Ioptron RC6 Focussing nightmare

[Tomservo]

Hi there! I am a beginner, and I realise thie could be 'Cockpit Trouble' but I've been struggling with this ever since I bought it.

I've never managed to get it in proper focus from brand new. The RC6 has 3 extension rings to adjust the 'Back Focus' but it never seemed to be quite enough.

I had it very roughly focussed on the moon by holding the Eye piece out from the mount by about 10mm.

I contacted Ioptron through RVO and they more or less said there was nothing wrong with the one they had there, and that was it!

I bought loads of stuff for it and managed to extend the back focussing a bit more.

I've now got the ASIAir Plus and ZWO ASI224MC Camera, and the  ZWO EAF (focusser) for it.

I bought a Laser Collimator and adjusted the Primary mirror and then the Secondary mirror, and as they were both considerably out of alignment I thought I'd cracked it....But No!

I'll do my best to attached some pictures I took on my IPad this evening of the focussing attempts; they will show by the position readout numbers on the focusser that it reaches best focussing position and then goes out of focus further on. The 'Best' focussing position (1831) is total rubbish.

I'd really appreciate some suggestions!! (aside from taking up knitting!)

Thanks.

[Paul M]

RC's are usually described as an experienced user's scope.

I got a 10" RC this time last year. I'm no expert but quite comfortable with collimation Newtonians. But still, I was cautious of the RC. They have a reputation!

There are some good tutorials on line for collimation of these things and I watched them all a few times!

Mine was out when I checked it using one of these: https://www.365astronomy.com/ts-optics-2-led-collimator-for-rc-telescopes-and-all-other-types-of-telescopes?path=119_416

I know now that such a fancy peice of equipment wasn't necessary but it was easy to use and reassuring.

It's not easy to diagnose your problem remotely, for me anyway. But you have confirmed that back focus is not the issue.

Normally the inside and outside focus images would be quite different if there is significant tilt or off axis aberrations.

I'd recommend watching a load of YouTube guides. This is the route I followed:

 

 

Edited November 25, 2023 by Paul M

[Carbon Brush]

Was the scope bought new? From RVO?

If that is the case, are you near enough to drop it into the shop?

An experienced eye will either sort the problems, or identify a 'back to maker' fault.

[bosun21]

I used to own the 6" Classical Cassegrain from the same manufacturer. I could reach focus easily with an extension piece(s) to spare. The distance between the primary and secondary mirrors has to be exact in an RC and is reportedly difficult to gauge. I know that @Chris owned the 8" RC at one time, so maybe he could give you his experience of reaching focus. Good luck 🤞.

[Anne S]

I have owned a Ritchey Chretien 6 inch for several years. I use 75mm of extension tubes. My imaging equipment takes up 17.5+37= 54.5mm. I don't have a reducer on. The focuser drawtube is just over 2 cm out. I would suggest you have too many extensions on. You've got  very long drawtube so make use of that to find the focuser position. Once you've found that, adjust the number of extensions to reduce the drawtube distance. The focuser can droop slightly with heavier loads. Make sure you use a long enough exposure as an out of focus star is almost invisible. The moon is your friend!

I have an instruction manual which contains some suggestions for using the extension tubes with different setup. Just do a search for Orion Telescopes. I notice the Ioptron site also has one but doesn't have the diagrams. You don't want Orion Optics!

When I bought my RC, I tried it out visually first. You'll need fewer extension tubes when you use a diagonal. Make sure you're looking at a distant object if doing in daytime.

i hope that helps. Incidentally, there are various versions of this scope with different suppliers but they are all made by GSO. Mine is an Altairastro version.

Anne

[Tomservo]

Thank you everybody for replying to my questions: 

Paul, I'll check out that link, the collimator I used was a laser one....that went back today and I'm waiting for a Cheshire to turn up tomorrow. I do tend to jump in at the deep ends, but I tend to enjoy the task. Thanks.

Carbon Brush, Yes it was new, but the problem is they are about 200 miles away from me, we seem to be starved of Telescope dealers these days where I am (Essex), But....If push comes to shove! Thanks.

