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RASA 8: Completely useless

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#1 Cerberus

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Posted 07 June 2020 - 04:02 AM

I've been beating my head against the wall for 3 months trying to get this god forsaken thing working. Have not gotten a SINGLE acceptable light frame. An entire season and $2000 completely wasted.

Camera swaps, collimation adjustment, attempts at fixing tilt, messing with focus shift/tilt of the mirror, buying various adapters to try and diagnose a bad part. There is no dedicated collimation tool available, and even more insulting the promised tilt adapter has not materialized. Only word from Celestron is "Pay to send it back and we will do a poor job collimating the scope for you".

Long story short I'm done. Do not buy this scope guys. Maybe some of them leave the factory fine but ANY TINY IMPERFECTION in your scope, ANY of the hardware in the imaging train or adapters, and the camera sensor itself will cause elongation or donut stars. The reduced exposure time is not worth the hassle in maintaining the system.


Edited by Cerberus, 07 June 2020 - 04:02 AM.

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#2 pyrasanth

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Posted 07 June 2020 - 04:14 AM

I understand your frustration. We should not have to put up with the poor mechanics offered by Celestron on most of the RASA range. The instruments demand very careful attention to component spacing and collimation adjustment which often is beyond the ability of most people to adjust without access to an optical test bench. Even my RASA 11" V1 was unable to be fixed by Celestron & they replaced it with a new unit. It is still in the returned shipping carton as I've not had time to retest it. I'm just tempted to sell the unit & cut my losses as I know the pain I might experience to get satisfactory images from it.

 

The RASA 14", I'm told, has none of the problems with the smaller instruments but it costs a lot more and everything has to scale significantly to mount that beast. I think Celestron tried to manufacture the RASA range on the cheap & put superb optics into poor mechanics. I have it on good authority that Celestron are only interested in the bottom line & the thought of spending money on good design does not enter into their philosophy- that's why we are still focusing the Cat products with a moving mirror.

 

If your unhappy I would reject the unit as not fit for purpose & move on.


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#3 luxo II

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Posted 07 June 2020 - 04:41 AM

They’re so mean that can’t be bothered insulating their OTAs, building in a heater strap behind the corrector or providing a sliding dewcap. As for fixing the wonky focusing no chance.

Imho it could probably be fixed if you’re brave enough to open the rear ended and install a strong spring pulling the back of the mirror sideways. The snag is doing so voids the warranty.

Edited by luxo II, 07 June 2020 - 04:46 AM.


#4 Benni123456

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Posted 07 June 2020 - 06:12 AM

I think these ventilation openings more problematic than the focuser...

 

 

Part of the appeal of the sct is that on a newton or rhichey chretien, dust builds up on the mirror and it often has to be cleaned and even recoated....

Opening an rc to clean it is a bit demanding...

 

So an sct, with a corrector plate that can be cleaned like a lens is quite appealing. 

 

When you open such a system to the elements, one has to think carefully what one does....

 

The filter mesh of celestron is pm50....

 

This is the measurement for particular matter with 10 mykrometers in the us:

 

https://www.epa.gov/...10-trends#pmnat

 

Depending on where one images, if there is much of that in the air, this stuff gets into the scope, together with moisture.. the fine dust creates condensation sources, and these are on the optical surfaces... Over time, this should clog your mirrors with an oily fog....

 

 

Any vacuum cleaner has hepa filters, which filter particles with a size of 0.3 mykrometers...

 

A rasa 14 is a device that is still handmade in california, i guess. So you may perhaps ask celestron to insert better filters with smaller mesh, or deliver it without any openings altogether..

 

Otherwise, you may buy a heavy rasa 14, and after a few years of use, depending on where you image, your mirrors may be clogged....


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#5 Ladyhawke

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Posted 07 June 2020 - 06:16 AM

That's why I've always preferred the Edge 11" with a Hyperstar. Never quite understood the appeal of a dedicated F/2 system when you can have the same thing as an add-on.


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#6 pyrasanth

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Posted 07 June 2020 - 06:31 AM

That's why I've always preferred the Edge 11" with a Hyperstar. Never quite understood the appeal of a dedicated F/2 system when you can have the same thing as an add-on.

I keep explaining this chestnut every-time it pops up to roast. The Hyperstar system is not a RASA & it never will be. It is a compromise and thus has limited field of view and its own set of unique problems. If the RASA worked as it was designed it would blow the Hyperstar out of the water but alas it does not due to its inherent mechanical issues.


Edited by pyrasanth, 07 June 2020 - 06:32 AM.

