250Px issue - is the supplied secondary mirror too small?

[Stargazer_00]

Following on from an issue another board member was having collimating his 250px, he said he couldnt see mirror clips. To date I've not had to remove either mirror in mine and have always used a lazer collimator so collimate so never checked mine.

I recently got a cheshire with my 150p and checked my 250px and noticed that the clips are indeed not visible at all, with or without a cheshire in.

I took a photo through the cheshire so show what is presented to the eyepiece.

I then removed everything and filmed through a 2" hole with my phone to show you how far you have to move before the clips become visible. this strikes me as quite alot of the primary mirror NOT hitting the secondary mirror. I'd estimate a good 10% ring around the outside of the mirror is being reflected right back out the end of the tube off into space!

http://www.youtube.com/watch?v=g_-f1QsdZUc

First video I ever uploaded to YouTube so I dont know if that's worked or not.

Am I missing something here or is there a real design flaw. I'm thinking i need a bigger secondary mirror now? Free aperture....

[Sirius Bizness]

I have the same issue with my SW MN190 so interested to know why I can't see the clips.

[WaveSoarer]

You probably wont be able to see the primary mirror clips with a Cheshire (nor do you need to in order to collimate) as your eye will be too far from the secondary, even with the focuser racked all the way in. You should be able to see the clips with a collimator cap though. I made my own colly cap and I could then see the clips - though the focuser had to be all, or at least most, of the way in.

[Stargazer_00]

watch the video, thats filmed through the 2" hole with nothing in the focuser, not through a collimator. It's literally pointing into an empty 2" drawtube.

[Stargazer_00]

If you do some math and make an assumption that 10% of the radius from centre to edge is not visible at all to the secondary then you get an alarming loss of aperture.

Mirror diameter is 254, so radius is 127. 127 less 10% 114.3

If full mirror was showing to the secondary then you have 50670mm²

With the loss as described the surface area of the primary visible to the secondary is approx 41043mm²

That, by my maths, is 19% aperture lost to space. Or rather the mirror is only performing to 81% of its capability because the secondary is too small.

thats awful, I hope im wrong.

[Stargazer_00]

thinking about it some more...its only from the dead centre the clips arent visible. If you move the camera about as much as I have you are probably still within the tolerance of an eyepiece field stop. Short focal length eyepieces dont use all the mirror, only small part of it. So long as all the mirror is visible within the field stop of a 2" eyepiece then none of the mirror is wasted. The reason I can't see it all from the centre, and the reason why the camera cant photograph it is because the aperture is too small. the camera is tiny and in day light my pupil is probably only 2-3mm wide. If I had an eye pupil as wide as a 2" field stop im sure id see it all?

I think that's probably the answer...

still makes positoning of the secondary a pain in the backside during collimation. Id imagine its going to be fun when I flock this OTA in a few days. Any tips would be useful.

[Moonshane]

I cannot tell from the video but is your secondary in the correct position down the tube (i.e. centred under the drawtube?). if it were too far down the tube then this might affect the visibility of the whole primary. also, have you put any stronger springs in or anything at the primary end? either way, tighten them fully and then back off a turn each bolt. this will hold your collimation better and will also take your primary slightly further away and may make a difference combined with the above.

[username]

Put the dimension of your newt on this web app and it will tell you the percentage illumated field of your newt. If your dimensions don't give any fully illuminated field the the secondary is too small.

There was a thread about this on cloudy nights. I'll see if I can find it.

Whats the minimum axis of the secondary on the 250px? A 63mm should be about right for them but if it's too close to the primary then it will lose light.

http://stellafane.org/tm/newt-web/newt-web.html

[Stargazer_00]

Thanks username. Can you post the web app too please?

Also, the secondary is an eliptoid (sp?), which measurement do I take?

[username]

hi the app link i edited back in as i forget whatever i'm doing within 10 seconds of starting it

the cn link is here, it's about a gary seronik article and he comments on the thread. he knows a lot about newts

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,6,7,8,9,10,11/Number/2822245/page/341/view/collapsed/sb/5/o/all/fpart/1

the secondary is only ellipsoid if you look at it from a normal direction, when it's at 45 degrees is profile is circular so take the minor axis, the short side

there's some suggestions in that link about what to do but it may not be much of a problem, you could sort out full illumination and find you got a turned down edge or something bad

[Stargazer_00]

Thanks username. I think I came to a round about understanding above but I want to check anyway, thanks for your help

Also how did you manage to edit a post after I had replied? I find the edit function is only available for a really short time after ive posted something even if someone else hasnt even replied after me.

[username]

I think I edited it back in whilst you were replying as I don't think you had replied by then. I clicked submit and realised that I hadn't done it.

I always send emails with 'Please find attached.......' with no attachments too. Always wake up to emails asking for the files

The edit feature is new to me anyway, before I had to litter threads with corrections.

[Michael11]

I have a similar problem in my 250mm Skywatcher newt (And i made a test photo exactly from a focal point). It appears since they introduced these slightly shorter tubes with dual focusers - they all have the same flaw of an undersized secondary.

I calculated that in my case I need a 63mm secondary instead of supplied 58mm for a manageable field. I even ordered and installed one from Orion UK - but it had quite strong astigmatism, and I still haven't sorted this out with it's supplier (the APM). For visual observing it doesn't make that much difference anyway. But it's frustrating

[SkyGibbon]

Some say a smaller secondary gives better contrast, some say it hides outside mirror issues. My 10" dob came with a 63mm secondary, yet my 6" only came with a 31 mm. Which is too small to me, but I can still see the entire mirror and part of the cell uniformly. The 6" is an f/8 and the 10" is a f/4. 7. Which is why I think I got a smaller secondary (if peoples contrast thoughts are correct) it being more of a planet scope. The mirrors are guaranteed 1/16 wave or better, so not sure of the outer edge misfigure or blemishes.

[username]

i think what everyone has got to remember with their newts is not too freak out if the secondary is a bit too small or bit too big. The synta scopes are mass produced optics and what makes them cheap is the manufacturers being able to standardise and stream line the production lines. That means standard tube sizes and secondaries which is going to limit the scope of compromise between illumination and secondary size.

If a 10" scope is really a 9.5" scope it isn't too bad and it's still a lot cheaper than what the equivalent would cost 20 years ago.

Plus as has been said before a small secondary might mask edge of mirror defects.