c11 + hotech coll.

[blink138]

evening members i am hoping someone familiar with firstly a c11 and secondly the (amazing) hotech collimator
to begin i watched the youtube tutes and to check focusser orthogonality the man undid the clamping ring at the front and removed the secondary, scary but i gave it a go but the whole unit turns in the corrector!....... oops!
anyhow really got to know the basics of it all
so we decided to bypass that bit and got to play with the the three collimating screws at the front, but one of the returned laser reflections was more out of focus than 'tuthers and had a doughnut hole in it (see picture)
also focussing on the the back reflector only seemed to show two laser point converging and not the third and it was not focussed in the middle of the screen (fucusser non orthogonality?)
any tips?

[brian nordstrom]

:mad2: Hi Pat, I hear your frustration .

I had and loved a beautiful Takahashi M210 ( but hate diffraction spikes , being a refractor man ) that was a pain to get the last 1% of columation and you knew it was just not quite right , the Japanese state and show well in the manual how to use a high power star image ( 500x plus) to acheave that final 1% , anyway mate I got it after many attempts and WOAW!! you knew , the images were like a 7 inch + APO , so good , I did not think a reflector was capable of such views ,, awesome optics , but I digress .

So after getting my M210 perfect ( by myself ) and it held it for over a year of transporting out to our dark sky site in Darwin then a fellow observer got one of these and we had a play with it on several Newt's and it worked extremely well , I was impressed with the ease it showed mis-columation in the newt's , and made it easy to remedy , great bit of kit ;).

So I suggested we try it on my Tak thinking it was still a tad out and it will perform even better after this :thumbsup: ,,, BIG!! mistake mate ! it showed the DK to be out of alignment by a smidge so we adjusted it to what the Hotech said and I did a quick look at Jupiter ,, what an abortion it was ,, like a vale had been put over the eyepiece ,, awful compared to what I knew this scope could do .

My thoughts Pat , don't use this on a compound scope , they certainly don't work on DK cassigrain's , probably SC's as well , but on Newtonions these work very well , so I suggest a high power star test , I will try and get to Peters when you are there next and give a hand if you want , PM me .

ps. my C9.25 could do with a tweek as well so maybe we can do both at the same time and its much easier with 2 people , make a night of it ?.

Brian.

[blink138]

good idea brian!
pat

[Star Catcher]

Hi Pat
Things look odd to me. I have run the ALC on my RC10 and you definitely get three clearly separate dots on the rear reflector with the intention of getting them eventually centered. There should also be three identical tight beams returning onto the target. By the way I think the Tak collimator is more convenient than the ALC.

Your return beams to the target appear huge relative to mine on the RC10, however that is likely due to the added focal length and your exposure settings for the image. It is a concern that one spot is different in size and shape as you have mentioned. I can't imagine the focuser could be that far off. Could mirror locks be causing an issue?

I do have an old C11 that I will run the ALC on to see what things look like. I have never found I needed anything more than a star test and a Howie glatter centering mask to get things looking good on the C11. I'll let you know.
Ted

[blink138]

any help or advice is great ted!
pat

[Star Catcher]

Hi Pat

Did a quick few shots of the Hotech on my non Edge C11 so you can compare. Quite different from what you are getting. The three lasers coalesce at the reflector and target lasers are close to being on a common circumference position. Maybe there is some tilt in your system, more likely the secondary, but it might also be the primary and use of mirror locks?

I just don't know enough to interpret what I see in your images. It would be worth while sending it to David Hotech who is very responsive to all technical enquiries.

Ted

[blink138]

thanks for trying this for me ted, the first pic i posted was to show the blurry laser reflection with the black hole centre at top, we did actually get it centred ok, but was wondering if the blurry laser may have something to do with all three laser images no all coming together on the screen at the back
we will go around to peters place (stardrifter) again and brian norsdrum may attend also, so maybe we can nut it out
BTW ted does your whole secondary housing rotate within the corrector if you turn it?
regards
pat

[Star Catcher]

No problems Pat. Over the years I have had 3 C11s and all the secondary holders have rotated over the years.

