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Mag-drop vs contrast

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#1 hakann

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Posted 20 October 2019 - 08:58 PM

When I was writing on the thread on ’big scope for faint fuzzes’, the topic come up to describe ( for Jon ) my way to see diganol size vs a allround scope and one for faint target in the center with a smaller pupil and rather narrow EP as ex Delites for a small as possible diagonal.

If do math on square inch minus the diagonal one can see from a ex 3.5” to a 4.5” or even a 5” on ex a 18” mirror the area don’t change much.
Ex a 5” get 17.25” and a 3.5” get 17.5”, so it’s marginal.
As I read most say a mag-drop at 0.2 and even to 0.4 and more really can’t be seen.
This one is told ; One need a 100% field at ex 0.5” and then whatever the mag-drop will be is not important.
Ex my 18” will most see 150-200X and rarley 500X.
As the scope has no tracking 500X start get hard to use.
So mayor work is low or mid powers in this scope for this sky.

So why not use a diagonal that give a 100% field at lowest power ? ( as it not effect the area of primary much )
In this case I need a 116 mm ( from CAD ) diagonal or a 4.5” ( 4.4” C-A ) is almost zero mag-drop.

But I heard to big diagonal is often used for visual so it’s not wrong to undersize it vs gain a better ’contrast’.

My question is ;
If the diagonal has not zero mag-drop it will limit the magnitudes some ( how much I can’t say*) and area of used primary is not much effected by use a bigger one.
-So what is the idea that a bigger one hurt the ’contrast’ ?
How can a area of 0.25 inch less get a better contrast.

* A mag-drop at 0.3 show at 100X ( low power ) from 18” it would be as a 15”.

http://www.cruxis.co...ngmagnitude.htm

Edited by hakann, 21 October 2019 - 03:26 AM.


#2 Jon Isaacs

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Posted 21 October 2019 - 03:29 AM

The contrast you gain with a smaller secondary is not deep sky contrast, it's fine scale planetary contrast, its scale is on the order of the Airy disk diameter, for a 30 inch scope, the diameter to the first minimum is 0.36 arc-seconds. This is far beyond the ability of the eye to resolve on a galaxy. 

 

With a 4.5 inch secondary, you'll still be at 15% CO which is tiny.

 

The way I see it:

 

- This is not a planetary scope but even it were, 15% is small enough. 

 

- The larger secondary means you can use low power eyepieces when you want to with no loss of capability at higher magnifications.

 

- The positioning of the secondary is far less critical. 

 

Design the scope around using a 31 mm Nagler and use a real focuser, a Feathertouch. There's no reason to compromise focusing in an attempt to minimize the secondary, that's for 8 inch F/8s where it's difficult to achieve a 15% CO and the smaller secondary that has some meaning.  

 

There will be times when an eyepiece like the 31 mm Nagler will be useful. A 0.4 magnitude drop with 31mm Nagler is reasonable, with a 9 mm Delite, a 0.4 magnitude drop is a not reasonable.

 

But at F/4.5, the depth of focus is 45 microns, half the thickness of a human hair, precise focus is critical.  That will never happen with a helical focuser. It would be easy with a Feathertouch.

 

I calculate that a 4.5 inch secondary with a Feathertouch focuser would drop about 0.15 magnitudes across the field of the 31 mm Nagler.

 

Jon


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#3 hakann

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Posted 21 October 2019 - 03:49 AM

Jon,
This thread was on diagonal for 18”, not the 30”, so I answer you there instead ( big scope for faint fuzzes )

But your answer here is that the smaller the better for planetarys ( contrast wise )
You did not comment on mag-drop vs low power if it limit the app. ( according to program )

#4 Asbytec

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Posted 21 October 2019 - 09:05 AM

Whether a 30", an 18", or an 8", the size of the diagonal obstruction affects "planetary" contrast on very small scales near the angular size of the Airy disc, as Jon is pointing out. Even Airy discs are small compared to the majority of resolvable deep sky detail. So, that larger detail is not affected by the obstruction on the scale of the Airy disc and a diffraction ring or two, which you may not resolve, anyway, at DSO magnifications and dim surface brightness. There is some reduction in light loss, but that's about it. 

 

You did not comment on mag-drop vs low power if it limit the app. ( according to program )

 

Well, Jon did say a 0.4 magnitude drop off at the edge of a low power, wide field like a 31 Nagler is okay, then he goes on to say if you're seeing the same magnitude drop off at the edge of a higher power, smaller field of a 9mm Delite, then the secondary is probably too small for many wide field applications. This means the magnitude drop off is also increased at the edge of all your wider field, lower magnification eyepieces. More so than most people plan for, maybe even up to a full magnitude or more (without doing the math, wagging it for effect) which seems very noticeable compared to 0.4 magnitude we normally accept at the field edge. 

 

I haven't read the other thread, so I may be missing some context of the discussion. Still...


Edited by Asbytec, 21 October 2019 - 09:33 AM.

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#5 Asbytec

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Posted 21 October 2019 - 09:22 AM

A mag-drop at 0.3 show at 100X ( low power ) from 18” it would be as a 15”.

