like so much), and still shows structure on good monitors
* Another where I pushed streching more, to ease showing on less good monitors, but it find it faded colors a bit and eventually I have mixed feeling about it.
I'm looking for others' opinion/preference/advice about which one to keep, or is more generally acceptable, as myself having good monitors at home I prefer the first one.

 

 

Thanks for looking.

I very much like the right-hand image (harder stretch) as the extra detail for me is worth a lot more than the slight changes in colour. They are both lovely though!

[Whistlin Bob]

Here's my Monday night effort- M52 and the Bubble Nebula. 

20x 5 min lights + flats, darks and bias.

130pds / HEQ5pro / Canon 600d / IDAS D2 Filter

Stacked in DSS and Processed in Pixinsight

[Stub Mandrel]

Nice pone Rob ?

[jjosefsen]

Hi.

 

I'm considering getting one of these bad boys (130pds) and modifying it over Christmas.

I'm looking at dewheater solutions, and wondering if a glued secondary mirror heater would be preferable to a dewshield.

Can any of you tell me the diameter of the secondary mirror stalk/mounting?

I was considering mounting an rca plug in the top of the ota for a more permanent solution instead of having a cable dangle lose up there.