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Adjusting focal length of 16 inch Ritchey Chretien scope?

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#1 Tom Gwilym

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Posted 08 June 2020 - 06:29 PM

Ok, I know I had this in another thread, but just wanted to keep topics separate and easily searchable if I get some good advice!

I've been fighting for -months- with a 16 inch RC trying to get it collimated.  I'm closer using a Takahashi scope now - long story, so back on the topic.


When platesolving an image using Astrometery.net, I get a focal length of 3297mm

The scope is advertised as being 3250mm

Do I stress over the 47mm, or do I try to move the secondary inward a bit to shorten the FL?  I've been reading a lot about trying to collimate these things, and do see that FL seems to be critical also.

Any tips on doing this?  I assume carefully loosen the "do not touch" middle screw, center the secondary again with the 3 screws, take a shot, test the plate solving, repeat until I'm closer.


Does this sound reasonable, or do I just leave it and keep chasing the mirrors and alignment of the focuser?

100606998_10221788634121709_560226947162
 

I feel VERY close, but still getting double stars.  This was an exposure of just a few seconds at about 120 gain with a ASI071MC Pro direct on the scope.

Thanks,

Tom



#2 Terry White

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Posted 09 June 2020 - 06:14 AM

I assuming that you're using a GSO store-branded RC here. I have an AT-16RCT myself. My understanding of the RC design is that it is only optimized for one design focal length, so yes, you should stress over the 47 mm difference you're measuring. That's a lot. Here's my take on the focal length. If it wasn't important, then GSO wouldn't be quoting the focal length to 4 significant figures. By default, you have to assume that GSO designed these RCs to an F/8 with the specified focal length to within ± 1-2 mm. Ultimately, the accuracy of your plate solving will limit how accurate you can set your focal length. There may be RCs shipped that are out of spec. on the focal length, but you will find out pretty quickly if yours is one of those.

 

I have read about people adjusting the secondary to change the focal length, with mixed results. I think you would be better off using the three primary mirror adjustment screws in tandem to change your focal length. If you try and adjust the secondary center screw, you run the risk of perturbing the secondary mirror factory centering. The OTA mechanical structure is made using numerical-controlled machining, so the tolerances should be ± .02 -.03 mm., but the screws are just standard tolerance stainless steel metric screws and not precision metric screws. Again, by default, you have to assume that GSO uses the mechanical tolerances of the OTA to locate the secondary near the optical axis of the primary, as there is no way to adjust that in the OTA mechanical design. There is always some small amount of lateral play in the center screw, so if you loosen it there's no guarantee that you will tighten it back in the same position. BTW, the three equally-spaced secondary screws mostly adjust tilt, not centering (offset).

 

The DSI Method seems to be the consensus cost-effective method to collimate an RC, short of an expensive laser interferometer collimator. The DSI Method is mute on adjusting the focal length, perhaps because the DSI scopes used a movable secondary to focus the image, which is different than the GSO RCs. According to the DSI Method, the secondary tilt adjustments are used to adjust off-axis astigmatism and to correctly balance the image at the corners, while the primary tilt adjustments are used to correct for on-axis coma. The DSI method goes into excruciating detail on why you shouldn't use collimation lasers or a Takahashi collimation scope. It's because the secondary optical axis and the secondary geometric axis (which these devices collimate to) aren't necessarily the same. You don't have this problem with SCTs because of their spherical secondary. I suggest you get Timo I 's advice as well, as he has a lot of experience collimating these RCs. He recently posted here https://www.cloudyni...t +another +gso


Edited by Terry White, 09 June 2020 - 08:51 AM.

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#3 Tom Gwilym

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Posted 09 June 2020 - 11:13 AM

Thanks for taking the time to write all that.   Yes, this scope is the 16 inch GSO (the big truss tube thing).  Good to know that the secondary center screw shouldn't be touched.  So from what I understand, I should just loosen the bolts on the primary and use the "push" screws to move it forward a bit to shorten the FL, then realign things?

If I have a 47mm difference, do I have to shorten it that much?  Someone else mentioned it would be 4.7mm change so figured maybe there is some multiplier in there somewhere with all the light bouncing back and forth.

