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Old 27-02-2020, 04:43 PM
TareqPhoto (Tareq)
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Achromatic scopes are bad for imaging at all?

Hi all,

To what degree is that achromatic scopes i mean the refractors here are bad for DSO imaging?

Let's say not using RGB at all, but only narrowbanding filters, mono camera only, how bad they are compared to APO refractors for example?

Also to compare, if you use 3 same exact scopes of achromatic refr and each has a mono camera the same exact and you put each with a filter let's say one with Ha or red, and second with green or SII and the third with blue or OIII, all in focus, and then used the data for stacking, will that be bad compared to using one APO refractor be it a triplet or a doublet with one mono camera only and using all filters with wheel or changing whatever?

All what i read about achro and APO is about how that APO is bringing all colors to one point, so it is about focus, and it is always about using a color camera or one camera and using all filters, so if using many scopes and focus with all individually, does that matter really? does APO only produce sharper results or better colors? Does it change FL or FoV or magnification or target look if refocusing? I mean someone said if those colors aren't in same plane then it will change the FL then, like 2 filters at 400mm while third could be like 395mm or 405mm something like that and then in stacking it won't generate a correct true final result?
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Old 27-02-2020, 04:53 PM
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Merlin66 (Ken)
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Tareq,
Using a mono camera and narrowband filters you won’t see any issue using an achromat, other than refocusing each filter.
The problem may come getting an acceptable L image......
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Old 27-02-2020, 05:24 PM
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Atmos (Colin)
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Ken is right, narrowband filters cut the band enough that bloating isn’t an issue as long as you refocus in between. As the focus shift is only on the order of maybe 100 microns or a bit more, registration isn’t an issue.

Where an APO really does well in narrowband imaging however is that most APOs have a considerably better optical figure than most achromats because they’re built to a much cheaper standard. THIS is where an APO will be better, smaller tighter stars and a better corrected field with less spherical aberration.
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Old 27-02-2020, 05:26 PM
TareqPhoto (Tareq)
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Quote:
Originally Posted by Merlin66 View Post
Tareq,
Using a mono camera and narrowband filters you won’t see any issue using an achromat, other than refocusing each filter.
The problem may come getting an acceptable L image......
Hi Ken,

While i like your answer, but sounds i don't see many or so so many who are using an achromatic scopes with mono really if there are not much differences, so either there are other things or i better wait and see all answers to see what else there.

I was going to add DSLR lenses in the question or equation here, but i decided to not so we focus on scopes only and no lenses.
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Old 27-02-2020, 05:29 PM
TareqPhoto (Tareq)
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Quote:
Originally Posted by Atmos View Post
Ken is right, narrowband filters cut the band enough that bloating isn’t an issue as long as you refocus in between. As the focus shift is only on the order of maybe 100 microns or a bit more, registration isn’t an issue.

Where an APO really does well in narrowband imaging however is that most APOs have a considerably better optical figure than most achromats because they’re built to a much cheaper standard. THIS is where an APO will be better, smaller tighter stars and a better corrected field with less spherical aberration.
I saw somewhere where they are selling individual optics for refrctor scopes so maybe there is one or two that is very good quality better than the standard or stock achcromatic scopes that are being sold as mass production in markets, so that might do some improvements or not?

Also about stars, so if using a high quality filters, won't help to bring that achromatic scopes to somehow near APO level? For example 6nm or 5nm/3nm NB filters.
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Old 27-02-2020, 07:51 PM
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Merlin66 (Ken)
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Tareq,
Not all AP guys are just using just narrowband filters - you have to consider the use of L filters and RGB filters if you're really "serious"
This puts extra demands on the already very good ED type achromats.
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Old 27-02-2020, 08:15 PM
TareqPhoto (Tareq)
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Originally Posted by Merlin66 View Post
Tareq,
Not all AP guys are just using just narrowband filters - you have to consider the use of L filters and RGB filters if you're really "serious"
This puts extra demands on the already very good ED type achromats.
I understand, and that is a good point, but does ED APO let's say doublets for example be good enough for imaging then or it must be a triplet? So is an ED APO scope small one for example about $300-600 nice for imaging or this is not bad but it won't do better as well?

