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Choosing a OSC and filters - need help with some practical questions!

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#1 phsampaio

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Posted 20 September 2021 - 10:11 AM

So far, I've been using my stock DLSR. I'm planning on upgrading it to a OSC with LP filters - I won't go into Mono right now, as I like the simplicity of OSC and it would go way beyong my budget, considering all the accessories (filters, filter wheel, etc). I'm considering the ZWO ASI2600MC - or its equivalents, the Altair 26C or even QHY268C, although the 268C seems hard to find in europe right now. 

 

So, on to the practical questions:

1. Any reason to choose one brand over the other? The 2600MC seems like a good choice. Although it comes with 256Mb RAM (compared to 512Mb in the 26C and 268C) it already comes with a sensor tilt plate and some extensor rings. It doesn't come with a power supply so I have to buy that one. The 26C does come with a power supply but doesn't come with extensors or tilt plate, the price becomes somewhat the same if you factor that in. The ZWO is compatible with ASIAIR Pro, in case I want to go that route in the future, though...

 

2. Do I need an UV/IR filter for the 2600MC? The 26C explicitly says that it has a UV/IR filter built in. The 2600MC only states a IR filter, but the manual shows a graph that resembles a UV/IR filter.

 

3. Regarding backfocus. I use a Flat6AIII 0.8x focal reducer with my WO GT71. In WO's website, it states that my backfocus is 64.1mm. That means it's around + 9.1mm more than the standard backfocus of 55mm, correct? Also, since I'll be using a 2mm thick filter (L-extreme), this needs to be around 0.66mm more, for a total of 64.7mm give or take right?

 

4. Also, regarding backfocus, let's see if my calculations are correct:
- Camera sensor to camera flange - 17.5mm
- TS optics filter drawer - 18mm
- Extensors need to be around 29.2 mm. Could I use the 21mm extensor that comes with the camera, and adjust the Flat6AIII to around 8.2 mm? That seems to me that it should work...

 

5. Again, regarding backfocus. I want to use the Optolong L-Extreme for narrowband targets. But for broadband targets, I'm still unsure what to use. I shoot in a Bortle 4 zone, so not a lot of LP. I could shoot without any filter in the drawer, but then my adjusted backfocus would change 0.66mm (compared to the BF with the filter on). I thought of using a simple UV/IR/Luminance from optolong that's also 2mm thick, just so the backfocus won't change. Or I could use the L-Pro (again 2mm thick) and cut a little bit of LP, although I've read It could mess up with my colors. What would you suggest?

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#2 blazek

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Posted 20 September 2021 - 11:36 AM

1. As you know, brand should stand for the quality of design, manufacturing, support etc.  Also the quality (design, manufacturing, support) comes with the price (vice versa is not always the case ). This type of camera is built around the same 

Sony IMX571 sensor by several camera manufacturers . ZWO, QHY having their own design but also Omegon, Lacerta, Altair that relabel the Touptek camera. The price of Touptek varries dramatically and one can get it for 40% cheaper than ZWO or QHY. I got Omegon version and did not regret so far, but the choice is yours. Touptek on Aliexpress is evencheaper than Omegon.

2. You need UV/IR cut, either "built in" or added

3. seems right, although it is likely that in real life it will be a bit different (mm or two more or less) . have that provision in your calculation . Adjustment on reducer gets very handy

4. yes, keep on mind above

5. There are many opinions about LP filters . My opinion is that if you can make it without them, do it . In Bortle 4 you can do a lot without them . I use Optolong UV/IR/Lum just to preserve backfocus of L-Extreme and for me it works fine


Edited by blazek, 20 September 2021 - 11:36 AM.


#3 blazek

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Posted 20 September 2021 - 12:13 PM

@ matt_astro_tx

O, yes , adding filter "adds" to your backfocus for roughly 1/3 of glass thickness compared to no filter.  For 2mm Optolong, that is around 0.6mm , and you'll se it if not accounted for, especially on a large sensor like 2600 is. 



#4 matt_astro_tx

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Posted 20 September 2021 - 12:37 PM

@ matt_astro_tx

O, yes , adding filter "adds" to your backfocus for roughly 1/3 of glass thickness compared to no filter.  For 2mm Optolong, that is around 0.6mm , and you'll se it if not accounted for, especially on a large sensor like 2600 is. 

