FlexTube 300p - Duller than me!

[russ.will]

I seem to be going round in circles with my 300p, but this isn't one of their well documented GOTO accuracy issues. No, it's just that I've never had a view through it as bright and crisp as my old 200p Explorer.

Last night was as clear as any since I bought it last May and there have been very few nights in that time where that has been the case AND and the moon is out of the way. With my 30mm ES82 wedged in the (correctly spaced) GSO Coma Corrector, I pushed it round to the M81/82 pair of Bode's Nebula sitting due north 15deg off Zenith, with high expectations. The good news is that the stars were absolutely pinpoint right to the edge of the FOV, so the coma corrector is a good buy, but I digress.

In my old 200p, it was perfectly possible to see the dust lanes impinging the edge-on disk of M82 and see the structure in the spiral arms of the oblique disk of M81. I was reasonably hoping that the xtra aperture would deliver a bit more resolution and support a bit more magnification otherwise, whats the point? The 300p just delivers a fuzzy cigar, partnered with a duller fuzzy oval nearby. I removed the Coma Corrector and the result was the same. Banging the ES82 18mm in, to bring a bit of magnification and a darker background just delivered bigger, less defined versions of the same. All this from a scope that, when loaded with a ES82 11mm and ES 2x Focal Extender, was allowing me to see swirls at the edge of Jupiters cloud belts. What gives?

The mirrors are clean and when they were out for cleaning and tube flocking, close inspection showed no signs of scratching. The centre spot is centred. Using a high quality Cheshire and latterly a webcam/Sharpcap/Mire de Collimation, I'm as sure as I can be that focuser is squared and collimation is as bang on as I can manage in the field, although it only every requires a very small tweak on the primary. However, I decided to double check by taking a photo through the Cheshire tonight and I'm not sure what I'm seeing.

Why am I seeing a double image of the spider vanes in the photos?

Star testing shows pleasing concentric rings outside of focus, although I do have trouble achieving the same inside of focus; The image seeming to be a lot more turbulent and disrupted. This was after the scope had been out for five hours and I even tried stepping well away from the scope and then rushing up with my breath held to minmise heat currents and this is with both the dew and light shields fitted.

Any help, much appreciated, because currently, there's not much I'm enjoying about the 300p experience except planetary and that's kind of completely not the point!

Russell

[MadAlbert]

I think I would expect a better performance, too.

Assuming your collimation is as good as, a couple of questions:

1. Do you have a shroud fitted (i.e. minimising stray light)?

2. Any chance you could pic the in/out focus patterns at reasonable high mag?

Might be barking up the wrong tree, of course...

[russ.will]

I've put it all away now, but I can certainly show you a good approximation of the images. Near the bottom of this page are some star test images.

Outside of focus, the images looked exactly like the top set on the first graphic. Inside focus, the images looked exactly like the middle example on the bottom line of the second graphic. I was using a real star in my case and there was no powermate, just the 11mm ES82!

Russell

[Damo636]

My inside and outside focus patterns never match either Russell so I'd also be interested to hear what others have to say on this.

[kirkster501]

Star testing shows pleasing concentric rings outside of focus, although I do have trouble achieving the same inside of focus......

I have that issue as well. I thought it was just me. I also cannot get the crosshairs to align precisely either. I need to look into it further...

[Stargazer_00]

My inside focus always shows the seconday slightly bias to one quarter or the prsented circle. I read somewhere this is due to the speed of the scope (F4.7). Outside of focus is perfect concentricity.

Also as the draw tube moves into the OTA that effects the concenticity of the rings, so that problably is effecting the infocus star test

[MadAlbert]

I have that issue as well. I thought it was just me. I also cannot get the crosshairs to align precisely either. I need to look into it further...

Would you describe your observations of DSOs as underwhelming, though?

[Moonshane]

it looks to me like your secondary needs to be rotated slightly. I cannot really see the position down the tube but does it appear far enough down the tube?

I'd do a full collimation starting with spider vanes but ignore the focuser for now as that's not really a problem as it can be adjusted out. I find images like this hard to interpret but do think that your secondary is not right and if it's not rotated correctly then a portion of the light is not reaching the focuser perhaps explaining the low light performance. the good planetary performance is perhaps explained by the fact that your primary collimation is not too bad.

re star testing, I never manage this effectively but you would be unlikely to see the drawtube on either side of focus as you only tweak it a minimal amount (perhaps 1/10th of a turn). you should do this on e.g. Polaris as it has to be (and remain) centered and all rings should be concentric. Not being so could be poor collimation or an uncentral star.