I did give the DSI method a try, but will have to revisit it since when I tried it I was chasing an uneven star and messed it up more. I read a lot of good things about that method, so I'll have to give that another try.

 

I've been using this method to find the actual FL of the scope:

  • Using the Full Sky Plate Solver program and the Settings Assistant.
  • Find the Pixel size from the FITS header
  • Upload and get the Pixel Scale from Astrometry.net
  • That then gives me the FL of 3297mm

Is this a good way to find that or is there a different method that works better?

 

 

I did get some better results in my testing and small secondary adjustments last night using the Takahashi (only the 3 screws, I'm still not messing with FL yet).  Stars looking better again.  Some elongation since I wasn't guiding and may need to work on the mount a little (Paramount ME) since I was getting some light drift between frames.  Camera is an ASI071MC Pro with no focal reducer or anything attached.  I do know there is probably some oversampling but that's something I'm not worried about just chasing round stars and good detail in galaxies.

102876042_3129609370486925_6479550469210

 

 

I assuming that you're using a GSO store-branded RC here. I have an AT-16RCT myself. My understanding of the RC design is that it is only optimized for one design focal length, so yes, you should stress over the 47 mm difference you're measuring. That's a lot. Here's my take on the focal length. If it wasn't important, then GSO wouldn't be quoting the focal length to 4 significant figures. By default, you have to assume that GSO designed these RCs to an F/8 with the specified focal length to within ± 1-2 mm. Ultimately, the accuracy of your plate solving will limit how accurate you can set your focal length. There may be RCs shipped that are out of spec. on the focal length, but you will find out pretty quickly if yours is one of those.

 

I have read about people adjusting the secondary to change the focal length, with mixed results. I think you would be better off using the three primary mirror adjustment screws in tandem to change your focal length. If you try and adjust the secondary center screw, you run the risk of perturbing the secondary mirror factory centering. The OTA mechanical structure is made using numerical-controlled machining, so the tolerances should be ± .02 -.03 mm., but the screws are just standard tolerance stainless steel metric screws and not precision metric screws. Again, by default, you have to assume that GSO uses the mechanical tolerances of the OTA to locate the secondary near the optical axis of the primary, as there is no way to adjust that in the OTA mechanical design. There is always some small amount of lateral play in the center screw, so if you loosen it there's no guarantee that you will tighten it back in the same position. BTW, the three equally-spaced secondary screws mostly adjust tilt, not centering (offset).

 

The DSI Method seems to be the consensus cost-effective method to collimate an RC, short of an expensive laser interferometer collimator. The DSI Method is mute on adjusting the focal length, perhaps because the DSI scopes used a movable secondary to focus the image, which is different than the GSO RCs. According to the DSI Method, the secondary tilt adjustments are used to adjust off-axis astigmatism and to correctly balance the image at the corners, while the primary tilt adjustments are used to correct for on-axis coma. The DSI method goes into excruciating detail on why you shouldn't use collimation lasers or a Takahashi collimation scope. It's because the secondary optical axis and the secondary geometric axis (which these devices collimate to) aren't necessarily the same. You don't have this problem with SCTs because of their spherical secondary. I suggest you get Timo I 's advice as well, as he has a lot of experience collimating these RCs. He recently posted here https://www.cloudyni...t +another +gso

 



#4 Terry White

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Posted 09 June 2020 - 01:14 PM

Do I stress over the 47mm, or do I try to move the secondary inward a bit to shorten the FL? 

Tom, according to equation 76 at https://www.telescop.../two-mirror.htm, if the final focal length needs to be decreased, then you need to increase the mirror spacing, not decrease it.

 

f = f1 f2/(f1-f2-s)

 

where f is the final focal length (negative), f1 is the primary focal length (negative), f2 is the secondary focal length (negative), and s is the mirror spacing (negative). The sign convention is that distances measured to the left are negative. If you plug in typical numbers for an F8, 400 mm aplanatic cassegrain (RC) found here https://www.telescop...et/appendix.htm and keep track of the sign changes, you'll find out that you need to increase the mirror spacing.  (I'm assuming that from the table, R1 ~ f1 and R2  ~ f2)

 

RC Focal Length.png


Edited by Terry White, 09 June 2020 - 01:28 PM.


