34 replies to this topic

[Henry from NZ]

Hi there

 

I like to share a new technique of collimating a GSO made RC during day time with a few simple tools.

 

Through the last few months I have received lots of help collimating my RC, and I thought I should return the favour by presenting a new technique which as far as I know has not been discussed before.

 

First of all, this technique is not for every one. If you can get good collimation with other techniques, such as DSI techniques, or Cheshire, or by removing the primary baffle - then you don't want to do this because this procedure is radical and may involve taking your OTA apart completely.

 

This technique is really designed for those, like myself, who have fiddled with the collimation a bit too much (i.e. have adjusted primary and removed secondary) and want to start with a clean slate. It is more meant to put it back into a factory setting - hence the "reset" method. You may still need some fine tuning after that but I was happy enough with the initial results that I did not bother to do any fine tuning.

 

Here is a FWHM plot to show the final results:

 

Here is a aberration inspector. Please note this is taken without flattener at 0.98 arc sec / px.

 

Here is an image from last night at 1355 mm FL, no flattener.

 

The technique is based on the premise that even though the mechanical and optical axis of these "cheaper" RCs do not necessarily align, they would still be better than many RCs out in the wild which have been fiddled with. By returning the RC back into a factory setting, many users will be better off than where they are now and sometimes that may be all that is required to make is functional (note - not necessarily perfect).

 

To start, you will need:

1. A good laser collimator

2. A good cheshire

 

STEP 1:

Reset the primary and secondary mirror distance to achieve the factory focal length.

You want to do this because according to Tommy at Telescope Austria, the Strehl value decreases as the spacing deviates from the required amount. If the distance is not correct you will get more spherical aberration. You really want to do this first before dialling in the collimation.

If you decrease the distance, you will increase the focal length and vice versa.

For every one mm change in distance you will increase / decrease the focal length by a factor of at least 10x.

To do this you will need to first establish the current FL by taking an image with no flattener / reducer and plate solving it.

This will give you a "direction" to adjust.

If you need to decrease the distance - you loosen all secondary collimation screws say by 1/4 turn; then loosen the center screw (be careful not to loosen it so much the secondary drops off) and then tighten the secondary screws by equal amounts.

If you need to increase the distance - you loosen all the secondary screws by the same amount, then tighten the centre screw until it is song again.

Your collimation will be off after this, but you should still have recognisable stars for plate solving.

Also note that if you decrease the spacing, you will increase the back focus and vice versa.

Keep repeating these steps until the FL is as close to the spec as you can. I started off with a focal length of 1300 mm and after multiple passes managed to get mine to 1368 mm (spec is 1370 mm) although after the subsequent procedure it went back to 1355 mm - but I am happy with that.

Edited by Henry from NZ, 25 January 2016 - 02:48 AM.

[Henry from NZ]

STEP 2:
Reset the primary: to reset the primary one just loosen all the locking (smaller) collimation screws and tighten all the larger silver collimation screws till home. This basically make the primary mirror and the focuser align with the mechanical tube axis within manufacturing errors.

I actually took my primary assembly off completely, removed the mirror, cleaned it, checked the plastic spacer it sits on and reinstalled everything as square as possible. Google "hnsky collimation" you will find a pictorial essay on how to take your RC apart.

 

STEP 3:
Align the secondary - here is a little know fact - the secondary mirror can be centred AND tilted. What this also means is that it can be de-centred as well as tilted. If the secondary mirror is de-centred, the alignment can still be compensated for within reasons by tilting, but it won't be perfect.

To accurately align the secondary, one needs to have it accurately centred first, then accurately tilted.

To do that, loosen the three peripheral collimation screws a bit (equal amount), then you can grab the secondary mirror holder and with a good laser in the focuser you can centre the secondary mirror so that the laser hits the centre spot. then you tighten the peripheral collimation scews to lock this in place. I found that with this I can even do away with the tilt plate after my collimation.

Once the secondary is centred then you can adjust the tilt by adjusting the peripheral collimation screws so that the laser returns to the collimator. You must loosen at least one screw before tightening one or both of the other screws. Make sure you do not de-center during the process.

