Category: Astrophotography

Flexure – The Astrophotography Image Destroyer

When you are trying to track down guiding errors with your astrophotography, look for flexure first.  The basic rule of thumb is that if it can move, it will move.  Any movement in your imaging train can wreck your pictures and make star trails and wild jerking movements.

Currently I’m using my Nikon 80 to 400mm zoom lens for most of my astrophotography.  It’s an ok but not perfect lens.  I’m waiting to buy a decent astrophotography telescope for wild field views.  In the meantime, I’m struggling to get long exposures in excess of 5 minutes in the 200 to 400mm range. I have done up to 20 minutes with this lens, but only if I mount the guide camera directly to the top of the lens.  In addition, I’m getting flexure in the lens itself due to the single mounting point design that was intended to be used in a camera tripod.

An example of flexure. Here the culprit is movement in the guide camera.

Differential Flexure

This refers to a difference in flexing from your optical train and the guide camera.  If I try to mount the guide camera off to the side, then I get differential flexure and can not take shots longer than 5 minutes.  Even at 5 minutes, the shots are inconsistent and many are ruined.

The Fix

I have finally got a system where I can consistency take long sub exposures. The solution was to add a second support bracket that bolts onto the universal vixen plate.  The difference this one change made is dramatic.  I can no longer feel any give at all when I press my finger down on the imaging camera.  Prior to the bracket, you could see visible movement as the camera train flexed on the tripod mounting bracket.

I added a custom 3D printed support bracket that screws directly into the imaging camera. It bolts onto the metal vixen plate which dramatically stiffens the optical train


 

In addition to this bracket, I went back to a piggy back system where I mounted the guide camera right onto the lens.  The reason I took it off to begin with is that I did not want to damage the vibration reduction switch on the lens.  Unfortunately, this is the only spot I could find to mount my custom 3D printed guide camera mount.

By make these two small changes, you can see the difference both in the final images and by looking at the RMS error reading on the ASIAir guide camera graph.  The following images shows the difference is guiding corrections between having the guide camera piggy backed, versus having it off the the side.

600 S Guide Camera Flexing
Higher RMS Readings
600 S Steady Guide Camera
Lower RMS Readings

Conclusion

Working out the bugs in your auto guiding setup has a lot of moving parts.  For me, the biggest issue is flexure and I think this is the first place you should start.  In the video above, I made a standardized test and show just one change at a time.  This is important so that you can narrow down the root cause of your issue.  

The Soul Nebula IC 1848

The Soul Nebula

Photo Details

 

Object

IC 1848, SH2-199 The Soul Nebula

Description

HOO Composition

The Soul Nebula is located right next the Heart Nebula and occupies a large field of view.  Best photographed with a wide field lens or telescope.  I shot this with a 200mm Nikon lens.

Camera

ZWO 1178mm Mono

Mount

Skywatcher EQM-35 Pro

Guide Scope

SVBony SV165

Guide Camera

ZWO 120MM Mini

Filters

ZWO 1.25″ Narrow Band

Integration Time

300S sub frames for a total integration time of 385Min or 6.4Hrs

Aquisition Date

2/8/2023

California Nebula SH2-220

Photo Details

 

Object

SH2-220, NGC 1499 California Nebula

Description

Shot in monochrome with H-Alpha narrow band filter.  The California Nebula looks like the state of California.

This is a Type HII emission with a lot of hydrogen and some sulfur gasses.

Camera

ZWO 1178mm Mono

Mount

Skywatcher EQM-35 Pro

Guide Camera

ZWO 120MM Mini

Filters

ZWO 1.25″ Narrow Band

Integration Time

5 Subframes of 1600S each for 50 Min total integration time

Aquisition Date

1/24/2023

Orion Nebula M42

Photo Details

 

Object

M42 Orion Nebula

Description

The Orion Nebula is the most photographed object in the sky. The nebula has significant amounts of Hydrogen, Oxygen, and Sulfur gasses.

In this composition, I am showing an HOO color palette with H-alpha as the luminance layer.

Camera

ZWO 1178mm Mono

Mount

Skywatcher EQM-35 Pro

Filters

ZWO 1.25″ Narrow Band

Integration Time

90 Min, 300S sub frames

Aquisition Date

2/15/2023

ZWO 1.25″ Narrowband Filter Set Review

I recently purchased the ZWO 1.25″ narrowband filter set.  The set include 3 filters H-alpha, S-II, and O3.

