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Imaging with the 130pds


Russe

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18 minutes ago, al-alami said:

Thanks.  It is a DSLR, but I'm stacking directly from the raw images.  Is there another way to do it?

Yes you can use a free program called iris to manipulate the raw files and save them as FITS prior to stacking. :)

Edited by Adam J
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Hi all, my first post in this long-time read thread. 1 year ago it finished convincing me and I bought a 130PDS, and don't regret it. These days I essentially use it for Alt-Az imaging though I also have an Eq (but don't use it often).

Incidentally I used some of my past months images to check and calibrate the pixel resolution with different combinations of filters, CC position, oculars (I also do projection imaging sometimes). It's pretty easy: knowing the sensors dimensions and number of pixels, and reference angular distances between stars (thanks to measure tool of Stellarium and catalogs), and just apply some basic trigonometry.

For reference I also calculated the resolution with no additional optics (the cam barely adapted to the OTA), and I found 632.5mm ± 1.3mm.  This is also confirmed indirectly by other optical combinations.

I know for normal that actual focal varies a bit from theorical / sold specification, mine is away by ~2.7%. Is it a normal value and what's your own values ?

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3 hours ago, rotatux said:

For reference I also calculated the resolution with no additional optics (the cam barely adapted to the OTA), and I found 632.5mm ± 1.3mm.  This is also confirmed indirectly by other optical combinations.

I tried a calculation that failed. Can you check my logic?

In one of my RAWs Tejat and Propus are 1971 pixels apart and my camera is 5.19um per pixel, so that's 10.229mm distance on the sensor

I estimated the angular separation of the stars at 1.9 degrees using Stellarium.

I worked out Focal length = (distance between two star images/2) / tangent(angle between the two stars/2)

This gives me an answer of just over 300, when it should be 585 ( I have a 0.9x coma corrector).

Can you tell me where I'm going wrong?

 

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3 hours ago, Stub Mandrel said:

In one of my RAWs Tejat and Propus are 1971 pixels apart and my camera is 5.19um per pixel, so that's 10.229mm distance on the sensor

I'm afraid you must not have measured the right stars :-)

In my Stellarium Propus and Tejat are 1.865° apart, so the approximate corresponding sensor distance should be on the order of tan(1.865°)x650mm = 21.17mm.

That's way off your measure, so either you missed the right stars in your image or you are measuring a resized image (maybe onscreen display rather than true-image pixels ?).

Apart from that I agree on the formula. The /2 division on both sides isn't really necessary because of small angles.

Edited by rotatux
Propos->Propus
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2 hours ago, rotatux said:

That's way off your measure, so either you missed the right stars in your image or you are measuring a resized image (maybe onscreen display rather than true-image pixels ?).

Weird!

I'm opening a RAW in Irfan View and saving as a Jpeg, then using another program to measure the distance.

But when Irfan view loads the image it's downsampling it to rather more than half the original dimensions!

I've used RawTherapee instead and the distance is now 3688 pixels not 1971!

That gives me a focal length of 588mm (using my formula) which is so close to 585 millimetres, well it's basically 0.5%.

I wonder what's up with Irfan view?

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14 hours ago, Stub Mandrel said:

I wonder what's up with Irfan view?

Maybe it's actually loading the JPEG  "thumbnail" embedded in the RAW, rather than the RAW itself ? It's a feature I have in my "ORF" RAW format, so other RAW formats may have it too.

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14 hours ago, Stub Mandrel said:

That gives me a focal length of 588mm (using my formula) which is so close to 585 millimetres, well it's basically 0.5%.

So yours is fully conformant. With the SWCC mine gives 565 or 590 depending on the spacing (shortest or longest, about 18mm between the two). Pretty cool actually, as it gives more FoV on demand to my smaller sensor :)

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15 hours ago, Stub Mandrel said:

Weird!

I'm opening a RAW in Irfan View and saving as a Jpeg, then using another program to measure the distance.

But when Irfan view loads the image it's downsampling it to rather more than half the original dimensions!

I've used RawTherapee instead and the distance is now 3688 pixels not 1971!

That gives me a focal length of 588mm (using my formula) which is so close to 585 millimetres, well it's basically 0.5%.