#5 Tom Gwilym

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Posted 09 June 2020 - 04:14 PM

Thanks.  I had to stare at that a bit, but see now that the negative numbers are left of the secondary in the diagram.

There sure is a lot to consider with these RC things.   I'll have to read your comments a few times and fully understand it.

 

Tom, according to equation 76 at https://www.telescop.../two-mirror.htm, if the final focal length needs to be decreased, then you need to increase the mirror spacing, not decrease it.

 

f = f1 f2/(f1-f2-s)

 

where f is the final focal length (negative), f1 is the primary focal length (negative), f2 is the secondary focal length (negative), and s is the mirror spacing (negative). The sign convention is that distances measured to the left are negative. If you plug in typical numbers for an F8, 400 mm aplanatic cassegrain (RC) found here https://www.telescop...et/appendix.htm and keep track of the sign changes, you'll find out that you need to increase the mirror spacing.  (I'm assuming that from the table, R1 ~ f1 and R2  ~ f2)

 

attachicon.gifRC Focal Length.png

 



#6 Terry White

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Posted 09 June 2020 - 04:59 PM

Yes, there's a lot to consider.foreheadslap.gif One of my fans stopped working and the only way to fix it is to take the backplane off. Brain surgerybawling.gifWhile it's apart I also plan to replace the heavy steel light shroud with a much lighter shroud made from 0.012" Mylar sheet. Another possibility is to remove the shroud completely and have an accessible primary that will facilitate cleaning. Stray light is pretty low in my dome.

PSX_20200609_181940.jpg


Edited by Terry White, 09 June 2020 - 05:27 PM.


#7 Terry White

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Posted 09 June 2020 - 05:49 PM

So from what I understand, I should just loosen the bolts on the primary and use the "push" screws to move it forward a bit to shorten the FL, then realign things?

Yes. The secondary is convex so it will amplify any adjustments you make on the spacing.



#8 Terry White

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Posted 09 June 2020 - 05:59 PM

I've been using this method to find the actual FL of the scope:

  • Using the Full Sky Plate Solver program and the Settings Assistant.
  • Find the Pixel size from the FITS header
  • Upload and get the Pixel Scale from Astrometry.net
  • That then gives me the FL of 3297mm

Is this a good way to find that or is there a different method that works better?

I'm no expert on plate solving, but in general, pick a target around zenith with some bright stars on a night with decent seeing and any of the solvers should be good to go. BTW, it looks like your latest adjustments did improve the star shapes. I see some astigmatism on the left side but the right side looks a little better. A little camera tilt maybe? Overall the stars look a little soft.


Edited by Terry White, 09 June 2020 - 06:04 PM.


#9 Tom Gwilym

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Posted 09 June 2020 - 10:32 PM

Thanks again for the comments.  Nice looking setup!  I like how you have the flat field screen up there too. That's on my long list of things to work out.   For my own personal observatory (Nexdome with Stellarvue SV102, Raspberry Pi 4 control EQ6R-Pro) I use an Amazon LED light tracing board which works very nicely. 

How critical are the fans?  We pretty much killed the batteries first night and I haven't used them. The ambient temperature is pretty close inside the dome, but I need to rig up some AC to DC power for those fans and get them running again. 

There could be some tilt of the camera, I have tried to level that out for now, but will get back to that also.

 

What method do you use to find your actual focal length?   Glad to hear that I can probably just tweak the primary, that's a lot less scary than the "forbidden" screw of the secondary!

 

As for the stars, yeah they are a little soft.  I had issues getting a good focus since the autofocuser chases around and stops in a weird place, then I manually adjust some more.  Probably not the 100% ideal camera for the scope since it's not a super huge sensor on the ASI071 camera. 

 

 

I'm no expert on plate solving, but in general, pick a target around zenith with some bright stars on a night with decent seeing and any of the solvers should be good to go. BTW, it looks like your latest adjustments did improve the star shapes. I see some astigmatism on the left side but the right side looks a little better. A little camera tilt maybe? Overall the stars look a little soft.

 



#10 Tom Gwilym

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Posted 14 June 2020 - 11:48 AM

Latest update.  Maybe I'm closer? 

I worked on the scope again last night working on another try at the DSI method.  Again, the DSI  method seems to only encourage me to totally mess up the primary mirror. 