Say what?  Why haven't I heard of this?  Honestly that would explain why my backspacing needed to be adjusted by ~1mm.

 

I humbly stand corrected.  bow.gif



#5 TelescopeGreg

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Posted 20 September 2021 - 12:41 PM

2.  The ASI2600MC's built-in UV/IR filter is sufficient to not need any additional filtering for star bloat.  Don't know about the other brands.


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#6 blazek

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Posted 20 September 2021 - 12:47 PM

Say what?  Why haven't I heard of this?  Honestly that would explain why my backspacing needed to be adjusted by ~1mm.

 

I humbly stand corrected.  bow.gif

No worries, I learned it same way .. That is exactly what this forum is all about



#7 blazek

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Posted 20 September 2021 - 12:53 PM

2.  The ASI2600MC's built-in UV/IR filter is sufficient to not need any additional filtering for star bloat.  Don't know about the other brands.

Indeed, just using something (clear glass would be ideal) to be parafocal with L-Extreme. In that respect mentioned  Optolong UV/IR/Lum could be used, no harm because of additional UV/IR cut . 



#8 OldManSky

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Posted 20 September 2021 - 12:54 PM

Actually, if you choose the ASI2600MC-Pro, you won't need a separate UV/IR block filter.  The color (MC) version of the camera has UV/IR block in the cover glass.  


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#9 phsampaio

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Posted 20 September 2021 - 01:19 PM

Actually, if you choose the ASI2600MC-Pro, you won't need a separate UV/IR block filter. The color (MC) version of the camera has UV/IR block in the cover glass.


Thanks, that's what I thought. But I would still need some 2mm glass in the optical path in order for the back focus to be parfocal with the optolong l-extreme. Otherwise, every time I take out the l-extreme, I would have to adjust the backfocus a little (0.6mm).
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#10 imtl

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Posted 20 September 2021 - 01:47 PM

Indeed, just using something (clear glass would be ideal) to be parafocal with L-Extreme. In that respect mentioned  Optolong UV/IR/Lum could be used, no harm because of additional UV/IR cut . 

More glass (with no benefit) = less throughput, possibly adding aberrations.

 

I doubt that 0.6mm difference in back focus will have such a big effect on your system. Did you actually try it?


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#11 blazek

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Posted 20 September 2021 - 02:01 PM

More glass (with no benefit) = less throughput, possibly adding aberrations.

 

I doubt that 0.6mm difference in back focus will have such a big effect on your system. Did you actually try it?

On my setup (Esprit 120 + 0.79 reducer +2600 clone camera) , unfortunately 0.6mm backfocus  difference is very clearly visible

As I said , very good piece of clear glass would be the best , but the closest I got with Optolong is their Lum filter which is quite parafocal with L-Extreme. Compromise, but I prefer it over elongated stars at corners or correcting backfocus every time I change the filter. Can't judge how visible would it be on the setup of phsampaio  who raised the question 


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#12 imtl

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Posted 20 September 2021 - 02:07 PM

On my setup (Esprit 120 + 0.79 reducer +2600 clone camera) , unfortunately 0.6mm backfocus  difference is very clearly visible

As I said , very good piece of clear glass would be the best , but the closest I got with Optolong is their Lum filter which is quite parafocal with L-Extreme. Compromise, but I prefer it over elongated stars at corners or correcting backfocus every time I change the filter. Can't judge how visible would it be on the setup of phsampaio  who raised the question 

That indeed sounds like the best compromise. So much for manufacturers of mass produced scope promising "image circle up to a FF".



#13 phsampaio

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Posted 20 September 2021 - 02:21 PM

On my setup (Esprit 120 + 0.79 reducer +2600 clone camera) , unfortunately 0.6mm backfocus difference is very clearly visible
As I said , very good piece of clear glass would be the best , but the closest I got with Optolong is their Lum filter which is quite parafocal with L-Extreme. Compromise, but I prefer it over elongated stars at corners or correcting backfocus every time I change the filter. Can't judge how visible would it be on the setup of phsampaio who raised the question


Using my 7D ii stock, 0.6mm is not a small change. My scope is F4.7 reduced, so the focal image plane is very thin.