 

At the edge, only.

 

You can probably get away with a very large diagonal for maximum field illumination and not worry about "planetary" contrast if you're limited to modest magnifications below 500x (1mm exit pupil) if seeing and tracking are holding you back. The surface area of the diagonal is small compared to the surface area of the 18" mirror, so shading will be minimal at worst (about 6% by area if I did the math correctly). The "contrast" you seek is found in the resolution of the aperture itself. You're gonna see more detail in DSOs with larger aperture, but it's not high resolution "planetary" contrast affected by the obstruction itself.

 

Here's a great read about all that.

 

Scopes optimized for high resolution lunar, planetary, and double star work normally have a smaller diagonal obstruction. In the case of a 4.5" obstruction, it'll be 0.25D, which is not terrible. Your planetary contrast will be approximately (1 - 0.25^2)^2 ~ 0.88 * the Strehl ratio of your scope or about 0.7 small scale intensity distribution at the diffraction limit. Basically, performance a little less than the diffraction limit that should not really matter on larger DSO scales, except possibly a bit on globular clusters and other near point source detail. And that does not including seeing effects. 


Edited by Asbytec, 21 October 2019 - 09:39 AM.


#6 hakann

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Posted 21 October 2019 - 10:29 AM

Asbytec,

 

Thank's for the site, I will read.

 

The thread ( that lead to this one ) was for a special designed telescope for faint galaxy's observing 'only'.
( if you like to read 'why', read on 'Big scopes for faint fuzzes' on this forum )

It will not be build for low power or Nagler 31 mm etc but has a pupil at 1.5-2 mm and Delites EP.
I'm not fanatic of a small diagonal, so don't get that wrong here, but one that hold 100% illuminated field at a FS at 9.6 mm.
To get there as I explained one use a L-distance needed vs the diagonal size whatever that be, so a 3.5" or a 4.5" so it still would be very small obstruction.
I also described ZT glass, thicker/stiffer/lighter and CF trusses in 3 sections.
I like to get feedback on that project. ( always good idea, and good things can come out )

 

This thread was something I came into try to explain that idea for Jon that I'm all ok with a bigger diagonal and coma corrector, 2" focusers and wide field EP etc, etc, but NOT to this project.
This thread was on diagonal size to the 18" scope I has whit a CZ Quartz 18" at f/4.
As OP text I noted I don't lose much if I'm going bigger on a diagonal to has 100% illuminated field even at low power.
From a mag-drop at +0.3 ( FS 36.2 mm ) using a 3.5" (3.3" C-A ) I has total inch area at diff at -0.25" if I would go to even a 5" diagonal.( A 5" give me 17.25" area )
Ok, obstruction is higher.
But ideas was to stay small on get the 'contrast'.

 

This 18" is a all-round deep-sky telescope. ( I has both a 3.5 and a 4" from Antares )

 

Right now Ostahowski sell 4.5" diagonal in Quartz at 750 USD at 1/10 w or better. ( what C-A on them I need to ask )


Edited by hakann, 21 October 2019 - 10:30 AM.

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#7 hakann

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Posted 21 October 2019 - 03:07 PM

I will bought get the Ostahowski Quartz 4.5" diagonal to, just to see and learn something out of it regards both see if one see fully illuminated field at low power as a 'gain', or if one see better 'contrast' with a smaller as a 'gain' at higher power.
I doubt in that scope I will has much more than 4-500X in it.

 

He say he test them all out to C-A or bevel.
In this case 1/50 or better RMS. ( normally one read it in ex 0.01 RMS )
Antares Always test them to a masked OD, ex -5 mm.

 

https://www.ostahows...icalflat-optics


Edited by hakann, 21 October 2019 - 03:09 PM.


#8 hakann

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Posted 22 October 2019 - 12:24 PM

My 18" Dob will use the 4.5" ( 4.4 C-A ) from Ostahowski and the tolerance was at PV 0.04, and that show on a great diagonal.

( tested all out to bevel in a new Zygo IF )
I will do some contrast test here later on vs a 3.5" ( 3.4" C-A ) diagonal with the same tolerance.

As I got all this, normally on low power the centre ( say 1/2" to 0.5" ) would be fully illuminated but the edge might drop.
So can we see up to 0.4 in mag-drop, if so just at the end - I can't say but in magnitudes that is instead of a 18" a 15" OD area.
Whit this one I has 100% even at low power, but will it hurt on mid or say higher powers ?
Maybe, but it's actually 'just' at 25% in obstruction, and that is not that bad, but 'maybe' one can see a better contrast with 15 or 20% obstruction.
But a Newton has always a mirror up there so it really can't be 100% illuminated, and this telescope will be more used on mid powers than highs vs the sky.
And as I use wide field EP and a TV Paracorr coma corrector, this will be very nice with a CZ Quartz f/4 optic's.


Edited by hakann, 22 October 2019 - 03:41 PM.


#9 Kunama

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Posted 22 October 2019 - 03:10 PM

I have an 18" F3.5 primary and both 4" and 4.5" secondaries (both better than 1/10th) I use the SIPS coma corrector tuned to exactly the recommended distance from my primary.