By the end of this step, you will have:
1. Reset the focuser to align with the OTA - within manufacturing tolerance.
2. Reset the primary to align with the OTA - within manufacturing tolerance.
3. Reset the secondary to align with the focuser which in turns is aligned to the OTA - within manufacturing tolerance.

FROM THIS MOMENT ON, YOU SHOULD AVOID ADJUSTING THE SECONDARY.
The secondary is done. Do not touch it again.

 

STEP 4.
Fine tune the primary.

If the factory tolerance is good you will actually be fairly close to being collimated - however this may not be the case. Also having the primary screws homed in means that there is no room for adjustment. So the next step I did was to loosen the silver primary collimation screws by equal amount (3 full turns - you will also need to tighten the smaller locking screws now). Looking down a Cheshire, you align the centre dot i.e. reflection of the Cheshire with the circle that is the secondary mirror centre mark by adjusting the PRIMARY.

You can further fine tune it following the instructions for the Vixen VC200L collimation of the primary (google it for the PDF), or the hall of mirror technique. Whatever you do, do not adjust the secondary

 

STEP 5.
Star Test and adjust the primary to eliminate on axis coma as per DSI technique - I did not need to do it.

Edited by Henry from NZ, 25 January 2016 - 03:24 AM.

[shawnhar]

Whoah hold the phone...that's an image with an RC and no corrector?

[Henry from NZ]

yes, no flattener. but atik 314 has a small sensor so curvature not so much a problem. When I get my hands on the Atik 460 I will test again. I suspect some fine adjustment will be required for the larger sensor

[pfile]

the FHWM plot looks great! i'd say that's perfect collimation.

 

only nitpick is with the statement: "For every one mm change in distance you will increase / decrease the focal length by a factor of at least 10x." i would say "increase/decrease the focal length by 10mm (at least 10x the change in primary-secondary distance)". when i read that the first time i was pretty confused.

 

generally speaking on an RC the secondary centering is really important - i didn't even know you could fix it on one of these GSO RCs. thanks for the tip.

 

rob

[jdupton]

Henry,

 

   I do not own an RC but do plan for one in the somewhat distant future so this is an interesting thread for me. Not owning one or knowing the mechanical details, I have a couple of questions regarding steps 1 and 2.

 

   In step one you set the separation of the mirrors by plate solving for the design specification. Are these RCs provided with individualized test sheets that give the specific focal length for a given scope? I would assume that there must be some tolerance coming off the assembly line and each scope's optics set might have a different "best image / least aberration focal length". Do you set everything to equal a parameter supplied with the scope or do you simply set the focal length to the published design point of F/9.000 focal ratio or something similar? 

 

   Secondly, in step 2 you unlock the primary and run the collimation bolts to their "reset" value. This squares everything up nicely within the mechanical manufacturing tolerances of the scope. However, doesn't it also change the primary to secondary separation again? I don't have a good mental picture of how collimation works on the GSO RCs but on most scope designs, bottoming the collimation screws would change the mirror position. After doing the step 2, I would have expected you to then perform the first part of step 4 -- backing off the collimation screws by an equal amount.

 

   It almost seems to me that doing step 2 first and then backing off the collimation screws by exactly equal amounts (as in the first part of step 4) would set the primary at a known "reset-referenced" point but still give leeway for later adjustments of the primary without changing primary to secondary separation. If the preliminary step 2 is then followed by step 1 to set the separation to give a specific focal length, you'd be in a better position to proceed with steps 3, 4 (last half), and 5.

 

   Your overall procedure seems logical and one certainly cannot argue with the results. Your test images are extraordinary! As I said, I don't own one of these so my observations may be *way* off-base.

 

 

Respectfully,

John

[Henry from NZ]

Rob, yes every 1 mm change in spacing will change the focal length and back focus by > 10 mm.

John, I agree with you. You can do step 2 ahead of step 1. But if you do that the scope may become so discollimated that plate solve fails. That's why i did it in that order. When you screw home the primary in step 2 you can always note down the average number of turns made by the three screws and turn the same number of turns in step 4. the difference is not great enough for me to worry about it.