I guess it's a compromise to use unnecessary glass, but unless I used a Petzval design telescope (one day, I'll have my takahashi...) using no glass at all would mean having to adjust my backfocus every time I changed filters...

#14 elmiko

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Posted 21 September 2021 - 12:30 AM

The L-Extreme filter is a great filter for osc. I've been using it for a year, and love it! I image from Bortle 8 skies. I use the Optolong Uvir filter. It doesn't mess with the colors with the Zwo 071 mcpro or my 533 mcpro.

But you don't need a Uvir filter with the 2600. It has a Uvir filter built in. I used to use an Orion Skyglow imaging filter....now that messed up the true colors of the stars. 



#15 phsampaio

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Posted 21 September 2021 - 09:28 PM

I think I needed to clarify some things... My current setup with WO GT71 + FR/FF is 4.7, and from my initial photos, it's very sensitive to changes in backfocus (relatively fast system with shallower focus plane, and all that). See this thread to see what I'm talking about

 

I plan to use an OSC with the L-Extreme or some other Duo NB filter (i'm now looking at IDAS NBZ as well) when imaging for nebulas, etc. For broadband targets, I won't use the Duo Band filter. For this reason, instead of threading the filter in the focal readucer, my plan is to use a filter drawer, and put in the L-extreme when imaging nebulas, and put it out when imaging broadband.

 

Problem: when addding glass to the image train, I automatically increase the backfocus a bit (see post #3 above). For filters, this increase in backfocus is about 1/3 of the filter thickness - around 0.6mm for the L-extreme. Therefore, the backfocus changes if I image with the L-extreme filter vs. without any filter. Only 0.6mm for sure, but in my system at F4.7 and shooting with an APS-C sized sensor, this small change does make a difference (it might not in other systems).

 

My question is: would it be ok to use a clear filter (like IDAS clear or Optolong's IR/UV filter - whichever is parfocal with the Duo Band), even though I already have and IR/UV cut filter built in the 2600MC? Is this asking for problems or is it something that people usually do?



#16 OldManSky

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Posted 22 September 2021 - 08:50 AM

I think I needed to clarify some things... My current setup with WO GT71 + FR/FF is 4.7, and from my initial photos, it's very sensitive to changes in backfocus (relatively fast system with shallower focus plane, and all that). See this thread to see what I'm talking about

 

I plan to use an OSC with the L-Extreme or some other Duo NB filter (i'm now looking at IDAS NBZ as well) when imaging for nebulas, etc. For broadband targets, I won't use the Duo Band filter. For this reason, instead of threading the filter in the focal readucer, my plan is to use a filter drawer, and put in the L-extreme when imaging nebulas, and put it out when imaging broadband.

 

Problem: when addding glass to the image train, I automatically increase the backfocus a bit (see post #3 above). For filters, this increase in backfocus is about 1/3 of the filter thickness - around 0.6mm for the L-extreme. Therefore, the backfocus changes if I image with the L-extreme filter vs. without any filter. Only 0.6mm for sure, but in my system at F4.7 and shooting with an APS-C sized sensor, this small change does make a difference (it might not in other systems).

 

My question is: would it be ok to use a clear filter (like IDAS clear or Optolong's IR/UV filter - whichever is parfocal with the Duo Band), even though I already have and IR/UV cut filter built in the 2600MC? Is this asking for problems or is it something that people usually do?

I have the same scope.  The WO GT71 and my other two main imaging scopes all have different focus points with the 2600 if there's a filter versus no filter.  I get it.

I'm using the ZWO filter drawer.  Currently I put in either the L-Enhance (I don't have the L-Extreme) or the L-Pro as my "not dual-band" filter.  That way focus point stays the same.

Your clear filter should work fine, but I would suggest getting one from Optolong that's the same thickness as the L-Extreme if possible -- best chance they'll be parfocal that way IMHO.

Good luck.



#17 TareqPhoto

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Posted 22 September 2021 - 09:01 AM

Actually, if you choose the ASI2600MC-Pro, you won't need a separate UV/IR block filter.  The color (MC) version of the camera has UV/IR block in the cover glass.  