[Moonshane]

if you look at the secondary alignment section here I think it is a good match for your scope. it could actually be a bit of rotation and a bit of tilt error, hence my suggestion to redo a collimation from scratch

http://starizona.com/acb/basics/using_collimating_newt.aspx

[Stargazer_00]

re star testing, I never manage this effectively but you would be unlikely to see the drawtube on either side of focus as you only tweak it a minimal amount (perhaps 1/10th of a turn). you should do this on e.g. Polaris as it has to be (and remain) centered and all rings should be concentric. Not being so could be poor collimation or an uncentral star.

I usually wind the tube in quite a lot to blow the image up. When it just starts to show concentric rings, very close to actual focus, i find in my scopes both sides are perfect. It's only when you continue to go out of focus you start to see clips, drawtube, bias in the concentricity etc.

[Moonshane]

ahh. sorry I misunderstood. the way out of focus point is possibly useful as it can allow you to see if there are still tube currents I suppose?

[Moonshane]

ps

with a flextube, you might find it easier initially aligning the secondary with the lower OTA covered with the tube cap. that way you won't get confused with the primary reflections

[Stargazer_00]

ahh. sorry I misunderstood. the way out of focus point is possibly useful as it can allow you to see if there are still tube currents I suppose?

No idea, honestly I only usually do this because seeing my hand in the image amuses me and i make stupid hand shadow puppets for myself. Whatever floats the boat I guess...

[russ.will]

it looks to me like your secondary needs to be rotated slightly. I cannot really see the position down the tube but does it appear far enough down the tube?

I'd do a full collimation starting with spider vanes but ignore the focuser for now as that's not really a problem as it can be adjusted out. I find images like this hard to interpret but do think that your secondary is not right and if it's not rotated correctly then a portion of the light is not reaching the focuser perhaps explaining the low light performance. the good planetary performance is perhaps explained by the fact that your primary collimation is not too bad.

re star testing, I never manage this effectively but you would be unlikely to see the drawtube on either side of focus as you only tweak it a minimal amount (perhaps 1/10th of a turn). you should do this on e.g. Polaris as it has to be (and remain) centered and all rings should be concentric. Not being so could be poor collimation or an uncentral star.

if you look at the secondary alignment section here I think it is a good match for your scope. it could actually be a bit of rotation and a bit of tilt error, hence my suggestion to redo a collimation from scratch

http://starizona.com...ating_newt.aspx

ps

with a flextube, you might find it easier initially aligning the secondary with the lower OTA covered with the tube cap. that way you won't get confused with the primary reflections

Thanks. Lot's of stuff to be getting on with there, so I did.

I took it back to basics and took the secondary and spider out, in order to double check focuser alignment. My original centre mark on the opposite tube wall was between 1-2mm out, but only along the main axis of the scope - So not pointing off to one side. The focuser was a millimetre or two out in both axis, so this was all re-aligned. I did all of this using a Xbox Live Cam with a 1.25" nosepiece attached, but modded so I can reinsert the original lens for precisely the purpose of focusing on the mirrors.

I overlay the image with Mire de Collimation, setting the reticle ring sizes to match the focuser tube. I then align the crosshair of Sharpcap to the centre of the circular reticle and then turn the circles off. You are then left with crosshairs to align with the mark on the tube wall. A quick note to Skywatcher - Why oh, why do you put two of the three sets of alignment screws beind the focuser wheels and then off set all three rotationally, so none are in a straight left/right or up/down plane? Idiotic in the extreme and no wonder people don't bother.

With this done, I reinserted the spider, carefully equalising the vane length to well within a milimetre using a steel engineering ruler with 0.5mm gradations. At this point I turn the MdC circular reticle on, align it to the centre of the crosshairs and realigned the mirror to be perfectly centred and round. The lower tube cap has been on at this point, mainly to protect the primary. The primary reflection was surprisingly close to being centred, so a quick tweak of the Bob's Knobs later, secondary collimation is spot on too.

I'll leave primary collimation until the scope is back on the mount under clear skies. I'll do it in daylight prior to observing so I can see if the double vane shadow is still present and then confirm it all with some observation a good few hours later under dark skies. Fortunately, the forecast for the weekend is hinting at this being possible.....