I have the camera mounted on the scope in the orientation which gives me normal steering (left, right, up, down) on the screen when using the arrow buttons.  When I use the DSI method, I put my hand in front of the mirror until I see that I'm blocking the bright part that I want to tighten. I go straight back to the nearest screw and try tightening it a little - then recenter the star - repeat - try losening the screw on the darker side of the star.  But I keep going and nothing works.  I then put the Takahashi back on and see that everything is a horrible mess again. 

 

This is after messing with it about an hour - really fouled up.

104303070_10221932369675008_739065733351

 

Then I put the Takahashi on and recenter everything as much as possible.  It's better, but still not there. The ring nebula is the large blob, so ignore that.

83043676_10221932645841912_5634263925182

 

Another closer view of the stars.  I'm SO CLOSE but not confident with the DSI method.  Do I need to change my camera orientation to have the up/down left/right oriented with the scope?  Am I missing something by putting my hand in front to fine the bright area to tighten?

104371049_10221932680602781_605434720906

 

I did line it all up as best as I could visually with the Takahashi, is it possible that the secondary is nicely centered but the support vanes off a tiny bit and still work?

Maybe this is the -year- that I get this thing working!? 

 

Tom



#11 Terry White

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Posted 14 June 2020 - 12:39 PM

Tom, before you revisit the DSI Method, I really recommend that you plate solve for your focal length again and get it close to 3250 mm before you jump into the DSI Method. The whole idea behind the DSI Method is that it already assumes you have the correct focal length, which you certainly do not. There's no telling what the DSI Method will yield if the RC is far off from the design focal length and you start twiddling with the collimation. Last time you said it was 3297 mm. What is it now?


Edited by Terry White, 14 June 2020 - 05:15 PM.


#12 Tom Gwilym

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Posted 14 June 2020 - 01:14 PM

Probably the same, unless messing with the primary last night changed it.   I'll have to check that again.  Good idea.

Is the method of using the All Sky Plate Solving uploading to Astrometry for the pixel info and letting it calculate the focal length a good enough method?

I can't even get that software to recognize the images, even with all 9 gigs of data installed.

 

Tom, before you revisit the DSI Method, I really recommend that you plate solve for your focal length again. Last time you said it was 3297 mm. What is it now?

 



#13 Terry White

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Posted 14 June 2020 - 05:16 PM

I'm using Pinpoint, http://pinpoint.dc3.com/ and it starts with astrometry.net but goes deeper to calculate distortions. It's free for 60 days. There are many tips on plate solving online, I was able to find this in just a few seconds. http://forum.mainseq...et-solves/545/6. I added some more reservations about skipping the focal length verification and jumping to the DSI method in my last post as well.


Edited by Terry White, 14 June 2020 - 08:51 PM.


#14 Darth_Takahashi

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Posted 15 June 2020 - 03:56 AM

I think the main point here is to be methodical. Don't change too much at once. Concentrate on one correction only. Then move forward to the next one.

 

I agree with Terry above. As I siad in the other thread your stars could be soft because of spherical aberration due to the focal length not being quite right but I'm not standing next to the telescope, you are.

 

The tools such as laser collimators and the Takahashi scope in my humble option only get you close you still need to refine that with a real star or artificial one when you have access to one!

 

You could also measure everything and then you know where you were and where the adjustments are taking you. This is what I did so that I could alway return to the begining when I received the telescope as new.

 

 

 

Finally, I would still collimate an on axis star and get that perfect before doing the DSI balanced image method. This should allow you to get a balanced image with some slight detuning off the on axis image. At least thats the way I understand it.

