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Good MN 190 FWHM Values?

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#1 John Miele

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Posted 23 September 2019 - 12:27 PM

Hi,

 

I just swapped my Edge8 for a Skywatcher 190 mak-newt and am setting up the new scope for imaging.

 

I think my collimation is not perfect but fairly close.

 

I have only had a chance to do a few trial runs and I am a little concerned about something. On my Edge8 and in my AT115EDT refractor I could get FWHM values as low as 1.6 arc-sec on the best nights and usually in the 2.5 'ish range on good nights.

 

So far, I had 3 nights to try the mak-newt using the same camera (ASI071pro with a Astronomik UV/IR cut filter) and my FWHM values were between 3.4 to 3.6 arc-sec each time according to CCDI. These were taken form individual short 20 second subs at zero gain focused using a Bhat Mask. So I was not overexposing and should not have had any meaningful tracking errors. Now maybe I just had three different nights of poor seeing, but I was expecting lower values. In addition the stars, while round, seemed a bit bloated.

 

Dumb question...if I had overstretched the image would that make the FWHM values larger? Because I did stretch the subs a fair amount to make sure CCDI would detect the stars.

 

To any mak-newt users, what type of FWHM values do you see on good or average nights seeing wise?

 

Thanks...John


Edited by John Miele, 23 September 2019 - 12:30 PM.


#2 jhayes_tucson

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Posted 23 September 2019 - 02:35 PM

First off, let's be clear:  You should always measure FWHM on linear data.  Once you stretch the data, you are no longer measuring the correct stellar profile.  The other requirement is to make absolutely sure that you are not measuring over-exposed stars.  You only get a correct measurement on stars that are not over-exposed (i.e. not showing a flat top) from linear data.

 

John



#3 John Miele

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Posted 23 September 2019 - 03:22 PM

Thanks John! I need to redo my checks then.

#4 Benni123456

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Posted 23 September 2019 - 06:20 PM

there are people who dissassemble some parts of the mn190, putting it back, and get better image quality

 

 

 Here is a post on how people improve and tune this thing.

 

https://stargazerslo...comment-1942792

 

That somehow prevents me from buying one...

 

Because i do not know whether i will have to realign everything after each transport...



#5 f300v10

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Posted 23 September 2019 - 09:24 PM

The subs for this image, taken under typical not great seeing here in GA, are in the 2.6 range according to APP.

 

M27-APP1-PS1-FullFrame-small.jpg


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#6 Benni123456

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Posted 24 September 2019 - 05:46 AM

that looks good.

 

How much backfocus has this mak newton by the way? I see no data from skywatcher on this...



#7 f300v10

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Posted 24 September 2019 - 07:39 AM

that looks good.

 

How much backfocus has this mak newton by the way? I see no data from skywatcher on this...

Benni, I can't give you an exact number, but I would say somewhere between 70 and 90mm.  I can just make it work using the Celestron OAG plus 17mm for the camera (ASI294MC-Pro).



#8 Benni123456

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Posted 24 September 2019 - 08:18 AM

hm, with 90mm backfocus, i could squeeze in my ao in between the off axis guider and the filter wheel, with 70, i could not....



#9 John Miele

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Posted 24 September 2019 - 08:31 AM

I'll measure mine tonight as distance from tube to focal plane. That's probably more useful since different focusers can have different draw tube length. I have a Moonlite on mine.

#10 Benni123456

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Posted 24 September 2019 - 08:40 AM

but yes im also looking for somethig with a flat field, better than f10 and around 1400-1000mm focal lenght.

 

As i find the celestron edge reducer disappointing, the alternative would be the mn from skywatcher or some larger apochromat.

 

A normal newton would not be optimal. First, i do not like the spikes, second, they have often only 50mm backfocus which does not fit to my ao, and, most importantly, they are open.

 

When i am outside with the scope, this is in some field. With much pollen and dust.

 

I can clean the corrector plate or the front lens quickly with a bellows but for a Newton, i would then need to disassemble it after every night in order to clean the primary...

 

For the mak newton:

 

For the skywatcher there are these forum descriptions about how to modify it in order to get finer stars.

 

But  I could would buy one, i guess i would need a tutorial with photos that show how to do every step. Not that i ruin something with my inexperience...

 

The images on astrobin with the mn190 from skywatcher are often better than those with the 150 comet hunter from explore scientific. I wonder why.



#11 Benni123456

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Posted 24 September 2019 - 08:42 AM

 

I'll measure mine tonight as distance from tube to focal plane. That's probably more useful since different focusers can have different draw tube length. I have a Moonlite on mine.

thank you. There is indeed no data about this...