Russell

[russ.will]

My inside and outside focus patterns never match either Russell so I'd also be interested to hear what others have to say on this.

I have that issue as well. I thought it was just me. I also cannot get the crosshairs to align precisely either. I need to look into it further...

My inside focus always shows the seconday slightly bias to one quarter or the prsented circle. I read somewhere this is due to the speed of the scope (F4.7). Outside of focus is perfect concentricity.

Also as the draw tube moves into the OTA that effects the concenticity of the rings, so that problably is effecting the infocus star test

re star testing, I never manage this effectively but you would be unlikely to see the drawtube on either side of focus as you only tweak it a minimal amount (perhaps 1/10th of a turn). you should do this on e.g. Polaris as it has to be (and remain) centered and all rings should be concentric. Not being so could be poor collimation or an uncentral star.

Good - It's not just me then. I find that the intra/extra-focal patterns are less than 1/8th of a focuser turn apart and that point is a loooonnnngggg way out from the point where the focuser tube impinges in the OTA lightpath.

So given that a wide range of people with different Newts, a number of different mirrors of varying qualities and differing expertise levels can't actually achieve both an intra and extra-focal test; Why is star testing always considered to be the 'ultimate' test of collimation?

Thanks for the input so far.

Russ

[russ.will]

No idea, honestly I only usually do this because seeing my hand in the image amuses me and i make stupid hand shadow puppets for myself. Whatever floats the boat I guess...

You would never, ever catch me doing that. Oh, no, no, no, no.......yes.

Russell

[neil phillips]

Hi Russel the primary reflection of the secondary vanes at first thought, looked to me like the spider might be ever so slightly bent. giving the illusion of double vanes, and the camera was picking that up. My vanes are also slightly bent. it happens often when they are tightened. I even tried loosening and re tightening, but it ended up in the same posision. But your photos show it looking far worse in the centre spot photos. So im unsure of that. I think your level of collimation sounds far to good to be creating what you describe in the views. at extremly high power planetary maybe. But this sounds unlikely to be caused by ever so slight mis collimation. i could be wrong. But did your memory of the superb view coincide with good seeing ? was it in the summer ? was there good transparency with the superb view. was the smaller scope cooled better ? Not as much heat from the houses disturbing the view ? I would be inclined to suspect one or more of these reasons than slight mis collimation. Though i could be wrong its just a guess based on things ive seen

[neil phillips]

Good - It's not just me then. I find that the intra/extra-focal patterns are less than 1/8th of a focuser turn apart and that point is a loooonnnngggg way out from the point where the focuser tube impinges in the OTA lightpath.

So given that a wide range of people with different Newts, a number of different mirrors of varying qualities and differing expertise levels can't actually achieve both an intra and extra-focal test; Why is star testing always considered to be the 'ultimate' test of collimation?

Thanks for the input so far.

Russ

You will be hard pressed to find a newt. with identical patterns, especially mass produced optics. My Orion has a superb set rated at 1/10th pv and is not identical either side. if one side looks good and the other only slightly less so that will be the norm i reckon. Have you thought about using a ronchi grating to test the primary. Good cheap ones can be got. Though i havent tried this myself yet

http://www.awrtech.c...htm#RONCHI_TEXT

[kirkster501]

Interested to know how you can make a camera, with lens, that can be used with mire de collimation. I have a lens for my SPC900. I have a nose-piece too. But cannot get lens in with nose piece attacthed. If remove nose piece and install lens how to attach a nose-piece?

[russ.will]

I don't know about the SPC 900, but with an Xbox Livecam, I simply glued the nosepiece off a kit Barlow onto the housing of the camera, taking time to make sure it was absolutely concentric. This means, with a bit of a fiddle, that the original lens screws in as normal.

I'll try and take some pictures for you later.

Russell

[kirkster501]

I don't know about the SPC 900, but with an Xbox Livecam, I simply glued the nosepiece off a kit Barlow onto the housing of the camera, taking time to make sure it was absolutely concentric. This means, with a bit of a fiddle, that the original lens screws in as normal.

I'll try and take some pictures for you later.

Russell

Thanks mate - Id appreciate that.

I am also thinking of removing the GoTo from mine as well. I don't like it - too much slop in the gears for it to be usable.

[russ.will]

There you go:

Russell

[neil phillips]

Hi Again Russel , Looking at it again i agree with moon shane. the secondary seems to have a slight rotation error, it can be seen at the top of the primary. And is magnified ( exagerated in the centre spot image )