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#7 Benni123456

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Posted 07 June 2020 - 06:50 AM

That's why I've always preferred the Edge 11" with a Hyperstar. Never quite understood the appeal of a dedicated F/2 system when you can have the same thing as an add-on.

 

the hyperstar has 35mm diameter, the rasa 60. So if you are a professional from nasa who can order a custom made large format cmos, or if you just want to add a medium format camera from hasselbladt or pentax or leica then the rasa has some advantages... However, I think most imagers do not have that money....

Time will tell if large middle format sensors become cheaper sometime.....


Edited by Benni123456, 07 June 2020 - 06:51 AM.

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#8 SandyHouTex

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Posted 07 June 2020 - 10:53 AM

the hyperstar has 35mm diameter, the rasa 60. So if you are a professional from nasa who can order a custom made large format cmos, or if you just want to add a medium format camera from hasselbladt or pentax or leica then the rasa has some advantages... However, I think most imagers do not have that money....

Time will tell if large middle format sensors become cheaper sometime.....

You make an excellent point.



#9 Benni123456

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Posted 07 June 2020 - 11:48 AM

You make an excellent point.

One should note that this 60mm image circle just holds for the largest rasa...

 

The smaller ones are, I think similar systems as the hyperstar, with just a bit better focusing mechanism, so one can, in theory, do longer exposures, provided one has a perfect mount....

The claims of the optimizations for the infrared wavelength are, I think rather marketing speak... If one looks at the spot diagram of the white paper, one sees large deviations between ordinary red and blue

 

 

And if you are from nasa (i.e. if this is not just a picture on your avatar), you should tell celestron immediately to close their rasa and edge hd scopes and remove these large 50 mykrometer filters.

 

Fine particular matter is smaller than 5 mykrometers sometimes. Even pollen is smaller than 50 mu and that ventilator just blows all this stuff on the mirror...

 

But well, i guess nasa, which uses rasa telescopes, can simply reorder them if the mirrors are fogging up because of this micro dust that flies through the filter, sits on the mirror and works as condensation source....

 

However, it is simply a waste of material to buy a rasa or an edge every year or to clean and collimate it just because they installed these stupid filters.....

 

It depends of course, where these things are located ... What one observes is that this fine dust decreases at a few 1000 meters height... So if one images in mountains, it may almost be fine and not fog up...



#10 RichA

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Posted 07 June 2020 - 04:09 PM

I understand your frustration. We should not have to put up with the poor mechanics offered by Celestron on most of the RASA range. The instruments demand very careful attention to component spacing and collimation adjustment which often is beyond the ability of most people to adjust without access to an optical test bench. Even my RASA 11" V1 was unable to be fixed by Celestron & they replaced it with a new unit. It is still in the returned shipping carton as I've not had time to retest it. I'm just tempted to sell the unit & cut my losses as I know the pain I might experience to get satisfactory images from it.

 

The RASA 14", I'm told, has none of the problems with the smaller instruments but it costs a lot more and everything has to scale significantly to mount that beast. I think Celestron tried to manufacture the RASA range on the cheap & put superb optics into poor mechanics. I have it on good authority that Celestron are only interested in the bottom line & the thought of spending money on good design does not enter into their philosophy- that's why we are still focusing the Cat products with a moving mirror.

 

If your unhappy I would reject the unit as not fit for purpose & move on.

A moving mirror.  Like a Questar?



#11 pyrasanth

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Posted 07 June 2020 - 04:28 PM

A moving mirror.  Like a Questar?

I have no experience with the Questar, however Celestron SCT's & RASA's focus by moving the primary mirror which is not the best way to focus an instrument.


Edited by pyrasanth, 07 June 2020 - 04:30 PM.

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#12 Menardi

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Posted 07 June 2020 - 05:11 PM

I understand your frustration. We should not have to put up with the poor mechanics offered by Celestron on most of the RASA range. The instruments demand very careful attention to component spacing and collimation adjustment which often is beyond the ability of most people to adjust without access to an optical test bench. Even my RASA 11" V1 was unable to be fixed by Celestron & they replaced it with a new unit. It is still in the returned shipping carton as I've not had time to retest it. I'm just tempted to sell the unit & cut my losses as I know the pain I might experience to get satisfactory images from it.

 

The RASA 14", I'm told, has none of the problems with the smaller instruments but it costs a lot more and everything has to scale significantly to mount that beast. I think Celestron tried to manufacture the RASA range on the cheap & put superb optics into poor mechanics. I have it on good authority that Celestron are only interested in the bottom line & the thought of spending money on good design does not enter into their philosophy- that's why we are still focusing the Cat products with a moving mirror.

