Rectangular shadows in defocused star test

[Suryaputha]

I have recently collimated one of my  telescopes the Orion SpaceProbe II Reflector Telescope. Spider vanes are equal in length, secondary mirror correctly centered to focuser tube and tilt. Completed correct collimation. When I have done the star test surprisingly there are rectangular shadows appearing at 5 o'clock and 11 o'clock position. I believe the 11' o'clock is focuser draw tube and the the other is secondary mirror clip. I could not take picture of it but I could able to find a picture of similar issue someone faced. The same I have attached. Please look in the issue and guide me. The shadows exactly protrude at 11 and 5 o'clock positions in the image attached.

I have taken couple of photos through focuser tube with draw tube in and out. Shows perfect cllimation. Pls look in to the photos.

1. Star test(Someone's photo facing similar issue)

2. Look through focuser when draw tube fully in( you can see protruding draw tube) 

3. Look through focuser when draw tube fully out( only secondary mirror clip seen)

4. Look through focuser when draw tube fully in with camera flash( you can see correctly aligned spider vanes,       correctly centered primary mirror reflection in center and draw tube). 

5. Look through focuser when draw tube fully out with camera flash(Only secondary mirror clip seen all others correctly aligned)

Interestingly I had observed that these rectangular obstructions are appearing when outward focus(I mean when draw tube fully out) draw tube not obstructing light path. So strange. Experts pls look in to the matter.

Earlier this issue is not there in star test the star test image is exactly like image number one except without rectangular shadows but little offset at 4 'o clock.  After adjusting the spider vanes and bring the secondary mirror holder exactly center to OTA this issue arrised. do the obscene of the offset causing the issue. But for slow telescopes offset is not an issue. One can align secondary mirror at the center of the OTA. What could be the issue then?

Thanks in advance.  

[vlaiv]

I believe that shadows are as follows:

In second image focuser shadow is not visible because it has been racked out to get out of the light path.

Out of all of these, I would only be worried by secondary clip.

Btw, welcome to SGL!

 

[Suryaputha]

Interestingly these shadows appear even when focuser racked out in star test. As I mentioned after adjusting spider vanes and aligning secondary holder at the center of OTA this issue raised. Before that star test is exactly shown in the image one without shadows you can inspect the image it is slightly offset to 4 'o clock.  After perfect collimation this issue raised. Any offset issues?.  But for slow scope it is not a concern then what is the issue.

[Suryaputha]

What I am thinking is after spider vane adjustments I believe focuser draw tube hole projecting as 9 'o clock shadow in the field of view in star test because even these shadows appear when focuser racked out. If it is the draw tube obstruction it should not show up when draw tube fully racked out. The draw tube hole may be appearing as a rectangular shadow. See the first image. Experts what is your opinion. If so how to correct it.  

[Suryaputha]

Hi tech gurus above post is my final thoughts after excessive thinking about the issue. pls analyse it help me out in solving the issue. 

[Suryaputha]

Can anyone help on this issue. Thanks in advance.

[Ricochet]

What you are calling a star test is not a star test, just the view of the inside of a very out of focus telescope. The amount you defocus for a star test is very tiny, maybe a millimetre of focuser travel either side of focus. It is the diffraction rings and effects on them that you want to be looking at. 

Looking at the photos down your draw tube it appears that your secondary is not centred under the focuser. At a glance it looks too "high" and too close to the primary (to the right in the photo). If centring the secondary and then aligning it with the primary consistently lifts it up above centred then I think you need to adjust the "vertical" spider supports to bring it down a touch. 

[Suryaputha]

Does the out focus reverse or change the orientation of Image.

[Ricochet]

38 minutes ago, Suryaputha said:

Does the out focus reverse or change the orientation of Image.

It is no different to the view in the photos you took in the daytime, except that the inside of the telescope is illuminated by starlight rather than sunlight. 

[Suryaputha]

I have done the star test again when focuser racked out from OTA there is no focuser obstruction only secondary mirror clip obstruction seen at 9 o' clock. I mean only one obstruction. When focuser racked in fully into OTA there is focuser obstruction at 9 o'clock  and secondary mirror clip obstruction at 3 o' clock . Two  obstructions. Interesting thing is that when focuser racked out position secondary mirror clip obstruction is seen in the place of focuser obstruction like explained in the above scenario "when focuser racked in fully into OTA". What phenomenon cause "outside of focus the obstructions seen in star test are reversed".

[Ricochet]

Oh yes, of course. Look at the Newtonian light path diagram: 

The light from the primary makes a cone with the point at the focal point. On the outside of the focal point it crosses over and so everything will be flipped 180°. 

As before, what you are doing is not a star test. I don't know that being so far out of focus gives any useful information about your collimation. 

Edited July 28, 2019 by Ricochet

[Suryaputha]

Hail Ricochet, thanks for explaining in graphical form and bringing this issue into final solution. So as the collimation is perfect nothing can be done. I will Never go to that much back focus I have Digltal eye piece WiFi. Out focus is not an issue we just focus currently. As long as collimation perfect my issue is solved. Once again thank for sharing your tremendous knowledge and time. Yes  I know it is not proper star test as you said I am just seeing in and out focus obstructions.