I wonder what's up with Irfan view?

The irfanview faqs say:

  • Q: Why is RAW loading slow (Camera RAW formats, e.g. CR2)?
    A: This depends which RAW plugin options are used in 'Properties->PlugIns'. The preview option is the fastest, Half-size is fast and if both are deactivated: the full RAW image is loaded => very slow.

So it looks like you might need to edit the settings :)

Louise

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  • 2 weeks later...

Hi,

just got a 130PDS and have attached it piggyback to mt CPC1100 - see pic.

Had to cut 20mm off the focuser, and fit a reatining ring to the bottom of the focuser tube so that it would not  fall out. The centre spot was not centred, so removed it and replaced it with a Catseye spot. Put a piece of flock opposite the focuser. Also had to shift the secondary position as it was way to low. Scope is now collimated and ready to go. Will be using the Skywatcher (GPU) F4 coma corrector. Hope this works so I don't have to buy a Skywatcher f5 coma corrector......

cheers

Gary

newt.jpg

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10 hours ago, gazza said:

Hi,

just got a 130PDS and have attached it piggyback to mt CPC1100 - see pic.

Had to cut 20mm off the focuser, and fit a reatining ring to the bottom of the focuser tube so that it would not  fall out. The centre spot was not centred, so removed it and replaced it with a Catseye spot. Put a piece of flock opposite the focuser. Also had to shift the secondary position as it was way to low. Scope is now collimated and ready to go. Will be using the Skywatcher (GPU) F4 coma corrector. Hope this works so I don't have to buy a Skywatcher f5 coma corrector......

cheers

Gary

Very nice, so will you be using both scopes at the same time to get place detailed objects within a wider field? 

I am starting to wonder if my centre spot is actually central....its just such a pain to remove the primary. How far out was it?

Either way I need to get shot of my terrible laser collimator as the laser is not parallel to the focuser before I do anything else. Anyone got a suggestion for a quality collimator....preferably still costing less than the scope?

 

 

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54 minutes ago, Adam J said:

Very nice, so will you be using both scopes at the same time to get place detailed objects within a wider field? 

I am starting to wonder if my centre spot is actually central....its just such a pain to remove the primary. How far out was it?

Either way I need to get shot of my terrible laser collimator as the laser is not parallel to the focuser before I do anything else. Anyone got a suggestion for a quality collimator....preferably still costing less than the scope?

The primary isn't too difficult to remove. The whole cell is held in place with just 3 - 4 screws. To center the center spot, it's good to have transparencies with a circle the size of the primary.

For collimation I use a self made cap and a barlow with my laser. It does require a center spot on the primary, however. If you google "barlowed laser collimator", you should reach a few explaining articles on the matter.

Good luck

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1 hour ago, Adam J said:

Anyone got a suggestion for a quality collimator....preferably still costing less than the scope?

My 2¢: Having gone the laser way and get lost, I would rather advise you to get a "Cheshire"... does not need batteries, much easier to use, good enough to very good results.

I've got a long version, but wonder if a short version may be better and easier when framing the primary edges (bigger view) to align the secondary.

1 hour ago, Adam J said:

I am starting to wonder if my centre spot is actually central..

Beware not to touch it unconsciously : the spot doesn't mark the geometrical center of the mirror, but it's optical center. The two are identical for a spherical shape but can be distinct for a parabolic shape such as the 130PDS'. It's supposed to be setup correctly at factory, so be sure to test your mirror on optical bench before thinking about moving it.

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6 minutes ago, rotatux said:

Beware not to touch it unconsciously : the spot doesn't mark the geometrical center of the mirror, but it's optical center. The two are identical for a spherical shape but can be distinct for a parabolic shape such as the 130PDS'. It's supposed to be setup correctly at factory, so be sure to test your mirror on optical bench before thinking about moving it.

?? Can you elaborate? The center spot of my mirror came off during cleaning, and I replaced it as per numerous articles on the interweb. Was this not right?

What's the cause if the difference?

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Yes for a sphere the perpendicular to any point on the surface is a symmetry axis. For a paraboloid only the perpendicular to the optical center is such a symmetry axis.

