10 replies to this topic

[Gschnettler]

Hello. I purchased one of these to go with my PVS-7 night vision goggles.

Baader Planetarium 1.25" IR Pass (685 nm) Filter

I have been having good success with removing the PVS-7 lens, replacing it with a c-mount and 1.25” adapter and then putting it in the focuser my 12” f/5 dob. Globular clusters look especially great.

Galaxies become much more obvious with night vision, but to me are still mostly just a bright center surrounded by some fuzziness. So then I tried adding the IR pass filter, and much to my surprise there was almost no difference. Maybe the whole image became slightly darker, but that’s about it. I’m sure the filter was doing it’s job and blocking tons of light, but from a usefulness perspective it didn’t do much if anything.

I thought this filter was supposed to make a pretty dramatic difference. I am wondering what I am doing wrong.

We were observing in the suburbs of Cincinnati about 15 miles from downtown where you can see some stars but there is still a lot of light pollution and you can’t see the Milky Way.

Does the filter only make a difference under heavy light pollution, like downtown in a city?

[bobo99]

A 685 IR pass filter is considered a light pollution filter. If you are that close to Cincinnati you would benefit using it almost in all your observation.

 

Try yours PVS7 at 1x and point up at the sky. Then add the filter. You should cut out a considerable amount of light pollution, get a darker sky, and be able to see a lot more stars. Using it with your dob when looking at globulars should improve contrast significantly as well. What happens when you try to do this ?

 

I too have found that using the 685 with galaxies doesn't help much. You'd benefit much more from going afocal with galaxies to get lower magnification creating a smaller object with a higher apparent brightness, that the NV can further help amplify. Try afocal (or step down your system somehow) + the filter, and your should see much better results.

 

The filter won't help with seeing halpha objects any better.

[GOLGO13]

The 685 works really good at 1x on the Milky Way. Cuts out a lot of light pollution. For use with the telescope you'd want to experiment with unfiltered, and the 685 on objects which are not nebulas. Then use an HA filter 12nm or less for Nebulas. 6nm is a good in-between HA filter. All of them are pretty much similar, but the more narrow they are the detail gets a bit more refined. Sometimes it seems to depend on how fast your scope is for how well they work. 

 

Agree that galaxies are a bit different with NV. Brighter cores, but otherwise not too much. At least in light pollution. I think galaxies only gain a moderate amount with NV...unlike most other objects. There are some nebulas that also are not picked up by NV. But most nebulas are very good.

[Gschnettler]

Thanks for the info. How would I do this?

“ go afocal with galaxies to get lower magnification creating a smaller object with a higher apparent brightness, that the NV can further help amplify. Try afocal (or step down your system somehow)”

Is there some sort of reverse Barlow that I could put into my focuser?

[a__l]

Is there some sort of reverse Barlow that I could put into my focuser?

Yes, TV67

https://www.televue...._page.asp?id=36

 

I bought this 2" filter but don't use it under relatively dark skies. H-alpha only.

Edited by a__l, 01 June 2022 - 03:18 PM.

[ButterFly]

Contrast is king.  Filters can only take away light.  A 685 longpass cuts out everything below 685.  That may or may not help with contrast, depending on how bad the skies are.  I prefer galaxies unfiltered under my dark skies.  IR pass filters cut out as much of the galaxies' light as they do background for me, so no boost in contrast.

 

H-alpha filters, on the other hand, offer dramatic boosts in contrast.  Over 90% of the light in the h-alpha band comes through, and almost 0% comes through outside the band.  That's a huge contrast boost.

 

685 is good for shutting off the h-alpha part and seeing only the reflection parts of the nebulae, if any, or the star clusters in them.  Most HII regions have star clusters.  Quite a few have substantial reflection portions.  Try comparing the Lagoon with h-alpha and 685.  I really like the 642 on that, letting through both.

[GOLGO13]

685 is for non nebula targets in light pollution. Not needed in dark skies. Great for the Milky Way.

[bobo99]

The TV 67mm linked above is a low power eyepiece. Attaching the PVS objective directly to the lens similar to here:

https://www.televue...._page.asp?id=36

will act as a focal reducer. Assuming the PVS7 lens is similar to the PVS14 you should easily be able to mount it, and the reduction ration is is the exit pupil of the 67mm eyepiece divided by 27mm(the focal length of the PVS14 objective lens. Again, check the values for the PVS7).

 

F/5 would get stepped down to F/2 at 67mm, and F/2.5 at 55mm. See attached table, you should be able to figure out the formulas pretty easy, last line is just exit pupil divided by focal length of or NV objective (27mm for the PVS14)

 

 

 

 

Or you can try Prime, but buy a separate focal reducer. Not sure if you'll be able to get the same focal reduction as the afocal method.

Thanks for the info. How would I do this?

“ go afocal with galaxies to get lower magnification creating a smaller object with a higher apparent brightness, that the NV can further help amplify. Try afocal (or step down your system somehow)”

Is there some sort of reverse Barlow that I could put into my focuser?

 

Attached Thumbnails

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Edited by bobo99, 02 June 2022 - 07:05 AM.

[Starchecker]

I have the 2" version of this and have no idea how to mount it to my Filter wheel or the filter tray on my 18" Dob.

 

There aren't any outer threads on the smaller circle side, just female threads on the larger circle side.

[ButterFly]

I have the 2" version of this and have no idea how to mount it to my Filter wheel or the filter tray on my 18" Dob.

There aren't any outer threads on the smaller circle side, just female threads on the larger circle side.

Sounds like someone sanded off the threads, for whatever reason. Get an empty filter cell and throw your current one in the trash.

[sixela]

An empty one or buy a cheap 2" polarsiing filter on AliExpress and chuck the filter in the bin and use the cell. Often cheaper.