 

Good luck & Regards

 

 

Neil

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#15 Terry White

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Posted 15 June 2020 - 06:04 AM

Tom, as Neil said, it's really important to keep track of all the adjustments that were made during each collimation step by keeping a log. I should have mentioned this earlier, but it is discussed in the DSI guide. The angular position of each tilt screw needs to be recorded before and after adjustment, otherwise you can never get back to your RC's "As-Received" factory adjustments in case you encounter difficulty during the collimation process. I find that a short-arm metric hex key set https://www.homedepo...10PCN/204153257 are the best tools, because it's hard to over-torque the screws with the short-arm set. Neil showed you another great tool, the digital caliper, which is very handy for measuring the primary travel. Mitutoyo makes the best. As mentioned in post #4, you need to increase the mirror spacing by moving the primary toward the back plate. To increase the mirror spacing you only need to be marking the pull screw positions. On the first primary tilt screw pair, loosen the small set screw (sometimes called grub screw or push screw) and turn the larger socket head cap screw (pull screw) clockwise. Start out conservative by a making small, 1/4-turn clockwise adjustment and lock it down with the push screw. Repeat this process for the other two tilt screw pairs and then plate-solve until you get a feel for the sensitivity of the adjustments on the focal length. Once you get comfortable you can always take bigger steps. You can also use the calipers to measure the primary position relative to the back plate and record it. Be careful not to over-torque the screws, as the stainless steel screws are much harder than the aluminum plates they are touching, so over-torquing may deform the aluminum plate. If you adjust the focal length to be too short, then you will be reversing the process by moving the primary in using the push screws (marking the rotation as you go) and locking down the adjustment with the pull screws. This is not the best drawing of the mirror cell https://www.cloudyni...eering-drawing/, but it does show that the three pull screws pass through the back plate and are threaded into the mirror cell support frame (and are also loaded with three compression springs). The three push screws are threaded into the back plate and push on the back of the mirror cell support frame.


Edited by Terry White, 15 June 2020 - 12:32 PM.


#16 Terry White

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Posted 15 June 2020 - 12:23 PM

How critical are the fans?  We pretty much killed the batteries first night and I haven't used them. The ambient temperature is pretty close inside the dome, but I need to rig up some AC to DC power for those fans and get them running again. 

The fans help in two ways. First, they speed up the thermalization of the mirror with the ambient temperature by pushing air through the back and around the edge of the mirror. Forced convection always has a higher surface heat transfer coefficient that free convection. Second, the fans help to break up the laminar boundary area in front of the mirror that leads to poor seeing. The third figure shows how this effect can lead to poor seeing https://skyandtelesc...ing-the-seeing/. Without fans, even if the mirror starts out in thermal equilibrium with the ambient air, as the night progresses, radiative cooling of the air proceeds at a more rapid pace than the mirror because the mirror has a higher thermal inertia. The mirror temperature will not drop as fast as the air temperature, leading to the formation of a boundary layer on the mirror surface. The fans forced convection cooling on the mirror helps to increase the rate of mirror cooling over what it would be if no fans were used (free convection), as well as to help break up any boundary layer that may form. It's just entry-level college thermodynamics. I read your previous collimation thread today and it looks like you have been battling collimation problems for some time. My sympathies.frown.gif


Edited by Terry White, 15 June 2020 - 12:40 PM.


#17 Tom Gwilym

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Posted 15 June 2020 - 01:31 PM

Other than the collimation issues, wouldn't the stars end up slightly soft since it's got a very long focal length and the camera is an ASI071MC-Pro?  I did understand that the smaller pixels will give some oversampling which would possibly soften them up some too (I could be thinking wrong - won't be a surprise!).

Good idea measuring with the micrometer, that would give an easy indication of movement.  

I worked on the DSI thing again last night and got the camera properly aligned and made -very- small tweaks.  I think I'm starting to understand that a little better now.

I did some test images and the stars looked better, but still have some tweaks to do.  

M20, 2 minutes, unguided - getting closer.  

104116606_3145186495595879_2417203373375

 

I think the main point here is to be methodical. Don't change too much at once. Concentrate on one correction only. Then move forward to the next one.

 

I agree with Terry above. As I siad in the other thread your stars could be soft because of spherical aberration due to the focal length not being quite right but I'm not standing next to the telescope, you are.

 

The tools such as laser collimators and the Takahashi scope in my humble option only get you close you still need to refine that with a real star or artificial one when you have access to one!

 

You could also measure everything and then you know where you were and where the adjustments are taking you. This is what I did so that I could alway return to the begining when I received the telescope as new.

 

 

 

Finally, I would still collimate an on axis star and get that perfect before doing the DSI balanced image method. This should allow you to get a balanced image with some slight detuning off the on axis image. At least thats the way I understand it.