 

If your unhappy I would reject the unit as not fit for purpose & move on.

There is a person (very good experienced imager) on the 10Micron forums that has horror stories with the 36cm RASA.

 

He send back his second 36 cm RASA that didn't work because the mirror flop wasn't allowing 60 second images. He even used an OAG and couldn't get it to work.

 

You'd think 13K dollar telescope (17K in Europe, rip) would have better QC.


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#13 JohnnyLingo

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Posted 07 June 2020 - 06:26 PM

Sorry to hear your frustration! I have always been intrigued by the RASA as I see it as the commercial manifestation of the venerable Schmidt cameras that have been used for large sky surveys of the 20th century. Of course, an f/2 instrument requires a great deal of mechanical precision, and it's too bad that it can be an issue for the RASA.

 

I have two questions/remarks to the OP:

- Wouldn't the situation improve by installing the Celestron motor focuser? I read that even autofocusing becomes possible using it.

- Wouldn't it be possible to give the scope to a third-party professional for collimation and tuneup before writing the scope off? Here in Germany where I live, there are some renown shops who collimate everything and anything at a reasonable flat rate (around 150 bucks plus shipping) on an optical bench. (They can even make uncollimatable systems collimatable by properly modifying them.) Some of them actually ship all new telescopes with top-of-the-line QC and collimation for no additional cost. So I assume that probably there should be a possibility for such specialized service in the US as well. In case the professional reveals that your scope cannot be properly collimated due to a non-trivial manufacturing flaw, you'd have that fact proven and would be eligible for a replacement/refund from Celestron.



#14 Cerberus

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Posted 07 June 2020 - 06:32 PM

Sorry to hear your frustration! I have always been intrigued by the RASA as I see it as the commercial manifestation of the venerable Schmidt cameras that have been used for large sky surveys of the 20th century. Of course, an f/2 instrument requires a great deal of mechanical precision, and it's too bad that it can be an issue for the RASA.

 

I have two questions/remarks to the OP:

- Wouldn't the situation improve by installing the Celestron motor focuser? I read that even autofocusing becomes possible using it.

- Wouldn't it be possible to give the scope to a third-party professional for collimation and tuneup before writing the scope off? Here in Germany where I live, there are some renown shops who collimate everything and anything at a reasonable flat rate (around 150 bucks plus shipping) on an optical bench. (They can even make uncollimatable systems collimatable by properly modifying them.) Some of them actually ship all new telescopes with top-of-the-line QC and collimation for no additional cost. So I assume that probably there should be a possibility for such specialized service in the US as well. In case the professional reveals that your scope cannot be properly collimated due to a non-trivial manufacturing flaw, you'd have that fact proven and would be eligible for a replacement/refund from Celestron.

Yep, motor focuser already installed. Autofocus is close but I still do it manually with Bahtinov when attempting an actual light frame.
I'm one of the 2-3 guys who often help collimate scopes here in this area. I have worked from time to time at the local Celestron telescope dealer.

Collimation is not the only issue with this scopes mechanical design, mirror shift, collimation, tilt, camera tilt, sloppy adapter fit, and lack of access to proper hardware (tilt adapter, collimation system you can manipulate without removing camera).


Edited by Cerberus, 07 June 2020 - 06:35 PM.

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#15 BKBrown

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Posted 07 June 2020 - 06:41 PM

I've been beating my head against the wall for 3 months trying to get this god forsaken thing working. Have not gotten a SINGLE acceptable light frame. An entire season and $2000 completely wasted.

Camera swaps, collimation adjustment, attempts at fixing tilt, messing with focus shift/tilt of the mirror, buying various adapters to try and diagnose a bad part. There is no dedicated collimation tool available, and even more insulting the promised tilt adapter has not materialized. Only word from Celestron is "Pay to send it back and we will do a poor job collimating the scope for you".

Long story short I'm done. Do not buy this scope guys. Maybe some of them leave the factory fine but ANY TINY IMPERFECTION in your scope, ANY of the hardware in the imaging train or adapters, and the camera sensor itself will cause elongation or donut stars. The reduced exposure time is not worth the hassle in maintaining the system.

Really? There are an awful lot of excellent images from a number of people out there on the net... your experience is not close to universal.

 

Clear Skies,

Brian snoopy2.gif


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#16 Benni123456

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Posted 07 June 2020 - 06:42 PM

im sorry but a tilt adapter can be simply bought...

ypu do not need to remove the camera when you move the tilting screws..