What is the problem of L filter if using with an achro refractor?
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Old 27-02-2020, 08:21 PM
TareqPhoto (Tareq)
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Also watch my first post, i ,mentioned like 3 scopes, so can't one of them be using L and the other two sharing RGB then? or say 2 scopes, one with L alone and the other is with RGB and refocusing always, i wasn't talking about a SINGLE achro against a SINGLE APO, but a bi or tri achro against single APO, because that APO is mainly having better optics inside and it is to provide like nearly same focus for all colors.
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Old 27-02-2020, 08:29 PM
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Quote:
Originally Posted by TareqPhoto View Post
I understand, and that is a good point, but does ED APO let's say doublets for example be good enough for imaging then or it must be a triplet? So is an ED APO scope small one for example about $300-600 nice for imaging or this is not bad but it won't do better as well?

What is the problem of L filter if using with an achro refractor?
The L Filter (Luminance filter) transmits light across a 300 nm band (~400 to 700nm) so will show up deficiencies in optics and in different focus points for differing wavelengths (colours) far more readily than a typical narrowband filter with a 3, 5 or 6 nm 50% bandwidth.

Best
JA

Last edited by JA; 27-02-2020 at 08:48 PM.
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Old 27-02-2020, 08:53 PM
TareqPhoto (Tareq)
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Originally Posted by JA View Post
The L Filter (Luminance filter) transmits light across a 300 nm band (~400 to 700nm) so will show up deficiencies in optics and in different focus points for differing wavelengths (colours) far more readily than a typical narrowband filter with a 3, 5 or 6 nm 50% bandwidth.

Best
JA
I still don't understand this point, so using a mono, L filter will give a mono result, where will be the problem then?

And what about those various LP filters nowadays that cut some of visible lights in between? I mean are those not good enough to be used instead of L filter let's say under LP skies? and if so then are those filters with achro scope isn't enough to cut something or produce good enough IR-UV cut result?
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Old 27-02-2020, 08:58 PM
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By the way, there is a filter that is designed to minimize the CA of a scope or lens, a Lum filter for that purpose, doesn't this help for example to minimize that blue halo or CA then it can be fine?

What iam asking about, if NB with achromatic scopes is just fine and can be done then to my understand that even with NB the colors are also on different bandwidth, i mean it is more apart together than RGB, OIII is in blue region, Ha and SII are almost close to each other, say using N filter which is nearly at green zone, i want to understand more what makes the difference if we neglect color cameras here at all? and if we use the filters separately and re-focusing for each?

Does the mirror then reflect all of them at the same point so that the mirror is doing better than APO top quality optics for example? Say with a mirror scope we don't refocus for all filters?
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Old 27-02-2020, 09:44 PM
Ukastronomer (Jeremy)
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Remember what people used BEFORE APOS were invented


.
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Old 27-02-2020, 10:43 PM
TareqPhoto (Tareq)
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Quote:
Originally Posted by Ukastronomer View Post
Remember what people used BEFORE APOS were invented


.
I really don't remember because maybe i wasn't born back then or i never know about what they were using.
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  #14  
Old 28-02-2020, 12:26 AM
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Originally Posted by TareqPhoto View Post
I still don't understand this point, so using a mono, L filter will give a mono result, where will be the problem then?
The problem I explained will still exist.

The light passing through the optics BEFORE the Luminance filter is full bandwidth light and as described previously the optics will still (depending on the quality/degree of apochromaticity) focus it at different focal points as a function of wavelngth (colour). This then passes through the Luminance L filter and is limited to a 400-700nm 50% bandwidth. Simply put it will appear to create a slightly defocused/smeared image as a result of the widebandwidth of light transmitted IRRESPECTIVE of whether a colour or monochrome sensor is used. It may help to recall that the monochrome sensor will still receive all the light passing through the Luminance filter in the ~400-700nm band and therefore display all the optic's potential faults, in a comaprison with a much easier job in the narrowband case.


Best
JA
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Old 28-02-2020, 01:21 AM
TareqPhoto (Tareq)
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Originally Posted by JA View Post
The problem I explained will still exist.

The light passing through the optics BEFORE the Luminance filter is full bandwidth light and as described previously the optics will still (depending on the quality/degree of apochromaticity) focus it at different focal points as a function of wavelngth (colour). This then passes through the Luminance L filter and is limited to a 400-700nm 50% bandwidth. Simply put it will appear to create a slightly defocused/smeared image as a result of the widebandwidth of light transmitted IRRESPECTIVE of whether a colour or monochrome sensor is used. It may help to recall that the monochrome sensor will still receive all the light passing through the Luminance filter in the ~400-700nm band and therefore display all the optic's potential faults, in a comaprison with a much easier job in the narrowband case.


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JA
Ok, got it, thank you!
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