5 replies to this topic

[WheezyGod]

My 10in dob is just under f/5. I’m aware that if I use the TV67 that brings my focal ratio down to just under f/2. I know that very fast scopes require more precise collimation because if collimation is off then it has a more detrimental impact on the view. Is this the case when using the TV67 or is it not because it’s affecting the f/ratio in a different way that doesn’t impact the significance of collimation?

Before NV I’ve always felt like collimation has more of an impact when using more magnification which I learned the hard way when visiting a bortle 2 site. I forgot a screwdriver and my secondary decided to go out of collimation for the first time in many months. Using more magnification resulted in a more noticeable impact from being out of collimation. I’ve since gotten Bob’s knobs.

With NV it feels odd to think that less magnification could potentially be making collimation more critical but since I haven’t seen this come up in the NV section then maybe this isn’t the case.

When I use NV in prime with a Barlow (96x) I feel like the views of anything get worse compared to without a Barlow (48x). Not because the view is dimmer but because stars look worse even though seeing conditions shouldn’t consistently be having much of an impact at 96x. Do most of you feel like stars look less sharp around this magnification when using NV compared to lower magnifications? If not then maybe my collimation isn’t as good as it could be or the cheaper 2x Barlow I’m using is the cause (Celestron Omni). Before I got NV I never felt like seeing had much of an impact around 100x, it wasn’t until 150x and higher that it did.

[Mazerski]

I have previously stated on fast / slow(er) f/ ratio that in my opinion (which means nothing) faster ratio provides better views as the stars look more jewel like. I don’t calculate magnification and at times I try a 2X Powermate ( all my scope viewing is in Prime Mode) but generally everything to me looks better without the PM ( a few of the large / bright globs look ok at f/9). 
 

Last night I threw the Antares 0.5x focal reducer on a Mod3 in 12.5” f/ 4.5 NewMoon for a ratio of f/2.25 (at face value as I don’t know if this is the actual reduction) and M16, M17, M20 and M8 looked great ( using the 6nm Astronomik Ha filter).

 

As for collimating - both my scopes hold up… if the are off I must not notice it since the DSO is in the center of FOV. 

[sixela]

The dominant aberration in faster scopes is edge of field astigmatism and vignetting (and the non-round entry pupil will cause diffraction spikes and change the diffraction pattern!), not coma, plus aberrations in the NVD objective (and under f/1.4 they tend to be quite severe).

We’re also using 40° AFOV eyepieces so even in my f/3.72 a coma corrector is frequently a wash (unless you maniacally place the coma corrector at the optimal distance it can even be worse!).

I often (esp. For H-alpha viewing) work without coma corrector just because there is less vignetting.

In a friend’s f/3 I do use a coma corrector at all times.

In other words: collimation is not that critical, and the aspects that are, are different. You really want the secondary optimally placed or one of the edges is going to vignette even harder than it should, and the NVD ( and a coma corrector) reacts really badly to focal plane tilt.

But a bit of coma on the on-axis object (I.e. slightly incorrect primary tilt) is the least of your worries.

[a__l]

WheezyGod, need a photo of the star field to understand what you are writing about.

[bobhen]

With NV you will be working at mostly low magnification (even 96x is rather low) so a "very minor" collimation issue should not be much of an problem, as all aberrations, with or without NV, are less seen at low magnifications. Of course, spot-on is always better.

 

An image intensifier delivers a huge light boost. This makes stars appear brighter. So, the magnification can be the same between NV and non-NV views but the brightness is not. So, one "possible" cause might be that you are noticing seeing impacts or optical impacts or filter impacts, on stars at the higher magnification with NV because the stars are brighter and aberrations are easier to see.

 

Bob

[WheezyGod]

With NV you will be working at mostly low magnification (even 96x is rather low) so a "very minor" collimation issue should not be much of an problem, as all aberrations, with or without NV, are less seen at low magnifications. Of course, spot-on is always better.

An image intensifier delivers a huge light boost. This makes stars appear brighter. So, the magnification can be the same between NV and non-NV views but the brightness is not. So, one "possible" cause might be that you are noticing seeing impacts or optical impacts or filter impacts, on stars at the higher magnification with NV because the stars are brighter and aberrations are easier to see.

Bob

Unless seeing impacts or star bloat are more profound when using NV at higher magnification.