 

Good luck & Regards

 

 

Neil

 


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#18 Tom Gwilym

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Posted 15 June 2020 - 01:45 PM

I'll have to work on powering those up.   Right now there is just a container for storing dead batteries hung off the scope. I know the Paramount ME has some power output on the mount that I could probably use for that.  

Yeah, I've been working on this thing for a long time.  The club bought it over a year ago, and originally had a Mallincam with it.   I tried it out and wasn't pleased with how that looked.  Very low contrast and all that.  Then we bought the ASI071MC Pro and an off-axis guider (still need to get that working but need good stars first) which I've been messing with now with better results - at least the distorted stars show up well!   Now with the pandemic, the observatory (and we have a nice Digitalis planetarium) are basically sitting idle sine we can't do any public events.  Not looking for research quality photos, but just decent enough (to meet my standards) for public outreach and other possible educational ideas that are dormant right now. 

 

The fans help in two ways. First, they speed up the thermalization of the mirror with the ambient temperature by pushing air through the back and around the edge of the mirror. Forced convection always has a higher surface heat transfer coefficient that free convection. Second, the fans help to break up the laminar boundary area in front of the mirror that leads to poor seeing. The third figure shows how this effect can lead to poor seeing https://skyandtelesc...ing-the-seeing/. Without fans, even if the mirror starts out in thermal equilibrium with the ambient air, as the night progresses, radiative cooling of the air proceeds at a more rapid pace than the mirror because the mirror has a higher thermal inertia. The mirror temperature will not drop as fast as the air temperature, leading to the formation of a boundary layer on the mirror surface. The fans forced convection cooling on the mirror helps to increase the rate of mirror cooling over what it would be if no fans were used (free convection), as well as to help break up any boundary layer that may form. It's just entry-level college thermodynamics. I read your previous collimation thread today and it looks like you have been battling collimation problems for some time. My sympathies.frown.gif

 



#19 Tom Gwilym

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Posted 15 June 2020 - 01:48 PM

Thanks for those tips.  Looks like it's not too extremely traumatic or dangerous to change the FL of the scope.  I probably have it pretty nicely messed up now that the factory settings are long gone.  I may work on stars a bit longer then try to lengthen the FL.  I'm feeling that I'm slowly learning more about this all the time, and the other night's mess was fairly quick to recover using the Takahashi to get me back to a fairly good starting point to mess a little more with the DSI method.

 

Tom, as Neil said, it's really important to keep track of all the adjustments that were made during each collimation step by keeping a log. I should have mentioned this earlier, but it is discussed in the DSI guide. The angular position of each tilt screw needs to be recorded before and after adjustment, otherwise you can never get back to your RC's "As-Received" factory adjustments in case you encounter difficulty during the collimation process. I find that a short-arm metric hex key set https://www.homedepo...10PCN/204153257 are the best tools, because it's hard to over-torque the screws with the short-arm set. Neil showed you another great tool, the digital caliper, which is very handy for measuring the primary travel. Mitutoyo makes the best. As mentioned in post #4, you need to increase the mirror spacing by moving the primary toward the back plate. To increase the mirror spacing you only need to be marking the pull screw positions. On the first primary tilt screw pair, loosen the small set screw (sometimes called grub screw or push screw) and turn the larger socket head cap screw (pull screw) clockwise. Start out conservative by a making small, 1/4-turn clockwise adjustment and lock it down with the push screw. Repeat this process for the other two tilt screw pairs and then plate-solve until you get a feel for the sensitivity of the adjustments on the focal length. Once you get comfortable you can always take bigger steps. You can also use the calipers to measure the primary position relative to the back plate and record it. Be careful not to over-torque the screws, as the stainless steel screws are much harder than the aluminum plates they are touching, so over-torquing may deform the aluminum plate. If you adjust the focal length to be too short, then you will be reversing the process by moving the primary in using the push screws (marking the rotation as you go) and locking down the adjustment with the pull screws. This is not the best drawing of the mirror cell https://www.cloudyni...eering-drawing/, but it does show that the three pull screws pass through the back plate and are threaded into the mirror cell support frame (and are also loaded with three compression springs). The three push screws are threaded into the back plate and push on the back of the mirror cell support frame.