And no, there is no factory issued specification re correct spacing or correct focal length. However these things are mass produced likely by machines so I suspect that they are made to have the specification focal length.

Note I don't think the collimation is perfect, because I use small sensor which may not show corner defects well. However I think it is a good base to start using more labour intensive technique such as DSI which I do believe is the best method. It just does not work that well if your scope is way out.

Edited by Henry from NZ, 26 January 2016 - 09:48 PM.

[akulapanam]

Honestly the best way to collimate a RC is to follow the manufacturer/dealer instructions and NOT touch the primary. I know its hard, I know people really believe they can do better, but when I was purchasing mine I talked with 3 different dealers and they all said the same thing DON'T touch it.  The primary is collimated at the factory and even with the laser approach it is hard to get it aligned again if you do.

 

IMHO the easiest way to collimate these things is a Cheshire eye piece followed by a star test with ccd inspector to get it under 5".

Edited by akulapanam, 27 January 2016 - 12:50 AM.

[pfile]

it's not really hard to get the primary aligned again. the easiest way is the DSI method where you adjust the primary to eliminate any on-axis coma. and any of the cheshire methods work fine as well.

 

for me the hardest part has been getting the corner shapes correct - the curvature on the 6" RC is enough that you can easily see elongated stars with an 8300M-sized sensor. but the curvature on the 10" is much less and so the shapes at the corners of an 8300M are a little subtle. using a full-frame camera can help but if it's not your imaging camera you're back in the situation where you're not collimating with your actual imaging train.

 

sometimes you have no choice but to adjust the primary - like if a previous owner messed up the collimation, or if shipping it or putting it in the back of your car jostled it enough that the primary moves.

 

rob

[bobzeq25]

Honestly the best way to collimate a RC is to follow the manufacturer/dealer instructions and NOT touch the primary. I know its hard, I know people really believe they can do better, but when I was purchasing mine I talked with 3 different dealers and they all said the same thing DON'T touch it.  The primary is collimated at the factory and even with the laser approach it is hard to get it aligned again if you do.

 

IMHO the easiest way to collimate these things is a Cheshire eye piece followed by a star test with ccd inspector to get it under 5".

The manufacturer no longer (they used to) demand you do not touch the primary, for the simple reason that sometimes you have to.   They tell you how to tell if you do need it.

 

https://www.astronom...ation sheet.pdf

 

That certainly was the case with mine.  Secondary only simply did not work.  It wasn't a case of wanting to do "better", it was a case of wanting to get to a reasonable collimation.

 

If yours didn't need it, congratulations.  But some do.

Edited by bobzeq25, 27 January 2016 - 01:24 AM.

[Henry from NZ]

The GSO made RC has push pull screws for the primary mirror.

I think if they are screwed tight the chance of shift is very small.

Therefore, if there is any change in collimation over time, it is likely coming from the secondary.

If the scope indeed has been collimated at the factory, and it is discollimated when you get it out of the box, chances are it is only the secondary that needs adjusting.

I think that's where the recommendation "don't touch the primary" comes from.

The design flaw of the GSO RC, besides the coupling of the focuser axis with the primary axis, is the way the secondary is mounted.

It is secured with a central spring loaded bolt with three tilting screws.

However as I discovered there is play with the central screw.

So the secondary mirror can be de-centered as well as tilted.

The secondary mirror has lots of freedom and lots of room for mal-alignment.

 

My suggestion is: if the primary has never been touched - leave it alone and tried to optimise the secondary first.If you really need to touch the primary, make a careful detailed note where the screws are before you do so so you can put it back if needed.

 

if the primary has been touched, this method I propose may bring it back to a usable state.

[anismo]

This is excellent info. I  have cleaned my primary mirror of my RC last year and haven't collimated it .

