Moving primary mirror

[Chris Stacey]

I live under beautifully dark skies, and when even using the standard 28mm that came with the scope I get some lovely views.......but as soon as I introduce a camera, nothing.....it's an f6 200 newt, whether is a spc900, or SDC 435 (the one I really want to play with) I can't get zip.....i've tried all manner of combinations, with barlows, focal reducers...you name it i've tried it......maybe an expensive focuser would help, but it's a lot to spend only to find out i'm no better off (knowing my luck).....so i'm thinking my only option now is to move the primary mirror up, make it an f5..........any advice on this? it looks easy enough....but is it as simple as moving it up 200mm, or is there a lot more to it?

[GazOC]

No Chris, baring regrinding the mirror will always be f6 and 1200mm focal length. Moving it up 200mm will just move the focus point up 200mm.

[yeti monster]

Moving the primary won't alter the focal ratio, that is to do with the curvature of the mirror. By moving the primary you can move the focal point within the focuser, should you need too. I had to do this with one of my TAL-1s, I moved the primary abouy 3/4" or more before I was happy with it. It is now a member of the Moscow State Circus acrobatic troup!

EDIT: Gaz beat me to it!

[ollypenrice]

If you point the scope at something bright like the moon you should be able to hold a card behind the eyepiece-less focuser and find the point of focus as a bright dot. This tells you where the chip of the camera needs to be. It may not be possible at present but you would know where you stood.

Olly

[Chris Stacey]

Thank you - I had no idea it was to do with the curvature of the mirror.

I'll try that trick Olly, that'll give me a better idea of what way to go!

[Chris Stacey]

Well i've decided to give this a go..as I have a day off today.

I don't think i'm to far away from focus, distant object and the moon are ok.....a few extra inches will hopefully solve my problems. I was hoping to have to not cut the tube, but i've just removed the mirror and the only secure away of refixing is to cut the tube, i'm going to go for 4 inches and hope i'm right....

This will either be a spectacular success for me, or a monumental failure.....if I disappear for a while, you'll know it was the latter....

Here goes nothing, fingers crossed.....

[haitch]

4" sounds a long way to move a mirror. Are you sure it needs to go that far? (Just don't want to see you do something irreversible to your scope)

[yeti monster]

That sounds VERY drastic! I'd make doubly doubly doubly sure, and then some more, that the tube requires cutting!

How much movement does the mirror require? I used longer screws, washers and springs to position the TAL-1 primary where I could use a barlowed webcam, I'm sure 4 inches is way too far!

Please be certain eh?

[Chris Stacey]

It does sound very drastic, I'd admit....i'm not sure how far the mirror needs moving since the clouds won't let me test it....but even with all attachments removed and the camera pushed into the focuser - nothing. I can obtain focus on a distant tree that way....but that's with a camera, focal reducer AND a 2x barlow.....so i'm a reasonable way out I suspect on getting focus on DSO's. I would rather go for a 4" and then add an extension tube if required.....that's my thinking anyway...

I could obviously go down the route of a better focuser.....but 300 quid or so...well, I think if I was to do that i'd rather keep the money and puut it towards a more suited scope....and the scope I have at the moment is not doing what I want it to do, so I don't really have much to lose, as it gathers dust whilst I can't get an image on my LCD through the SDC 435....

At the end of the day, if it doesn't work then it doesn't work....i'll have to get another scope to put on what I already have......time will tell....but i'd rather give it a go then put up with the frustration i've encountered so far...

[dph1nm]

Something weird here. If you were 4" off focus then you would not be able to focus with an eyepiece, never mind a camera! Depends on the eyepiece type, but roughly speaking the chip in the camera shouldn't need to be any nearer the scope than a distance equal to the focal length of the eyepiece plus the position of the bottom piece of glass in the eyepiece, so about an inch for your 28mm.

NigelM

[Chris Stacey]

Something weird here. If you were 4" off focus then you would not be able to focus with an eyepiece, never mind a camera! Depends on the eyepiece type, but roughly speaking the chip in the camera shouldn't need to be any nearer the scope than a distance equal to the focal length of the eyepiece plus the position of the bottom piece of glass in the eyepiece, so about an inch for your 28mm.

NigelM

Sorry, that was a bad description on my part.....I focus fine with eye pieces, no problem at all......I can only focus on distant images (like a tree) with the camera up close to the focuser, and the focuser wound fully in....I can't get focus on anything else though.......