 

collimation for this system means, i think, just centering the schmidt plate...

is this even necessary?



#17 Cerberus

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Posted 07 June 2020 - 07:09 PM

im sorry but a tilt adapter can be simply bought...

ypu do not need to remove the camera when you move the tilting screws..

 

collimation for this system means, i think, just centering the schmidt plate...

is this even necessary?

This is the RASA 8, there is no room for a conventional tilt plate. Even the little thin plastic index shims that come with cameras affect the focus significantly.

Yes you DO have to remove the camera to adjust collimation when using a cooled camera.



#18 freestar8n

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Posted 07 June 2020 - 07:12 PM

I have not used RASA but I have used C11, EdgeHD8 and EdgeHD11 and have had no problems with the focuser - at f/10, f/7 or f/2.

 

I use automatic focus based on multi-star parabolic fit - and I focus with a simple stepper on the primary focus knob

 

I make sure to pull back the mirror and take the focus curve moving the mirror upward against gravity during the focus run - and when I return to focus I do the same - with a good amount of backlash compensation and final move pushing up against gravity.

 

Unless I hear that people are doing the same thing - my guess is that problems they have are due to poor focusing practice and not loading the mirror against gravity.  If you load it against gravity the allignment will be repeatable and there will be less drift.

 

With both hyperstar and at f/10 I can focus about every 40 minutes - so focus isn't drifting rapidly.

 

If people are manually focusing RASA by twiddling the knob back and forth - that will likely result in focus and alignment drift.  Good automatic focus with the primary knob should make it much more reliable.

Frank


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#19 SandyHouTex

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Posted 07 June 2020 - 07:16 PM

So I just did a random search for the Celestron 8inch RASA, and this guy had no issues.  He also never used a Schmidt-Cass or RASA for imaging before, he used to use APO refractors. Here’s the review:

 

https://astrobackyar...-rasa-8-review/
 

He also mentions that the image circle is only 22mm.  He also says pixel size is important, and you can’t use a DSLR.  You need to use a dedicated round astro camera of 100mm or less.  He doesn’t even guide as such, but he loves it.  There are some example images that look really good.



#20 Cerberus

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Posted 07 June 2020 - 07:18 PM

So I just did a random search for the Celestron 8inch RASA, and this guy had no issues.  He also never used a Schmidt-Cass or RASA for imaging before, he used to use APO refractors. Here’s the review:

 

https://astrobackyar...-rasa-8-review/
 

He also mentions that the image circle is only 22mm.  He also says pixel size is important, and you can’t use a DSLR.  You need to use a dedicated round astro camera of 100mm or less.  He doesn’t even guide as such, but he loves it.  There are some example images that look really good.

Success stories do not negate the issues many of us here on the forums have been having.


Edited by Cerberus, 07 June 2020 - 07:54 PM.

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#21 Benni123456

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Posted 07 June 2020 - 07:54 PM

This is the RASA 8, there is no room for a conventional tilt plate. Even the little thin plastic index shims that come with cameras affect the focus significantly.

Yes you DO have to remove the camera to adjust collimation when using a cooled camera.

What?
I read here:

 

https://www.teleskop...cher-Tubus.html

 

Working distance from the T2 thread: 25 mm (T2 adapter is included in the scope of delivery)
Working distance from the connection at the front side (dovetail ring): 29 mm for self adaptions

 

My asi 1600 has 6.5mm.

 

A conventional zwo tilt adapter is 11 mm, so 17,5mm for the asi and a tilt adapter...

https://www.teleskop...ompensator.html

 

But yes, I can't install a filter drawer... they are all larger than 7,5mm... although...

 

the chinese claim to make a filter drawer with 5mm but I do not know if that is good...

 

https://www.aliexpre...2315866747.html

 

 

One problem could be the screws of the tilter... They would collide with the filter drawer, most likely....

But without the filter drawer it would work, although not with the screws that come with the zwo tilter..

 

On my tilting adapter, i also replaced them with standard hexagon screws... they barely stand out and will easily fit with 7 mm additional space.... One can access them from the side with a wrench...

 

So there would not be any need to remove the camera... 

A problem is more to make the tilt precise enough... but having worked with it, I suspect this may be possible and durable...

The question is what happens when you unmount the camera...Since the tilt means collimation only for a specific orientation... It is therefore likely that the imaging train needs to stay on the telescope and if removed everything needs to be tilted again. 

 

But I think this should go quite fast if the goal is just to remove donuts.. Half a turn with one screw or something.... and then its done...