 

I was going to collimate it before loading it for the galaxy season and this post is right on time

[Henry from NZ]

Tonight I have done some test with my DSLR (much larger sensor than my Atik 314)

 

Because my DSLR is not set up for the OAG, I have no ability to guide, so the test images are comprised of 10 second subs unguided at 0.655 arc sec per pixel. Again, no flattener is used.

 

FWHM plot - this shows a skew to one side.

Edited by Henry from NZ, 30 January 2016 - 05:37 AM.

[Henry from NZ]

Now the aberration plot. The stars are getting sparse towards the periphery, so I made a 5x5 mosaic.
The corner stars are obviously astigmatic, but the direction of tilt is symmetrical (pointy end towards centre)

 

[Henry from NZ]

However - despite the apparent tilt on the FWHM plot, I think I am happy with the result (down sampled to fit within limits):

 

Edited by Henry from NZ, 30 January 2016 - 05:48 AM.

[Henry from NZ]

Lastly - this is an image of defocused stars to demonstrate any on axis coma / off axis astigmatism, as per the DSI technique.

 

 

Conclusion

The "reset" technique proposed here can get your RC back to a functional state, all done during day time. It probably will not produce perfect result, but is likely to get you very close to the ball park and can form the basis of further fine tuning with more advanced / time intensive techniques.

 

For me, I will probably leave it at this, as I intend in the long term to be using my yet to arrive Atik 460 with it. Since the Atik 460 has a smaller sensor than my DSLR and I will be mostly be using a flattener, this degree of collimation would be sufficient for my purpose.

Edited by Henry from NZ, 30 January 2016 - 06:01 AM.

[pfile]

the decentering thing is really key - my AT10RC is currently in storage and is definitely out of collimation - based on some images i posted in the cats/casses forum it was diagnosed as having a decentered secondary. i wasn't sure how this could happen, but you've shown that it can happen. i have had the secondary very loose with the OTA in a horizontal position, so that's probably what did it. next time i work on it i will try to recenter it. i think with the OTA pointed straight down and standing on a ladder i can probably get it centered up properly with the glatter laser or the tak scope. problem is mine is old enough that the secondary has no factory center spot and the spot was hand-drawn by a previous owner. so it's hard to know if it's really mechanically centered by looking at it, which complicates things a bit.

 

rob

[Jon Rista]

I'm working on getting my AT8RC into optimal imaging condition. I brought it out tonight, and before the wind kicked up (bah, wind is my bane!), I had a tough time getting the kind of results I felt I should have been able to, as I was guiding at 0.65" RMS to my image scale of 0.793"/px, peaks of about 1.8" max. The stars are just not sharp, even though they appear fairly round across the frame. I did a blind plate solve at Astrometry for one of my M46 subs, and it indicated my image scale was 0.791"/px, which would indicate a focal length of 1629mm (vs. the 1625mm I believe is correct). That is a deviation of ~4mm, which I am not sure I want to mess with...I kind of doubt I'll be able to keep it that close if I follow this full procedure.

 

Anyway...I pulled off the primary mirror assembly, as ever since I got this scope, the primary mirror has looked quite dirty to me. It's definitely got something on it, something sticky spattered fairly evenly around the mirror...maybe sap (?), I honestly don't know what, but I think I am going to need to follow a cleaning procedure Anismo shared some time back (distilled water, cotton balls, and a very careful touch). After cleaning the mirror, I figured I would go through this procedure, because I've tweaked and tweaked, and while I can get pretty round stars edge to edge with my TSRCFlat2 flattener, they just aren't as sharp as I think they could be. Even the diffraction spikes after focusing as ideally as possible with a bhatinov mask end up looking soft when I scale the images to full size...instead of the razor sharp they seem to be in images from most others on this forum that seem to have AT8RCs. I'm wondering if anyone who has an 8" GSO type RC has followed this procedure, and has some examples, at 100% scale, of what I should be expecting with this scope? Maybe I just have perpetually horrible seeing, and I just cannot get the kind of sharp stars I'm looking for...but mine just seem to be VERY soft, frustratingly so:

 

[ekallgren]

Hi Jon,

I,m curious about something that doesn't get mentioned very much when collocating one of these scopes.  What is the measurement from the rear cell to the sensor when in focus?