#12 John Miele

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Posted 24 September 2019 - 08:38 PM

I apologize...after work I decided to work on collimation again and I adjusted the focuser position before I remembered to take the measurementconfused1.gif ! I don't want to give you a wrong number so I need to get the camera back into the correct focus position in order to take the measurement. I probably won't have time until this weekend to do it. The scope is back inside the house and off the mount...



#13 John Miele

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Posted 24 September 2019 - 08:41 PM

The subs for this image, taken under typical not great seeing here in GA, are in the 2.6 range according to APP.

 

attachicon.gif M27-APP1-PS1-FullFrame-small.jpg

Scott,

 

Looks awesome!

 

I wish I could have you come over and collimate my scope!  I just checked it with an eyepiece after work today and it is nowhere close. I have astigmatism all over the field. No wonder my FWHMs were so high!

 

The view in my cheshire looks close but obviously something is way out of whack. 



#14 f300v10

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Posted 24 September 2019 - 09:24 PM

John, adding the autocollimator was the KEY for me to finally get nice stars edge to edge with my MN190.  That, and getting the focuser exactly in the right spot, which for me happens to be over the mirror dot on the secondary.  My routine is:

 

1) Use the laser to get the secondary aligned.  After making sure the laser itself is collimated.  My laser was off in the beginning, which was part of the problem

2) Laser again to get the primary as close as possible

3) Autocollimator to nail it

 

I use the newest version of the Farpoint autocollimator.



#15 John Miele

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Posted 25 September 2019 - 08:11 AM

I think when I installed my Moonlite I didn't get it exactly over the center dot on the secondary. I'm having a hard time seeing the dot and the sight tube crosshairs at the same time.

#16 f300v10

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Posted 25 September 2019 - 08:39 AM

John,

 

I know the feeling.  It was very difficult to get my eyes to find and focus on the secondary spot, and when I did the crosshairs were a blur.



#17 John Miele

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Posted 25 September 2019 - 11:56 AM

I saw a video where someone used a Microsoft LifeCam to adjust the secondary. This looked like an awesome way to get your eye out of the loop and use video and crosshairs to be more precise. But you also needed an adapter to hold the lifecam and it is apparently no longer for sale. I'm thinking of trying to homebrew my own adapter. The other problem is the inside of the tube is so dark I cant really see the outline of the secondary. Not enough contrast between the edges of the secondary and the tube wall. But I am determined to get it right!



#18 Benni123456

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Posted 25 September 2019 - 06:44 PM

stupid question:

 

can one use the hotech advanced cassegrain collimator also for newtonians?

 

because they are both telescopes with 2 mirrors that have to be aligned, this should be possible or is it not? perhaps it may be impossible because of the focal length of the primary, or would that work? then i could buy that mak newt and would perhaps not need an autocollimator or something...



#19 f300v10

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Posted 26 September 2019 - 07:51 AM

stupid question:

 

can one use the hotech advanced cassegrain collimator also for newtonians?

 

because they are both telescopes with 2 mirrors that have to be aligned, this should be possible or is it not? perhaps it may be impossible because of the focal length of the primary, or would that work? then i could buy that mak newt and would perhaps not need an autocollimator or something...

Benni,

 

I struggled to achieve good collimation on my MN190 until I added the autocollimator.  Even when the laser collimator showed perfection, the images were still off in one corner at least.  But once I added the autocollimator, and made sure to double check/adjust it after mounting the scope (I travel to image), I finally got good stars across the entire field.  

 

As for using the Hotech SCT collimator I don't see how that would work.  To collimate a Newtonian/Mak Newt, you must insert the laser into the focuser and have it be perfectly square in the focuser tube.  I initially purchased the Hotech 2" laser collimator for Newtonians, and to my disappointment, the unit itself was WAY out of collimation.  I returned it, only to find the second unit was also off. I finally tried an inexpensive unit I found on Amazon ($25), and it was spot on.  The Farpoint auto collimator is $80, so for a total of $105 you would have all the tools you need to collimate an MN190.



#20 John Miele

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Posted 14 October 2019 - 04:37 PM

I apologize...after work I decided to work on collimation again and I adjusted the focuser position before I remembered to take the measurementconfused1.gif ! I don't want to give you a wrong number so I need to get the camera back into the correct focus position in order to take the measurement. I probably won't have time until this weekend to do it. The scope is back inside the house and off the mount...

Sorry it took so long but I focused my camera on the Moon and measured from the tube surface to the focal plane and it is 137mm. Or with my Moonlite focuser, it is 77mm above the fully racked in drawtube.




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