Help! F4 Newtonian Collimation/Oversized Diagonal

[Anthony RS]

Hello, I've already posted this issue on cloudy nights but I'll post it here also maybe someone has some other ideas.

I just received the TS UNC 6" F4 newtonian. According to TS,The scope is perfectly collimated before shipping, but will probably need to be recollimated before use.. However when checking the scope, I noticed that the secondary mirror is way off center and slightly elliptical (attached image). Is it normal for the secondary to be in such a position in an F4 or should I adjust it so it 's centered in both the x and y axes? According to TS, they calculate the perfect offset for the scope, and I'm not sure if that's the result.

My main problem is it's impossible to get the secondary mirror to appear at the center of the drawtube. The reason being when I loosen the diagonal's center screw as far as I can, first the spring between the holder and the diagonal gets too loose to the point where it's useless, secondly, the 3 adjustment screws won't even reach the diagonal even when fully tightened. The furthest I could position the diagonal is around 3/4 the way the the center. I'm not sure if "offset" has anything to do with it but from what I know, offset is automatically set once I center the secondary under the draw tube. 

The diagonal is an oversized 70mm mirror and here's a link to the scope https://www.teleskop-express.de/shop/product_info.php/info/p5881_TS-Optics-UNC-150-mm-f-4-Newton-Teleskop-mit-Carbontubus.html. According to TS, they manufactured the scope specifically to give full illumation with my ASI 1600mm and SW F4 Aplanatic Coma Corrector.

I've shared two images. The first one is just an image of how I received the scope taken through the draw tube with my phone without a cheshire or a collimation cap showing how far the diagonal is from the center ( I know it's not a reliable way, but the diagonal was so far off that it's really apparent). Note that the manufacturer claims to collimate and test the scope before shipping. I do trust TS, I love everything I purchase from them, but I feel there's something really wrong going on.

The second image is after doing my best to center the secondary (which remained far off-center) and collimate the scope. I took the image using my asi 120mc with its lens inside the focuser and used mire de collimation for the crosshair layout. The green circle is the current position of the diagonal, the purple when is where it should be.

The third image shows a test image I've taken with my DSLR though sincec I haven't received the ASI yet. It shows all the edges and the center of the image. The stars are terrrible. Guiding error was less than half my image scale so it's not the issue. Backfocus spacing for the coma corrector and DSLR needs to be 52mm but I have it at 55mm since I can't adjust it without a thinner t-ring but I don't think it's the cause for this since even the stars at the center are elongated.

I'm not sure, but the lower left corner does suggest some pinched optics as well?

Am I doing something wrong, or is the scope defected? 

I will share some images below of the scope, maybe they could be helpful.

Cheers!

Anthony

Edited July 26, 2020 by Anthony RS

[Anthony RS]

Here are some images for the scope.

 

 

[Ricochet]

7 hours ago, Anthony RS said:

Is it normal for the secondary to be in such a position in an F4 or should I adjust it so it 's centered in both the x and y axes?

You should adjust it so that it appears central. The offset is towards the primary, your secondary appears to be too far away from the primary.

Assuming that TS calculated the correct length of secondary collimation bolts and collimated your telescope as stated, I would start by looking at the focuser and checking whether it is square on the tube and pointing at the correct point on the opposite tube wall. If it has taken a knock during transit it is possible that it is no longer pointing where it should. The point where the focuser connects to the adaptor plate is probably a weak point so I would loosen off the grub screws around the outside of the plate, reseat the focuser and tighten them up again to see if it makes any difference. 

[Anthony RS]

1 hour ago, Ricochet said:

You should adjust it so that it appears central. The offset is towards the primary, your secondary appears to be too far away from the primary.

Assuming that TS calculated the correct length of secondary collimation bolts and collimated your telescope as stated, I would start by looking at the focuser and checking whether it is square on the tube and pointing at the correct point on the opposite tube wall. If it has taken a knock during transit it is possible that it is no longer pointing where it should. The point where the focuser connects to the adaptor plate is probably a weak point so I would loosen off the grub screws around the outside of the plate, reseat the focuser and tighten them up again to see if it makes any difference. 

I loosened the grub screws and reseated the focuser, nothing changed. I'm not sure how to square the focuser and how to check if it's pointing to the correct point. 

[Ricochet]

2 hours ago, Anthony RS said:

I loosened the grub screws and reseated the focuser, nothing changed. I'm not sure how to square the focuser and how to check if it's pointing to the correct point. 

Unfortunately, you have to remove the secondary/spider from the tube. Once that is done you should take a piece of paper/card thin enough that you can bend it around the inside of the tube, but thick enough that it isn't flopping all over the place. With the OTA horizontal place the paper inside the tube and butt one edge to the edge of the focuser or focuser hole and push the paper so that it follows the curve of the tube. Use the other end of the paper to draw a line on the inside of the OTA tube. Do this once with the paper going around the bottom curve and once around the top curve as you look into the tube. you may have to cut notches to avoid any screws protruding through (eg for the finder shoe). Once you have your two lines place the paper inside the tube so that you can transfer the lines on the tube onto the paper. remove the paper and measure the distance between them. Make a new mark on the paper half way between the two lines. Now put the paper back inside the tube, align the outer marks on the paper with the marks on the tube and then transfer the centre mark onto the tube. This line should be roughly inline with the secondary spider hole opposite the focuser.

Secondly measure the distance from the front edge of the OTA to the centre of the focuser. You may find it easier to measure from the OTA edge to either side of the focuser and then average the result. On the inside of your telescope mark this distance in from the edge onto the centre mark you made earlier so that you have a cross on the far wall of the OTA from the focuser. You can now put a sight tube into the focuser and compare the location of the crosshairs to the cross you have drawn on the far wall. This alignment doesn't have to be exact, but the closer it is, the easier it is to collimate the telescope.

While your secondary is out of the telescope, roughly measure the distance between the centre of the secondary mirror (the front, mirrored side, not the back), to the plane of the threaded part of the spider arms that protrude through the tube. Also measure the distance from the edge of the OTA to the holes that the secondary spider passes through. At maximum extension you want 

Secondary spider "Length" > Distance from OTA edge to focuser centre - Distance from OTA edge to spider hole + 5mm

I hope that makes some sense without pictures.

Edited July 26, 2020 by Ricochet

[SamAndrew]

Coma corrector spacing is important, I've found with the Skywatcher Apalanatic coma corrector, being even 0.2mm out wrecks the corners.

Take a short exposure of a defocused star at the centre of the FOV to verify your colimation and eliminate tracking errors.