 



#20 akulapanam

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Posted 23 June 2020 - 10:11 PM

Just a couple comments:

-My understanding is GSO sets the best focal length for a particular mirror set which varies slightly
-On a 10” RC you can move your secondary 8mm or 80mm of backfocus and still be 1/4th wave / diffraction limited. See http://interferomete...en-gso.html?m=1
-I’m not positive that there is one right setting here either. I’m looking into this myself out of curiosity. Certainly a lot of resources reference that fact but most high end manufacturers also offer secondary focusing on RCs
-DSI method, sure on a high end scope. GSO has great optics and horrible mechanics. I certainly played around with the DSI method my GSO and concluded it was a non-starter. You looked a long ways off on the 14th so I would focus on fundamental methods - laser + Tak scope first . DSI works best for fine tuning
-What focuser are you using? Honestly for anything but the lightest camera you need a Moonlite or better yet Feathertouch or Optec or FLI
-Remember there is not necessarily just one combo of primary, secondary, focuser that will achieve good combination. I saw this in several high end manuals but didn’t really understand that. Once I did, it really helped my skills


Edited by akulapanam, 24 June 2020 - 12:04 AM.

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#21 rgsalinger

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Posted 23 June 2020 - 10:26 PM

I use a 23" F8 RC scope. We focus using the secondary. Hence, as we focus we change the focal length we move a few hundred microns between the coldest nights and the warmest nights. Yet stars are perfect (we have an engineer who knows how to collimate and RC) across the entire 36x36 chip. It's really hard to imagine that every single mirror set on a GSO scope comes with an identical optimal focal length. I think I'd focus on getting it collimated and worry about every other aspect until much later in the optimization process.

 

Now, I'm biased about this as I gave up on the GSO scope that I owned - I just could not get it correct - and paid up for the CDK that I now own. 12.5" is a much scope as is useable in most locations. The 12.5 gets the same results (FWHM) as the 23" does 75 feet away from each other. When I read about these struggles with RC's I really wonder how much savings there is over just biting the bullet and getting a PlaneWave or AG Optical in the first place. 

 

Apologies for hijacking - I just read this and feel so sorry for the OP. Still, maybe he's having fun with all this! YMMV as they say. 

 

Rgrds-Ross


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#22 Tom Gwilym

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Posted 24 June 2020 - 03:25 PM

So are the scope's FL tuned at the factory depending on how it's made?   If that is the case, is the 3297mm possibly 'normal' from what I calculated using All Sky Plate Solver / Astrometry.net?

I think I had closer to better results using the DSI method, but that came to a temporary halt when I accidentally "synced" a star and found that the scope can't find stars again.  (See my other thread about SkyX frustration).

Just tossing more facts out....the Glatter laser we have has the grid pattern attachment.  Seems that since Howie died, it's been harder to find the ring pattern. 

 

 

Just a couple comments:

-My understanding is GSO sets the best focal length for a particular mirror set which varies slightly
-On a 10” RC you can move your secondary 8mm or 80mm of backfocus and still be 1/4th wave / diffraction limited. See http://interferomete...en-gso.html?m=1
-I’m not positive that there is one right setting here either. I’m looking into this myself out of curiosity. Certainly a lot of resources reference that fact but most high end manufacturers also offer secondary focusing on RCs
-DSI method, sure on a high end scope. GSO has great optics and horrible mechanics. I certainly played around with the DSI method my GSO and concluded it was a non-starter. You looked a long ways off on the 14th so I would focus on fundamental methods - laser + Tak scope first . DSI works best for fine tuning
-What focuser are you using? Honestly for anything but the lightest camera you need a Moonlite or better yet Feathertouch or Optec or FLI
-Remember there is not necessarily just one combo of primary, secondary, focuser that will achieve good combination. I saw this in several high end manuals but didn’t really understand that. Once I did, it really helped my skills

 



#23 akulapanam

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Posted 24 June 2020 - 11:50 PM

So are the scope's FL tuned at the factory depending on how it's made?   If that is the case, is the 3297mm possibly 'normal' from what I calculated using All Sky Plate Solver / Astrometry.net?

