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Imaging with the 130pds


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On 02/12/2021 at 17:39, Jim Smith said:

Thank you. A little surgery coming up!

Just a note - you can do a reversible mod by moving the primary a short way up the tube instead of sawing the focus tube. There's others in this thread with pictures of the process, but it means if you ever wanted to you can undo the change.

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2 hours ago, sagramore said:

Just a note - you can do a reversible mod by moving the primary a short way up the tube instead of sawing the focus tube. There's others in this thread with pictures of the process, but it means if you ever wanted to you can undo the change.

Too late...the deed is done! Now awaiting a clear night to see if my star shapes will improve.

Just a thought...won't you need a larger secondary if the distance between primary and secondary is shortened?

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On 06/12/2021 at 13:17, Jim Smith said:

Too late...the deed is done! Now awaiting a clear night to see if my star shapes will improve.

Just a thought...won't you need a larger secondary if the distance between primary and secondary is shortened?

i thinks so too, yes, but i dont think the difference in vignetting would be visible.

I never shortened my tube and i dont have those star shapes. (also with the MPCC3)

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  • 2 weeks later...

I know I'm changing too many variables at once here but in addition to shortening the focus draw tube I have replaced the standard eyepiece holder with the compression ring version and I have recollimated. I think the secondary was not as well aligned as I first thought.

Still no sign of a gap in the clouds though...:-(

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Finally, the clouds parted for a few minutes. I managed to get one 30s, ISO400 shot of the Pleiades using my usual Z50 on the 130 pds on a SW  AZ-EQ5.

But as you can see, even with the latest modifications, the triangular stars persist!

I am using a Baader MPCC mk III. I have a ring baffle fitted to the primary. I have shortened the draw tube. I have fitted a compression ring eyepiece holder. I collimated very carefully and focused with a Bahtinov mask. I think I will check out the primary clamps next!

Uncropped...

pleiadedTriangles01.thumb.jpg.c934987c95977fa0ceac700362abc5be.jpg

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@Jim Smith Did you give your OTA enough time to cool down outsite before shooting? You could be seeing a reaction to thermals from the primary mirror.

I've had this in the past and pulled my hair out over it for a while, collimating again and again thinking it was that. My 130PDS needs a good 30 minutes to an hour cooling time to ambient temperature before use.

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15 hours ago, Jim Smith said:

I am using a Baader MPCC mk III

Hi

Lovely shot. This is going to be a very nice 130.

Baader cc? Before going further, perhaps best to tackle the astigmatism, which to a greater extent is helping keep the stars triangular in shape.. Unless you're accurately square and have exactly the correct sensor distance, there'll be astigmatism. 

The only way we've found with the Baader cc is to lose the -awful- m42 adapter and space it properly from the m48 shoulder. The best spacing is around 58mm to 59mm. The method is described here, but check carefully the spacing you need for the z50. If you've one of the old compression ring fittings with only 2 screws, you'll also need to drill and tap a third. However, due to the undercut on the cc -which causes tilt-  it's best to just use the old ring with a third screw, as here. Push the camera assembly tight up against the collar as you tighten each screw a little at a time.

HTH

 

Edited by alacant
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Hi Jim, I think you're almost there. All of the latest modifications you have done are narrowing the problem down, plus they should help keep problems away going forward 😊.

A star test using a high powered eyepiece would be interesting. My guess is that you're primary clips were overtightened when fitting the baffle. This happened to me before. The triangular shaped stars correspond to the location of mirror clips. There should be slight gap between the clip and mirror face. It's worth popping the rear cell off for a quick look. It would only take a few minutes, just be sure to re-collimate before use.

Tapping the compression ring with a third thumbscrew helps to keep the camera or eyepiece more centered and is something I think you should definitely consider getting done down the line. It will also give a tighter grip on your expensive camera, cc, filters, etc.

Best of luck Jim

Hope you get it sorted soon

IMG_20211028_204414.thumb.jpg.0e9ce5a1c6e8ecb9323bdae0c36616c7.jpg

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3 hours ago, Jamgood said:

Did you give your OTA enough time to cool down outsite before shooting?

Thanks for the idea. My 130pds lives in the shed which, on a cloudy day, is no warmer than outside. So, I don't think that's the problem.

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3 hours ago, alacant said:

The only way we've found with the Baader cc is to lose the -awful- m42 adapter and space it properly from the m48 shoulder. The best spacing is around 58mm to 59mm.

Thanks. I am using an m42 adapter but I have increased the spacing slightly with a 1mm thick, m42 ring. That seemed to help the corner stars a bit. I will look at this potential issue after I have checked the mirror clips. (Checking the mirror clips won't cost anything!)

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2 hours ago, Pitch Black Skies said:

The triangular shaped stars correspond to the location of mirror clips. There should be slight gap between the clip and mirror face. It's worth popping the rear cell off for a quick look. It would only take a few minutes, just be sure to re-collimate before use.

Thanks. I'm pretty sure I screwed the clips down quite tightly when I fitted the ring baffle. They were already tight. I will loosen them and see what happens.