It gets more difficult to catch the point of the optimal hfd at every edge. I suggest this would take hours....


Edited by Benni123456, 07 June 2020 - 08:15 PM.

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#22 SandyHouTex

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Posted 07 June 2020 - 08:12 PM

Here’s another successful review:

https://www.skyatnig...sa-8-telescope/
 

Maybe the issue is with what equipment you’re putting on it?  What camera are you using?
 

Just a suggestion.


Edited by SandyHouTex, 07 June 2020 - 08:15 PM.


#23 Cerberus

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Posted 08 June 2020 - 12:53 AM

What?
I read here:

 

https://www.teleskop...cher-Tubus.html

 

Working distance from the T2 thread: 25 mm (T2 adapter is included in the scope of delivery)
Working distance from the connection at the front side (dovetail ring): 29 mm for self adaptions

 

My asi 1600 has 6.5mm.

 

A conventional zwo tilt adapter is 11 mm, so 17,5mm for the asi and a tilt adapter...

https://www.teleskop...ompensator.html

 

But yes, I can't install a filter drawer... they are all larger than 7,5mm... although...

 

the chinese claim to make a filter drawer with 5mm but I do not know if that is good...

 

https://www.aliexpre...2315866747.html

 

 

One problem could be the screws of the tilter... They would collide with the filter drawer, most likely....

But without the filter drawer it would work, although not with the screws that come with the zwo tilter..

 

On my tilting adapter, i also replaced them with standard hexagon screws... they barely stand out and will easily fit with 7 mm additional space.... One can access them from the side with a wrench...

 

So there would not be any need to remove the camera... 

A problem is more to make the tilt precise enough... but having worked with it, I suspect this may be possible and durable...

The question is what happens when you unmount the camera...Since the tilt means collimation only for a specific orientation... It is therefore likely that the imaging train needs to stay on the telescope and if removed everything needs to be tilted again. 

 

But I think this should go quite fast if the goal is just to remove donuts.. Half a turn with one screw or something.... and then its done...

It gets more difficult to catch the point of the optimal hfd at every edge. I suggest this would take hours....

That's the thing, filter drawer is not optional for me it has to be there. I'm using the Starizona one because it seems stronger than the Baader UFC. I have the correct spacing and distance.

 

Here’s another successful review:

https://www.skyatnig...sa-8-telescope/
 

Maybe the issue is with what equipment you’re putting on it?  What camera are you using?
 

Just a suggestion.

I have tried ZWO 1600MM-Pro, 294MC-Pro, 294MC, 120MM-S, 224MC for collimation, tilt adjustment, etc. With and without filter drawer, with and without 3d printed spacer, with original spacer and without, I swear I have tried at least 100 combinations and adjustments several times.
 


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#24 Benni123456

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Posted 08 June 2020 - 04:47 AM

I see, yes it is more for one shot color cameras... this is true...

 

 

But the 294 mc that you have used is a color camera with 6.5mm backfocus. Adding a tilter should not be a problem....

Any eggs that can not be removed by the tilter may come from a decentered corrector or a problem with the lenses of the rasa...


Edited by Benni123456, 08 June 2020 - 04:48 AM.

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#25 freestar8n

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Posted 08 June 2020 - 07:59 AM

That's the thing, filter drawer is not optional for me it has to be there. I'm using the Starizona one because it seems stronger than the Baader UFC. I have the correct spacing and distance.

 

I have tried ZWO 1600MM-Pro, 294MC-Pro, 294MC, 120MM-S, 224MC for collimation, tilt adjustment, etc. With and without filter drawer, with and without 3d printed spacer, with original spacer and without, I swear I have tried at least 100 combinations and adjustments several times.
 

If you don't clarify how exactly you set the mirror for collimating and later at focus for imaging - you are leaving out key details that are critical.

 

If you just wiggle the mirror around for focus and collimation - and then later wiggle it around for imaging - its state in both cases is unclear and it can flop around.  But that is because poor practice has been used both in collimation and in focus for imaging.

 

It is common for people to complain about slop in telescope components - but it amounts to a craftsman complaining about tools and ignoring best practice.

 

Even high end equipment can yield sub-par results if not used properly.  Mid-range equipment benefits even more from good technique.  But with mid-range equipment, good technique can yield a large ratio of performance vs. cost.

 

So - did you take the basic steps for a good result - by using automatic focus and backlash compensation to make sure focus was achieved by pushing the mirror up against gravity?  It needs to be done both for alignment/collimation - and for imaging.

Frank


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