Bosun21, I may ask him the question after I get a result tomorrow, thank you for that, I'm assuming I just leave @Chris a 'Public message'? (not sure how it works) Thanks

Anne S, Thats interesting stuff, I worked my way up to using all the extensions and found I still needed another 10mm; I achieved that by pulling the camera out by 10mm at which time I managed to take a few images of the moon. Although they were 'ok' I had to stack a lot of them to get it anywhere near reasonable. I was at that stage I contacted RVO, and I was less than impressed with their answers. Then Moon was almost in focus but then its a lot closer and bigger than the stars. I'm beginning to get concerns about the mirror distances now but I don't know enough about that aspect yet, (it may explain why I needed to use all the extensions) after I collimate it again tomorrow I'll do some research on that. Thank you

[Oddsocks]

On 25/11/2023 at 21:27, Tomservo said:

 

I had it very roughly focussed on the moon by holding the Eye piece out from the mount by about 10mm.

13 hours ago, Tomservo said:

 

Then Moon was almost in focus but then its a lot closer and bigger than the stars. I'm beginning to get concerns about the mirror distances now but I don't know enough about that aspect yet,

Having to hold the eyepiece out from the mount, as you described, is expected.
To use the telescope visually you would normally attach a diagonal holder and insert the eyepiece into that, which would bring the eyepiece out around another 20mm, or so, and you would have to rack the focuser inwards a little to reach the focal plane from there.

The distance you needed to pull the eyepiece out of the focuser to reach the focal plane tells you where the camera sensor needs to be as both the eyepiece and camera will reach focus at the same distance and when setting up the back-focus spacers for an imaging setup a quick method to find the additional spacers needed is to rack the focuser to the half-out position and starting with the eyepiece fully inserted in the eyepiece holder you gradually slide the eyepiece out, into free air, until you reach visual focus and (easiest with the help of a second person) measure the distance between the field stop of the eyepiece and the back of the eyepiece holder on the telescope and that distance is (roughly) the amount of additional spacers you need to add so that the camera reaches focus approximately in the middle of the focuser’s travel range.

Picking up on your comment re’ distance to the moon being closer to the stars, so far as focussing a telescope is concerned they are both at infinity and will both reach focus at the same point.

As mentioned in an earlier reply, with the RC telescope design the distance of the focal plane from the back of the telescope body is greatly dependent on the distance between the primary and secondary mirrors and a very small change in primary-secondary distance will have a big effect on the distance of the focal plane from the back of the OTA, but, although the distance of your particular telescope appears excessive there could be some variation in manufacturing tolerances of the mirrors that have resulted in an unusually long focal plane distance for your particular instrument.

On the other hand it could be that your instrument left the factory assembly line having been mis-collimated, or had a particularly rough journey, and that has pushed the collimation and back-focus out of tolerance.
Whatever the reason, it is quite easy to put right with no specialised tools needed although it’s rather disappointing to hear that your supplier has provided no helpful advice or pathway to resolve the problem.

I don’t know your particular model telescope but several of the bigger RC’s back-focus standard spacers (of a few years ago) were calculated to allow for the inclusion of an OAG, filter wheel and flattener as well as the camera to reach prime focus. When I briefly owned an RC8 around twelve-fifteen years ago I needed an extra 25mm spacer in addition to the three spacers supplied with the OTA, and that was using a camera, OAG, filter-wheel and a flattener (although my camera had an integrated filter-wheel and OAG and had a smaller back-focus requirement than if all those components were individual elements bolted together).

A final tip, many beginners to imaging struggle with focussing a camera because they forget that as the camera sensor gets closer to the focal plane the photons from the object being focused, Moon, stars, whatever, are falling on fewer and fewer pixels, which saturates them, and this is particularly noticeable with big bright objects such as the Moon and planets. As you move the focuser position and the focussed object appears to shrink on-screen and grow brighter, you must also reduce the camera exposure time to prevent the camera pixels saturating, otherwise you’ll never be able to tell if you have really reached focus or not.

When the camera exposure time is too long, or the camera gain set too high and the camera pixels are saturated you could move the camera all the way from an intra-focal position, through prime focus and out to extra-focal position and never see a significant change on the monitor. You have to continually reduce the exposure time, and if necessary the camera gain, as you bring the camera into focus and try and keep the image brightness constant.
 

You will find that when visually focussing a camera the image has to appear quite dim on the monitor/screen to have any chance of detecting the prime focus position, which is not easy to judge if the monitor has other bright graphic elements in the same field of view because your eyes naturally adjust to those bright objects and the tendency is to try keep the object you are focussing on at roughly the same brightness as those other elements, and that will almost certainly lead to pixel saturation and you won’t be able to tell when the object is really in focus.

It’s a bit of a juggling act at first, adjust focus to reduce the apparent size of the object on-screen while at the same time reducing the exposure time, or camera gain setting, to keep the object brightness under control so that the camera pixels are not saturated.