This, I think sets the FL of the optics. I might be wrong.

I have the 6" and I just bought an 8" RC and the 8" states , " The image plane is located 10” behind the rear cell". The 6" states that, “the infinity focus of the AT6RC falls 9-3/4" outside the back of the telescope".

The 8" that was just delivered measured 10 -1/4" from the rear cell to the sensor when in focus. I haven't had a chance to get it outside as yet to test it.

 Just as a reference the 6" measurement was at 9-1/4 with bad stars in all the corners. I have since reset that measurement to the 9-3/4" as stated in the literature. But again I haven't had a chance to get it out to test it.

 

I'm wondering if a few people could take a measurement from the rear cell to the sensor and post there results here? Either in millimeters or inches.

 

Maybe this should be a different thread?

 

Thanks in advance.

Edited by ekallgren, 28 February 2016 - 12:51 PM.

[Jon Rista]

Hi Eric,

 

What you are referring to is backfocus, which is actually mentioned quite a bit with these scopes. My backfocus is just shy of 1mm off, from the 109mm allowed by the TSRCFlat2 flattener. It is only off by 1mm because my imaging train doesn't exactly allow me to eliminate that extra millimeter:

 

 

The OAG there is the Celestron OAG, which is configured for use with a DSLR, which consumes just a touch too much backfocus for the flattener in use. Nothing I can do about that, but I don't think it matters enough to really worry me, since I am using a full frame DSLR, and the fact that I get even remotely round stars in the corners is kind of amazing to me.

 

The thing that frustrates me is my stars are so bloated. The scope sat untouched in a closet since the last time I used it, which was months ago during fall last year. Last fall, my stars still seemed a bit soft, although before that I was able to get the scope properly collimated and collimated pretty well, such that the stars seemed to be a good deal tighter (looking for those subs...not sure where they are). At the very least, I remember resolving many more small stars than I seem to be able to now. I think they are there...it's just that they are blurred so much they either don't appear, or appear as barely visible blobs in the noise. Anyway...it just doesn't seem to be performing well, but I cannot figure out why. My focal length seems to be off only by a few millimeters (whereas I've hear some people have been off nearly 100mm before), my collimation seems to otherwise be decent at the very least...yet I still have these very soft stars. 

[Jon Rista]

Well, I haven't fully dialed in collimation yet. I followed the procedure listed to center the secondary (I could not shift it around to center, there was less than a millimeter of play laterally, so I only adjusted the tilt), and left it. I don't think that is going to work for me, as there is clearly some off-axis astigmatism now that I've got the primary tweaked. After resetting the primary and secondary, there was a small but definite amount of on-axis coma, however the stars at the periphery looked fairly consistent. Since fixing the on-axis coma, the off-axis stars now have issues...I don't know of any way to fix that other than to adjust the secondary, which will require subsequent adjustment to the primary... 

 

Guess a reset didn't quite work for me? 

 

Anyway, there has been an improvement in my stars:

 

 

Before and after GIF:

 

http://i.imgur.com/Ve2Ks5w.gif

 

I just hope I can maintain this improvement when I fix the off-axis stars... 

[pfile]

Hi Jon,

I,m curious about something that doesn't get mentioned very much when collocating one of these scopes.  What is the measurement from the rear cell to the sensor when in focus?

This, I think sets the FL of the optics. I might be wrong.

I have the 6" and I just bought an 8" RC and the 8" states , " The image plane is located 10” behind the rear cell". The 6" states that, “the infinity focus of the AT6RC falls 9-3/4" outside the back of the telescope".

The 8" that was just delivered measured 10 -1/4" from the rear cell to the sensor when in focus. I haven't had a chance to get it outside as yet to test it.

 Just as a reference the 6" measurement was at 9-1/4 with bad stars in all the corners. I have since reset that measurement to the 9-3/4" as stated in the literature. But again I haven't had a chance to get it out to test it.

 

I'm wondering if a few people could take a measurement from the rear cell to the sensor and post there results here? Either in millimeters or inches.