Difference comes from making / grinding imperfections. That's an approximation, but think of the optical center as the mirror's center of rotation during grinding. If the mirror plate is not perfectly centered on the rotation center from the start, you end up after grinding with an optical center that could be a few mm off from the geometrical (disk) center.

However I don't know of a simple way to find the optical center. Since you completely lost it during cleaning, better to have the geometric center approximation rather than nothing !

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44 minutes ago, rotatux said:

Yes for a sphere the perpendicular to any point on the surface is a symmetry axis. For a paraboloid only the perpendicular to the optical center is such a symmetry axis.

Difference comes from making / grinding imperfections. That's an approximation, but think of the optical center as the mirror's center of rotation during grinding. If the mirror plate is not perfectly centered on the rotation center from the start, you end up after grinding with an optical center that could be a few mm off from the geometrical (disk) center.

However I don't know of a simple way to find the optical center. Since you completely lost it during cleaning, better to have the geometric center approximation rather than nothing !

I could see this being an issue with older hand ground mirrors, not so sure I would expect the mirror to have been ground off centre with modern automated equipment. I'll have a read about Cheshire colimators, my issue it is currently that the laser spot moves as I rotate the collimator on my current bargain basement model. 

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11 hours ago, Adam J said:

I've actually set it up this way so that I can do a mixture of visual and imaging. I want to look at an object, take a pic, record my observations. Having the c11 for visual gives me aperture, and the piggyback scope for imaging without having to disassemble anything so I can go whereever I fancy on a given night.

The spot was a bit over 2mm out of centre. As Have a Catseye template it was easy to check, and easy to replace. I've had half a dozen GSO/Skywatcher newts an only one was accurately centre spotted. I learned to check after hours trying to collimate and getting tilted fields and an edge out of focus. Once I centre the spots collimation is easy - I use a webcam to centre the secondary mirror, A Glatter laser to roughly point the secondary, and Catseye tools - cheshire and autocollimator to nail it. If your laser is collimated itself, get a Glatter Parallizer which will ensure the laser sits correctly in the tube.

cheers

Gary

 

Very nice, so will you be using both scopes at the same time to get place detailed objects within a wider field? 

I am starting to wonder if my centre spot is actually central....its just such a pain to remove the primary. How far out was it?

Either way I need to get shot of my terrible laser collimator as the laser is not parallel to the focuser before I do anything else. Anyone got a suggestion for a quality collimator....preferably still costing less than the scope?

 

 

 

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10 hours ago, rotatux said:

 

Beware not to touch it unconsciously : the spot doesn't mark the geometrical center of the mirror, but it's optical center. The two are identical for a spherical shape but can be distinct for a parabolic shape such as the 130PDS'. It's supposed to be setup correctly at factory, so be sure to test your mirror on optical bench before thinking about moving it.

I wonder where you heard this? I've never heard or seen any evidence of this factory testing for optical centre? On the contrary I've seen many scopes that were impossible to critically collimate accurately using a factory placed spot, which became easy once a centre spot was accurately placed - indicating that the optical centre was the mechanical centre......Could you please provide a reference, I'd love to know how they determine the optical centre.....

cheers

Gary

Edited by gazza
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Digressing briefly. For those who don't take Astronomy Now they had a lengthy article on the 'scope and some improvement that can be made. I'm not sure if the 'Tune Up' makes sense as I do not have a 130PDS but for those who do it might be worth a look. - John

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2 hours ago, Grierson said:

Digressing briefly. For those who don't take Astronomy Now they had a lengthy article on the 'scope and some improvement that can be made. I'm not sure if the 'Tune Up' makes sense as I do not have a 130PDS but for those who do it might be worth a look. - John

What edition was it?

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Hi,

disappointingly my new 130pds is showing triangular stars. The focuser has been trimmed and is not impinging upon the mirror. Anyone else had this? Pinched primary in its cell? Secondary problem? Using Skywatcher f4 coma corrector. It is consistent right across the field. Also note the reflections on smaller stars. Have never seen this before....It was focused using a bahtinov mask, yet looks almost out of focus

Suggestions appreciated.

 

cheers

Gary

triangles.JPG

Edited by gazza
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