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14 minutes ago, Jim Smith said:

an m42 adapter

The z's flange to sensor is 16mm, so the adapter should put the m48 shoulder of the cc at a distance of 43mm from the flange on the camera.

If you still want to keep the Baader t2, this type is the closest we found:

nikonz.jpg.647ce2d99c85843a30d1eaa7419972a7.jpg

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18 hours ago, Jim Smith said:

Finally, the clouds parted for a few minutes. I managed to get one 30s, ISO400 shot of the Pleiades using my usual Z50 on the 130 pds on a SW  AZ-EQ5.

But as you can see, even with the latest modifications, the triangular stars persist!

I am using a Baader MPCC mk III. I have a ring baffle fitted to the primary. I have shortened the draw tube. I have fitted a compression ring eyepiece holder. I collimated very carefully and focused with a Bahtinov mask. I think I will check out the primary clamps next!

Uncropped...

pleiadedTriangles01.thumb.jpg.c934987c95977fa0ceac700362abc5be.jpg

On my 200pds I had this effect, the clips were the culprit it siliconed the primary to holder using a few blobs of low modulus silicone , baader cc I have using m48 fitting is 57.1mm .

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54 minutes ago, bottletopburly said:

siliconed the primary to holder using a few blobs of low modulus silicone

Why did you do this? Will the mirror be loose if I loosen the clip screws? I suppose I'll find out when I take another look at the mirror cell! I wonder if the 130pds mirror cell is the same design as the 200.

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3 hours ago, bottletopburly said:

siliconed the primary

+1

It prevents lateral mirror movement and in so doing represents the only way we've found to retain collimation during and between sessions. 

No need for mirror clips (and hence no aperture mask) either.

 

Edited by alacant
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2 hours ago, alacant said:

No need for mirror clips (and hence no aperture mask) either.

The primary mirrors edge will show slight diffraction spikes if the primary baffle is removed. It's not as extreme if the 3 clips are removed but it's noticeable none the less.

See the thread here.

Just my 2c.

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1 hour ago, Pitch Black Skies said:

primary mirrors edge will show slight diffraction spikes

Maybe on older models. The 130 mirrors we've seen recently have had a bevel, so introducing fewer irregularities that an aperture mask.

Easy to test without the clips and with/without the aperture mask of course. 

Cheers

Edited by alacant
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33 minutes ago, alacant said:

Maybe on older models

Maybe, I'm not sure.

Do you use an older model yourself or newer bevelled? I'm pretty sure my one has a bevelled edge. I've never tested it without the baffle though, just assumed they were all the same.

35 minutes ago, alacant said:

The 130 mirrors we've seen recently have had a bevel, so introducing fewer irregularities that an aperture mask.

What irregularities are noticeable with the aperture mask?

Cheers

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14 hours ago, Pitch Black Skies said:

irregularities

Fatter stars and diffraction spikes due to -presumably- the edge of the baffle introducing thousands of tiny intrusions, gaps and reflections of its own. But IIRC, there's little in it. 

We haven't had a 130 for a while, but every example we've seen behaves differently You'd have to test it yourself. If you have only one though, it's a pain. In the end, if you're satisfied with what you have, just leave it.

Anything though, is better than the protruding focuser and the mirror clips;)

Edited by alacant
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Progress! 

I searched back through Lightroom and found a shot from before when I fitted the mirror edge baffle. (Also before I shortened the draw tube.)

The stars, in the centre at least, are circular! I'm pretty sure loosening those clip screws is going to be the fix. I have ordered some silicone sealant in case I decide to remove the clips.

I have a feeling that with everyone's help I am going to end up with decent stars across 90% of the field...which I think is a reasonable goal. Thanks again for all the helpful suggestions.

beforeBaffle01.thumb.jpg.a2628f0f2c90a463ee1c6ae0d59645e4.jpg

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2 hours ago, alacant said:

Anything though, is better than the protruding focuser and the mirror clips;)

Absolutely, I really like the idea of the silicone method though. Not only does it prevent lateral movement from the mirror and might eliminate the need for buying a baffle, but it also leaves the focal ratio intact and maximises light gathering.

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20 hours ago, Jim Smith said:

I'm pretty sure I screwed the clips down quite tightly when I fitted the ring baffle. They were already tight. I will loosen them

Yes my first reaction seeing your shot was about pinched mirror. The clips, if you keep them, must *not* be tight but just screwed enough to secure the mirror and avoid any play and vibrations. Of course other answers (astigmastism, CC spacing) are also valid.

BTW about an aperture mask, to avoid reducing the aperture I'm thinking about masking just the clips with half-circles of cardboard or hard paper, which would be sandwiched in the head of the clips. Would it have a chance to work ?

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On 23/12/2021 at 18:40, Jim Smith said:

But as you can see, even with the latest modifications, the triangular stars persist!

I had a triangular star problem with my 130PDS, it was fixed by loosening the primary mirror clips a little, they were too tight.

Good luck !

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