 

Maybe this should be a different thread?

 

Thanks in advance.

 

this is true - the mirror spacing sets where the focal plane is, which also sets the focal length of the telescope. it varies from design to design but generally speaking the distance to the focal plane changes in a ratio of about 10:1 with the mirror spacing. so if the mirrors change by 2mm, the backfocus changes by 20mm.

 

if the telescope is mostly working properly, you can take and image and then solve it for the plate scale. this should tell you the current focal length of the telescope and whether or not you need to move the mirror spacing. IIRC too long of a focal length means the mirrors are too close together, and vice versa.

 

i dont have an RC8 so i can't make that measurement for you, but the plate scale should tell you everything you need to know...

 

rob

[akulapanam]

Well, I haven't fully dialed in collimation yet. I followed the procedure listed to center the secondary (I could not shift it around to center, there was less than a millimeter of play laterally, so I only adjusted the tilt), and left it. I don't think that is going to work for me, as there is clearly some off-axis astigmatism now that I've got the primary tweaked. After resetting the primary and secondary, there was a small but definite amount of on-axis coma, however the stars at the periphery looked fairly consistent. Since fixing the on-axis coma, the off-axis stars now have issues...I don't know of any way to fix that other than to adjust the secondary, which will require subsequent adjustment to the primary... 

 

Guess a reset didn't quite work for me? 

 

Anyway, there has been an improvement in my stars:

 

M46_420s_200iso__22c_20160228_21h21m42s568ms_L_mosaic.jpg

 

Before and after GIF:

 

http://i.imgur.com/Ve2Ks5w.gif

 

I just hope I can maintain this improvement when I fix the off-axis stars... 

 

How much of the improvement do you think has to do with seeing in the M46 image.  The star shape looks a bit better but the improvement also looks like it could be seeing related.

 

 

Hi Jon,

I,m curious about something that doesn't get mentioned very much when collocating one of these scopes.  What is the measurement from the rear cell to the sensor when in focus?

This, I think sets the FL of the optics. I might be wrong.

I have the 6" and I just bought an 8" RC and the 8" states , " The image plane is located 10” behind the rear cell". The 6" states that, “the infinity focus of the AT6RC falls 9-3/4" outside the back of the telescope".

The 8" that was just delivered measured 10 -1/4" from the rear cell to the sensor when in focus. I haven't had a chance to get it outside as yet to test it.

 Just as a reference the 6" measurement was at 9-1/4 with bad stars in all the corners. I have since reset that measurement to the 9-3/4" as stated in the literature. But again I haven't had a chance to get it out to test it.

 

I'm wondering if a few people could take a measurement from the rear cell to the sensor and post there results here? Either in millimeters or inches.

 

Maybe this should be a different thread?

 

Thanks in advance.

 

this is true - the mirror spacing sets where the focal plane is, which also sets the focal length of the telescope. it varies from design to design but generally speaking the distance to the focal plane changes in a ratio of about 10:1 with the mirror spacing. so if the mirrors change by 2mm, the backfocus changes by 20mm.

 

if the telescope is mostly working properly, you can take and image and then solve it for the plate scale. this should tell you the current focal length of the telescope and whether or not you need to move the mirror spacing. IIRC too long of a focal length means the mirrors are too close together, and vice versa.

 

i dont have an RC8 so i can't make that measurement for you, but the plate scale should tell you everything you need to know...

 

rob

 

 

Another way to do this is use the 12dstring FOV calculator to get your arc second resolution and then upload to astrobin for comparison.

[Jon Rista]

Well, I haven't fully dialed in collimation yet. I followed the procedure listed to center the secondary (I could not shift it around to center, there was less than a millimeter of play laterally, so I only adjusted the tilt), and left it. I don't think that is going to work for me, as there is clearly some off-axis astigmatism now that I've got the primary tweaked. After resetting the primary and secondary, there was a small but definite amount of on-axis coma, however the stars at the periphery looked fairly consistent. Since fixing the on-axis coma, the off-axis stars now have issues...I don't know of any way to fix that other than to adjust the secondary, which will require subsequent adjustment to the primary... 
 