Ok, so regardless that ZWO did mention about it, how do you know that the camera is having one without taking the description or ZWO mention?

 

I have ToupTek one and I requested it to be with AR window not UV/IR block one, but I don't know if it is that or this only trusting the seller, how can I test to be sure?



#18 OldManSky

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Posted 22 September 2021 - 01:30 PM

Ok, so regardless that ZWO did mention about it, how do you know that the camera is having one without taking the description or ZWO mention?

 

I have ToupTek one and I requested it to be with AR window not UV/IR block one, but I don't know if it is that or this only trusting the seller, how can I test to be sure?

Aside from ZWO confirming it in writing, I can tell by the images.  Since I also have a monochrome camera, I know what my images through my scopes look like without UV/IR cut.  One in particular (the 61 EDPH-II) shows more variation in star size by color than the others without a UV filter.  With one, it's got nice tight blue stars.  The 2600 produces nice tight blue stars on it :)



#19 KungFood

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Posted 22 September 2021 - 01:46 PM

I shoot with the 2600mc, and have started recently shooting from bortle 9 with the L-eXtreme. I can confirm that its a great combo, and does NOT need a separate UV/IR cut filter. The front window on the camera is a cut filter. Some earlier ZWO cameras did not have this, the new crop of camera bodies does.

 

Example of this combo from bortle 9:

 

get.jpg?insecure

 

Stars are fairly tight, and star color is manageable.


Edited by KungFood, 22 September 2021 - 01:48 PM.

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#20 TareqPhoto

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Posted 22 September 2021 - 02:34 PM

Aside from ZWO confirming it in writing, I can tell by the images.  Since I also have a monochrome camera, I know what my images through my scopes look like without UV/IR cut.  One in particular (the 61 EDPH-II) shows more variation in star size by color than the others without a UV filter.  With one, it's got nice tight blue stars.  The 2600 produces nice tight blue stars on it smile.gif

So i have only to test it for stars??? Any examples around for that with and without filters?

 

My camera is the color version, and later if i can buy a mono version i will ask for AR window as well if possible, i always use filters with the mono cameras anyway.



#21 TareqPhoto

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Posted 22 September 2021 - 02:35 PM

I shoot with the 2600mc, and have started recently shooting from bortle 9 with the L-eXtreme. I can confirm that its a great combo, and does NOT need a separate UV/IR cut filter. The front window on the camera is a cut filter. Some earlier ZWO cameras did not have this, the new crop of camera bodies does.

 

Example of this combo from bortle 9:

 

get.jpg?insecure

 

Stars are fairly tight, and star color is manageable.

Bortle 9 means that all LED lights are pointed on your scope or your equipment and your sky is bright like a daylight?!!!



#22 KungFood

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Posted 22 September 2021 - 02:53 PM

Bortle 9 is heavy urban/large urban metro. I live in the center of Dallas. DFW is reported as bortle 8/9, where I am is very central, so 9. I don't think the bortle scale has anything to do with ground light levels, only sky brightness, but as always, I could be wrong.



#23 TareqPhoto

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Posted 22 September 2021 - 03:38 PM

Bortle 9 is heavy urban/large urban metro. I live in the center of Dallas. DFW is reported as bortle 8/9, where I am is very central, so 9. I don't think the bortle scale has anything to do with ground light levels, only sky brightness, but as always, I could be wrong.

I also could be wrong as looking at the LP map it is showing me at red zone and stating as Bortle 8, but that was from 2015 or 2016 one, and when i changed to latest one it sounds that changed more, so i don't know if i am under Bortle 7 or 8 or 9 actually, and this is even before they will turn on the LED streetlights in the street front of my house.



#24 KungFood

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Posted 22 September 2021 - 03:50 PM

Yup, according to https://www.lightpol...FFFFTFFFFFFFFFF I'm smack in the middle of the white zone



#25 sbharrat

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Posted 22 September 2021 - 03:55 PM

Yup, according to https://www.lightpol...FFFFTFFFFFFFFFF I'm smack in the middle of the white zone

in which case, bow.gif for the M51 in your profile pic. 




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