Once you have the manual focus technique mastered, and the prime-focus distance accurately determined, then you’ll be able to use autofocus instead of manually focussing, which is a whole lot easier, but it is important to master manual focus techniques first to help understand how things should work so you’ll know how to resolve any issues that may arise with autofocus.

HTH.

William.

Edited November 27, 2023 by Oddsocks
Usual, poor spelling and punctuation….

[Clarkey]

From experience, I would suggest that the video method above is fundamentally flawed. This assumes that the primary mirror is perfectly aligned with the focuser - which in most cases on these 'budget' RC's is not the case. I tried this method once on my RC and it was awful. Personally, I would suggest you align the secondary as described in the video (step 1), but do NOT use a laser to try and align the primary. Providing your collimation is not too far off, you can use a star test to get the primary right. You can do this following this method:

A Procedure for Collimating Ritchey-Chrétien and Other Cassegrain Telescopes (deepskyinstruments.com)

This is the most reliable method I have used. You can get it pretty close using the outer ring (as shown in the video at the beginning of step 2, before the laser). This works on the RC6 (but not on the RC8).

[Tomservo]

Thank you both for your information and suggestions and I do appreciate  the time spent here. It is quite a learning curve but it's very interesting and I tend to be persistent with things like this so i'll keep at it. I will be taking your advice on board.

I find a lot of the videos online showing how to collimate using the stars, assume that you can already focus on them, which I've not been able to do, however........

I did a collimation  today with a Cheshire eyepiece, and it was quite an eye opener (if you'll forgive that); what I thought I had achieved with the Laser collimator before was probably making it worse!

I did manage to get the required result according to the pictures in the instructions.

I will be testing it out on the stars, as soon as I've found a way to see through the clouds.

Thanks again

Tom

[Paul M]

I think the most critical point are, that the secondary/primary distance is correct and that the axis of the secondary is aligned with the center spot of the primary. This only needs an accurately collimated laser. As Clarkey says, the rest is best done on a real star. The fancy collimator I linked to earlier is perhaps more of an educationally tool! It helps you understand just how much is going in within the optical train.

I haven't worried to much about tilt adaptor for the focusser.

Life is short and contains a very low proportion on clear nights! If you find yourself drifting into the higher echelons of visual observing or Imaging, then yes, it matters. But worry about getting a better focus first!

[Tomservo]

Thanks Paul, that fancy collimator IS a nice piece of kit though! I often wonder why some kind of software can't be written to use Asiair for collimation? (I haven't wondered too much though)

I was outside last night getting it to focus on Polaris, (now I have an awful neck ache) It was recommended I think on the Sky at night site that I use a med - to high magnification eyepiece, so I bought a 4mm, but I couldn't even find it with that; so I put a 23mm in and found it with that. Now...because it was terribly out of focus, the first thing I saw was the doughnut, which was large; I focused it up and changed to the 4mm; I focused on that, which was a relief! and took it past it to get a doughnut but it was quite small and difficult to make out the rings; what I think I saw was some reasonably concentric rings with a small glare of light at the bottom (like a diamond ring) does that sound like a big issue? 

Using Polaris is an obvious choice if you can't use a tracker yet, but it definitely ain't practical for making adjustments, and its hard on the knees!

Is there a solid reason why I shouldn't us the 23mm eyepiece for this? 

I'm wondering now if I mixed the eyepieces up! but I'm pretty sure I didn't.

It's reassuring to learn that a tilt adapter isn't necessarily needed; my setup seems very solid, I guess I'll find out.

I plan on setting it up again tonight with all the gear back on it and see if I am indeed getting a proper focus now, if I am I'll use a star a bit lower down to check the collimation.

Thanks again

[Tomservo]

I have another question!!

Following instructions for Collimating, that I got from the Ioptron RC6 user guide; I did it without any extensions on, basically not setup for use. I used the Cheshire eyepiece and adjust the mirrors to get a good result.

I took the telescope out and attempted a Star Collimation which showed concentric rings in a circular position but seemed to have a glare coming from it at the 6 oclock position.

Today I put the Cheshire in, in place of the camera, (so the scope is now setup as it were for use) and the collimation is out.

Now I know that a Tilt adjusting ring maybe required (more money!) but it made me wonder if the collimating should be done under normal use conditions? i.e. all extensions etc. attached?

waddya think?

Edited December 2, 2023 by Tomservo