Guess a reset didn't quite work for me? 
 
Anyway, there has been an improvement in my stars:
 
M46_420s_200iso__22c_20160228_21h21m42s568ms_L_mosaic.jpg
 
Before and after GIF:
 
http://i.imgur.com/Ve2Ks5w.gif
 
I just hope I can maintain this improvement when I fix the off-axis stars...

 
How much of the improvement do you think has to do with seeing in the M46 image.  The star shape looks a bit better but the improvement also looks like it could be seeing related.

Seeing was actually worse tonight. My guide RMS was 0.65-0.7" RMS last night, and it was 0.85-0.95" RMS tonight. Not a huge difference, but the stars were jumping around a bit more tonight than last night (at least, before the wind picked up last night...after that, my RMS was 1.3" last night.)

I am actually suspecting something else now, though. I forgot that I'd taken off my IDAS filter to do the collimation. I am now wondering if the filter has been affecting things. I don't think it messes with backfocus, as before my collimation gave me round stars to the corners of my full frame 5D III...however maybe it is blurring things more than I suspected? The filter is at the front of my imaging train, before the flattener, OAG, and camera...

I am not sure what to think about using the filter. I measured my subs, and the subs WITH the filter were actually noisier than those without. I bumped my ISO all the way down to 200 when testing tonight, and I only went down to 400 last night. The ISO 200 subs were a bit cleaner than the ISO 400 subs, even though they were the same exposure length. I may just try imaging without a filter and see how things go. My skies are far from good lately...around 18.5mag/sq", but there is ALWAYS a bit of thin high cloud cover which mucks up the works. It's just crappy skies, and the filter doesn't seem to be doing much other than costing me resolution and SNR. The 18e- worth of read noise doesn't seem to matter at all with my skyfog levels as they are.

Edited by Jon Rista, 29 February 2016 - 02:25 AM.

[Jon Rista]

Hi Jon,
I,m curious about something that doesn't get mentioned very much when collocating one of these scopes.  What is the measurement from the rear cell to the sensor when in focus?
This, I think sets the FL of the optics. I might be wrong.
I have the 6" and I just bought an 8" RC and the 8" states , " The image plane is located 10” behind the rear cell". The 6" states that, “the infinity focus of the AT6RC falls 9-3/4" outside the back of the telescope".
The 8" that was just delivered measured 10 -1/4" from the rear cell to the sensor when in focus. I haven't had a chance to get it outside as yet to test it.
 Just as a reference the 6" measurement was at 9-1/4 with bad stars in all the corners. I have since reset that measurement to the 9-3/4" as stated in the literature. But again I haven't had a chance to get it out to test it.
 
I'm wondering if a few people could take a measurement from the rear cell to the sensor and post there results here? Either in millimeters or inches.
 
Maybe this should be a different thread?
 
Thanks in advance.

 
this is true - the mirror spacing sets where the focal plane is, which also sets the focal length of the telescope. it varies from design to design but generally speaking the distance to the focal plane changes in a ratio of about 10:1 with the mirror spacing. so if the mirrors change by 2mm, the backfocus changes by 20mm.
 
if the telescope is mostly working properly, you can take and image and then solve it for the plate scale. this should tell you the current focal length of the telescope and whether or not you need to move the mirror spacing. IIRC too long of a focal length means the mirrors are too close together, and vice versa.
 
i dont have an RC8 so i can't make that measurement for you, but the plate scale should tell you everything you need to know...
 
rob

How much does that matte in practice? Wouldn't you be able to absorb most of that with the focuser?

I use the TSRCFlat2, which specifies 109mm backfocus from the flattener flange. How much would a shift in backfocus affect that? Previously, my focal length was either 1627mm or 1629mm, depending on which image I plate solved (which makes me wonder about how accurate the image scale computations from plate solved images are...two frames measured differently). That would have left me with either a 20mm or 40mm shift in backfocus...yet my stars were round corner to corner before.