I have spent a few months experimenting and swapping between the two diagonals and in the end decided that the gain in contrast and image is not enough to warrant worrying about so I have now left the 4.5" in permanently and offset it accordingly for the larger FIF. This way I know the full primary is being used and I have even illumination for all my eyepieces.


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#10 hakann

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Posted 22 October 2019 - 03:40 PM

Matt,

So you has the 25% to and don't worry over contrast at higher power.

How much lip radial in mm do you has on the diagonal ?



#11 hakann

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Posted 22 October 2019 - 05:51 PM

I did found some good stuff by read & learn from Mel Bartels program now.

http://www.bbastrode...r.html#diagonal

 

Searching for contrast.

 

One might has a fully ( or almost ) field at low power, but the bigger it will be little that will effect transmission as we has a mirror there ( bigger or smaller )
In this case it’s my new Ostahowski 4.5” I’ll just bought.
I tested it and it is almost full field and I masked it off some so it is a 112 mm OD.
I used low power FS at 36.2 mm.
One can read it is ; 0.07 mag off.
So this diagonal mirror block of 0.07 mag.

 

If I set it at higher power ( ex my new Delite 7 mm or equal at Ethos 3.7 mm = FS at 7.5 mm ) and  it is still the same 0.07 mag.
That’s not much but it will make my 18” area less. ( and that’s the reality in a Newton )

 

 

Ok, now I test the 3.5” diagonal.
I use the same low power FS at 36.2 mm and now it is 0.19 mag off.
Well, that’s actually something one 'can' be see as its lower the app around 2".

 

Now over to the interesting part.
I now set in the 7.5 mm FS and now it is 0.05 mag off = a hair better than the bigger diagonal..

 

So what can we say about this.

The 4.5” is the winner.
It’s actually way better at low power and it is near as good on power at 300 or 600X. ( diff is only 0.02 mag )

I hardly believe one can call that a winner to use the 3.5" and one see that in contrast.


Edited by hakann, 22 October 2019 - 05:54 PM.


#12 Kunama

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Posted 23 October 2019 - 12:35 AM

Matt,

So you has the 25% to and don't worry over contrast at higher power.

How much lip radial in mm do you has on the diagonal ?

I trimmed the holder on my diagonal so now it covers less than 1.5mm of the edge leaving 112mm clear.



#13 hakann

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Posted 23 October 2019 - 02:06 PM

If one read Gary Seronik's repotage one take L-distance by focal as the old forumula.

https://garyseronik....nian-secondary/

 

In my case that is 339.08 / 4.02 = 84.34 ( 3.32" ) so with lip etc that is a OD 3.5".

But he mean for deep-sky it's not uncommon to use +0.5" extra, so a 4".

That's the reson I bough a 4" actually, but as that one had a hair ugly edge so the deal was to mask it off some as C-A was only 3.6".

As Ostahowski had a new batch ongoing now with Quartz 4.5" he picked up a good one at PV at 0.04, so I bought that one.

Gary went into to 'to big will hurt the contrast', and a 4.5" might is a overkill, but he also mean dropp at the edge can be seen, but not in that one…

But will I has a contrast issue, well let's see, and I has the 3.5" to test.

 

Gary also went into 1/2" fully illuminated.

What he mean is one take the smallest possible diagonal and then one add a 1/2" = then one has it '1/2" fully illuminated'.

 

I has hard understand that deal here based out of that formula on the 'smallest', or how one can line trace it up.

 

---------------------

 

The diameter of the fully illuminated field is usually governed by the size of the secondary mirror. For example, if a ½ -inch fully illuminated field is desired, simply add this amount to the minimum diagonal size to figure out the approximate diameter of the secondary. So, if you have calculated your minimum diagonal size to be 1.5 inches, using a 2 inch secondary will yield a fully illuminated field of ½ inch across.
To determine the approximate size of the fully illuminated field in an existing instrument, simply subtract the minimum diagonal size from that of the actual secondary mirror. A telescope equipped with a 1.83-inch diagonal that requires only a 1.5-inch secondary will have approximately a 0.33-inch-diameter fully illuminated field.


Edited by hakann, 23 October 2019 - 04:42 PM.


#14 hakann

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Posted 24 October 2019 - 01:16 PM

I went over the issue of the old tale of a ½” illuminated field and as I got it was complex and few can tell, if any, and what we know are doing is check for a acceptable mag-drop.
( visual use and deep sky observing )
One thing that came up is that has been blind test done to see on the famous contrast gain for go smaller if it can be seen.
As I got it, it’s very diffuse issue, and  it was actually show the opposite – bigger was better.
One idea ‘why’,  is a tiny ( or minimal ) diagonal is hard to get in correct position so telescope can actually be less in aperture vs the position in the in the light cone.
Few diagonal are also ok all out to bevel as most might think.
Even if they might be ok, there is still a bevel there so what a not perfect all out diagonal can actually cause the telescope do worse ( as the tolerance goes down )
The idea was gain more contrast going smaller vs a bigger.
Blind test with mask was tested and several could see. Results was not showing what we might think.
But to big is not what it is about, but limit at 25% seems work all ok.




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