I think I had closer to better results using the DSI method, but that came to a temporary halt when I accidentally "synced" a star and found that the scope can't find stars again.  (See my other thread about SkyX frustration).

Just tossing more facts out....the Glatter laser we have has the grid pattern attachment.  Seems that since Howie died, it's been harder to find the ring pattern. 

That is my understanding.  A current GSO dealer would know more.  I would suggest reaching out to either the host of this site or Teleskop Express.  Nick Altair (Altair Astro) knows a ton about this too because they were a reseller and then doing custom mechanical work on them.

 

This is the circle adapter.  Honestly though a Tak scope is just as good, especially if you reach in and unscrew the primary baffle.

http://starlightinst...&product_id=152

 

I think your experience will really improve if you buy a Starlight 3" focuser to replace the stock one and check to see if anything is moving (I usually check 5 key points).  Probably want to replace the focuser first because if anything is moving its probably the focuser.


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#24 Darth_Takahashi

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Posted 30 June 2020 - 07:48 AM

I use a 23" F8 RC scope. We focus using the secondary. Hence, as we focus we change the focal length we move a few hundred microns between the coldest nights and the warmest nights. Yet stars are perfect (we have an engineer who knows how to collimate and RC) across the entire 36x36 chip. It's really hard to imagine that every single mirror set on a GSO scope comes with an identical optimal focal length. I think I'd focus on getting it collimated and worry about every other aspect until much later in the optimization process.

 

Now, I'm biased about this as I gave up on the GSO scope that I owned - I just could not get it correct - and paid up for the CDK that I now own. 12.5" is a much scope as is useable in most locations. The 12.5 gets the same results (FWHM) as the 23" does 75 feet away from each other. When I read about these struggles with RC's I really wonder how much savings there is over just biting the bullet and getting a PlaneWave or AG Optical in the first place. 

 

Apologies for hijacking - I just read this and feel so sorry for the OP. Still, maybe he's having fun with all this! YMMV as they say. 

 

Rgrds-Ross

Ross,

 

These are not the same thing! I have a Mewlon CRS = CDK or Corrected Dall-Kirkham and you need lots of other optical components to achieve the same/similar results as an RC. Furthermore, you can read people complaining about collimating even these scope with a spherical secondary!!! I mean how easy does this need be?

 

RC are designed to be aplantic with only two mirrors and no other optical components. If you buy one of these with your eye's open then go for it. Being an engineer and always wanting one I took my opportunity. Does that mean I will sell the Mewlon, hell no! However, I do expect to get better results from the RC in the end.

 

Mine was delivered in great condition from new and the collimation was 90% when it arrived. I have it now at 95% and I'm happy with it. I'm not going to fight it for that extra 5%. That will now happen over time and many imaging seasons. Also, I have logged / measured everything, every adjustment.

 

Are these scope difficult to collimate, yes, but not impossible and the results in the end are worth it. I would only say that you need to adjust the telescope slowly and methodically. If you make too many adjustments too quickly you will lose track of what you changed and whether or not it had a positive/negative impact.

 

Anyone not wanting to collimate should buy a refractor as all reflectors need collimating. Company's selling magical collimation tool's for hundreds of Dollars are also part of the problem in my humble opinion. They're only part of the solution not the whole solution as some would like us to believe. A good set of eyes and a cheshire work wonders. Yes, I also use a laser but understand its purpose and limitations.

 

Regards

 

 

Neil
 


Edited by Darth_Takahashi, 30 June 2020 - 08:23 AM.

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#25 Darth_Takahashi

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Posted 30 June 2020 - 08:01 AM

Regarding the focuser put your laser collimator in against the backplate and tighten equally the two screws. Rack it in and out around your focus while monitoring the red dot on the secondary. When everything remains stable the focuser is OK. Otherwise, see if you can adjust/fix it before you replace/upgrade it?

 

I don't know how deep your pockets are?

 

One further point, I have the "Far Point Laser" I like it becuase its heavy, almost as heavy as my imaging train. If you have a light weight laser collimator put some additional stress on it with your finger while racking the focuser in and out to simulate your imaging train. Rotate and repeat until you're satified with the results.

 

I have the generation 3 version and I'm happy with my 3inch GSO "Omegon branded" focuser.

 

Regards

 

 

Neil




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