Now that I am thinking about in my experience, the centre hole cut in the corrector plate can be substantially oversized. What this means is that when the sandwich coupling between the back section on the inside and the outer section of the secondary holder loosens, the whole assembly of the secondary mirror holder not only rotates but also slips under gravity i.e. it can become substantially de-centered and not optically co-aligned with the mechanical axis.

I think as a starting point I would get a pair of calipers to check centration of the secondary housing. If it is not centered, it means it has slipped. The secondary assembly will need to be tightened so it stays in position on the corrector, otherwise moving the scope about the sky will make it shift again. Once secondary assembly is tightened, the whole corrector plate must be adjusted to ensure the secondary is re-centered. Note before removing the corrector check for corrector position/orientation points, refer to your manual.

Ted

[blink138]

ted you are a veritable font of optical goodness ha ha!
now listen ted i broke out in a cold sweat because i had never turned my bobs knobs collimation screws up until a few nights ago so attempting to remove the corrector will surely give me an instant anxiety attack!
regards
pat

[multiweb]

I have some shots of the Hotech board here (http://www.astropic.net/astro/C11/article.sub&productid=1) that might help. Accurate registration of the board is 99% of the work. You should spend a lot of time in making sure everything is co-registered and square or your scope will end up in a worst state of collimation.

If you have rotated your secondary in respect to the primary you should have a look at the back of the secondary glass. There is usually a line from center to edge. There is the same line at the back of your primary which should point at 3 o'clock looking from the front of the scope so opposite the focuser knob.
If you are worried about which side is in or out when you've removed the corrector, there should also be a '>' mark on the edge pointing towards the primary, so the peak is pointing towards the inside of the scope. The correctors are only figured on one side, so the curved surface is usually out. In some rare instances both sides were figured. Although I was also told it doesn't really matter which side is in or out it's best to keep the original orientation. More importantly there should be an etch on the corrector glass that should also point at 3 0'clock. That's really important. Finally the corrector retaining ring is just 'touch' only. DO NOT tighten up. There are 12 screws and the retaining ring acts as a flange. You can apply a lot of pressure on the glass over the whole ring surface if you're not careful and break it with changes in temperature when the scope goes out in the cold and back indoors. The glass needs room to expand or contract.

So to recap, if you remove anything: Secondary, corrector and primary marks are all facing the 3 o'clock position (opposite focuser knob)

[Star Catcher]

Pat, not an expert, just someone who has gone through the same frustrations. The hyperventilation bit eventually disappears, just be methodical in approach and do your homework.

Looking at the manual, seems the Edge scopes have a different secondary assembly to my old C11 so that they are fast star compatible. But..... it seems the secondary holder's rotation on the corrector plate is still an issue! :(

See a thread discussing solutions to 8inch Edge HD rotation here on Cloudy Nights (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6149296/Main/6122283).

Your secondary may still be centered if the corrector plate hole closely matches the fit of the secondary holder. I would still check by measuring from corrector edge to out rim of secondary holder all the way around. If it is still centered you should be able to converge three beams onto the center of the cross hairs at the hotech reflector and get three similar dot sizes on target. If it is not centered, you will have compromised collimation.

If all else fails, then apply KISS. Come back to a known position by using a simple star test, bringing it back to a symmetrical donut.

As far as the rotating corrector secondary assembly goes, it will need to be addressed if found to be decentered. This movement will need to be stopped to give you confidence that collimation will stick.

Ted

[blink138]

very helpful thanks marc, i did read your collimation posts a good while ago so i will revisit them again
the screws at the front of the corrector were quite tight already though mate, i know because i loosened a few as per instruction on the hotech youtube tute but i decided to leave that bit
pat

[blink138]

thanks again ted i will look over the cloudy nights blog
my c11 is the carbon fibre fastar version though!
pat