I bottled doing 4" though, I've just cut 2" off, putting it back together now....I will see what the result is shortly...

[John]

I would not use a tree (even a distant One) to judge how much the mirror needs to move. Astro objects are more or less at infinity for focussing purposes - that's a big difference from the focus position needed for the tree !.

[yeti monster]

No! STOP! Don't do it!... ....... Too late!

[Chris Stacey]

I know John....i'm not explaining myself very well though, I know what i'm trying to say, and i'm pretty sure this will work, and is what's needed......but we'll see soon...

LOL @ yeti ....maybe i'm secretly hoping it fails....i'll just have a buy another scope then....*whistles*.....

[dph1nm]

don't think i'm to far away from focus, distant object and the moon are ok

If you are in focus for the moon then this is the correct focus for ALL astronomical objects. As far as your scope is concerned the moon is at infinity.

NigelM

[Chris Stacey]

Well it's done now...and reassembled.....so hopefully the clouds will clear tonight, and i can see what's what....

If it doesn't work, it's not the end of the world, it can just be added to my list, which already has the folowing:

1) Never buy a Meade LPI (ever)

2) Leave EQ Mod well alone...

3) When having the thought "I wonder what will happen if I plug that lead into there...?" Leave it as a thought, always.....

4) Never cut your telescope tube ?????

[Catanonia]

Good luck.... but I have a feeling 4inches was too much especially as said the moon was in focus.

Sounds more like a colimation problem rather than focusing problems.

[Chris Stacey]

Good luck.... but I have a feeling 4inches was too much especially as said the moon was in focus.

Sounds more like a colimation problem rather than focusing problems.

I went for 2 inch in the end.....even focus on the moon required the focuser all the way in bar about 2 or 3mm....i'm hoping the 2inch raise just gives me that little bit of play I need....

Wall to wall cloud now though, so it doesn't look like i'll be testing it tonight.....*sigh*.......

The collimation on this scope is very good though.....for visual I get lovely images looking back at me....can't fault it at all in that regard...

[Catanonia]

I went for 2 inch in the end.....even focus on the moon required the focuser all the way in bar about 2 or 3mm....i'm hoping the 2inch raise just gives me that little bit of play I need....

Wall to wall cloud now though, so it doesn't look like i'll be testing it tonight.....*sigh*.......

The collimation on this scope is very good though.....for visual I get lovely images looking back at me....can't fault it at all in that regard...

Always best to under estimate when cutting things. Good luck.

[Macavity]

Well, all is not (never) lost! I was astonished to find that I couldn't get my TS 8" F4 "photographic" Newt to focus visually, without even *more* extensions! But, if the worst comes to the worst, you can get a bigger secondary too. <G> Personally, I'd like to go back the other way! Making a tube longer, is rather harder though...

[Chris Stacey]

Well, all is not (never) lost! I was astonished to find that I couldn't get my TS 8" F4 "photographic" Newt to focus visually, without even *more* extensions! But, if the worst comes to the worst, you can get a bigger secondary too. <G> Personally, I'd like to go back the other way! Making a tube longer, is rather harder though...

Well if you ever do give it a go...I'd sure like to see it!

Was meaning to ask you, since I see your thread....are you using the video/in from your camera to the LCD, or going through the scart? Just wondered if you noticed any difference in picture quality with one or the other......I haven't got the SDC 435 to focus properly yet, so not been able to try it...

[Macavity]

...are you using the video/in from your camera to the LCD, or going through the scart?

More devious yet. Basically, it's a very OLD computer LCD with 15-pin VGA input only! FWIW, it works OK with a (small grey box) "VGA converter" with input from the camera's BNC video.

[Chris Stacey]

More devious yet. Basically, it's a very OLD computer LCD with 15-pin VGA input only! FWIW, it works OK with a (small grey box) "VGA converter" with input from the camera's BNC video.

I would never have thought of doing that...I like it!

I also like the splitter for viewing on LCD, then vid capture on the laptop......i've got a belkin capture card, so will be following your thread with interest..

[haitch]

Plug your scopes dimensions into this to figure if everything is still operating optimally (eg if you need a bigger secondary or if the focuser tube is getting in the way)...

Dale Keller's Newt Software page

[Chris Stacey]

Interesting link Haitch.....thanks for that!