Narrowband filters are a great way to battle light pollution when photographing emission nebula. I live in a Bortle 8-9 area so for me narrow band is a must have. Even in light polluted skies, you can achieve highly detailed color images with a monochrome camera.

Specifications

Thickness – 2mm (glass)

Cost – $369 as of post date

Light Pass – up to 90% (see chart) The manufacturer says “up to 80%” but if you look at the chart, it clearly say 90%.  I suspect it’s just a typo on their site since the chart is more reliable.

Band Pass – 7 +/- .5nm

Wavelength

H-alpha – 656nm

S-II – 672nm

O3 – 500nm

Mounted – yes (standard 1.25″ filter thread male)

The ZWO filter set has up to 90% transmission.

Installation

The filters are pre mounted onto standard 1.25″ male filter threads.  I have used them individually and also with the ZWO Filter wheel.  All you have to do is to screw them into the filter holder. 

Performance

The following images were taken in my back yard using all three of these filters. 

Camera – ZWO 1178mm

Mount – Skywatcher EQM-35 Pro

Guide Software – AsiAir Plus

Guide Scope – SVBony SV165 

Orion - H-alpha Stacked Frame 50 Min Integration
Orion - S2 Stacked frame 100Min Integration
O3 50 Min Integration Time
4.6/5

Customer Reviews

Agena AstroProducts has a rating of 4.3 stars with 6 reviews. 

High Point Scientific has a 5 star rating with 2 reviews.

Most of the feedback posted from customers has been positive. The 4 star ratings mention difficulty focusing and not fitting the filter wheel.  For me they fit perfectly so I cannot replicate any issue that the customer mentioned. As far as focuing goes, I can attest that it is more difficult to focus with the filters.  The issue is that there is a significant reduction in the amount of light passed to the camera.  To overcome this, I use a 15 to 30 second exposure time in the focus and preview mode.  With the addition of a Bahtinov mask, I can use the preview function with long exposure to fine tune the focus.

Focusing

As I just mentioned, focusing is a challenge.  Here is how I get to focus fast with my setup.  I use a filter wheel with my kit.  I bought a cheap CLS filter and added it to the wheel.  To get close but not yet perfect, I just switch to the CLS filter and increase my exposure time to 2 to 5 seconds.  This get’s me very close to what the narrow band filter need to be at.

After this step, I switch to the filter I intend to use and use the preview mode with 30 seconds.  With the mask, I can achieve perfect focus this way.

Here I use the AsiAir. Even in focus mode, I can clearly see that this is in focus. I use the CLS filter first to get it close before moving onto narrow band.

Horsehead and Flame Nebula

There was a recent break in the weeks of rainy weather.  I jumped on this opportunity to capture one of my most favorite nebula – IC434 also known as the Horsehead Nebula.

Since I was using a wide field lens of 200mm, I added the Flame Nebula to the composition.  The Flame Nebula is an emission nebula which is unlike the Horsehead Nebula.  The Horsehead is a dark nebula that does not emit much of it’s own light.  Instead, this nebula is illuminated from behind where there is a type HII emission nebula.  This arrangement creates a mystical looking scene with horse head appearing from the shadows. 

This region of the sky is located near the infamous Orion Nebula.

Emission Gases

This region of space has significant amounts of hydrogen and S-II gases with some Oxygen 3.  I captured all three of these with narrowband filters that I purchased here.

Using these filters allowed me to get strong data even in my Bortle 8 light polluted back yard.  

I just love Hydrogen Alpha monochrome images.  They have so much detail and show a great deal of the emission.  

Color Palette

There are many color palettes to choose from when creating images from monochrome data.  I get the best results when I use Hydrogen-alpha as Red.  For the Blue and Green channels, it depends on what the next strongest signal I have obtained.  Since this image has so much Sulfur, I used the S-II for both the Blue and the Green channels.  I have found that also adding a luminance layer greatly increases the color and detail.  For this image I used the H-alpha as the luminance layer.  All of this data was processed using Siril, the free open source astrophotography imaging processing program.

Taken with Nikon 200mm lens, 300S exposures, 8.25 total integration time

How To Mount An Astrophotography Camera To A Nikon F Mount Lens

Refracting telescopes are expensive, especially those suited for photography.  A great way to get started is to use an existing camera lens.  There are pros and cons to doing this, but you can experiment and get started with narrow band imaging by using the right adapter.

This can save you some money upfront by allowing you to use a quality lens that you probably already own without buying a new telescope.  You can always upgrade later after you get all the bugs worked out of your workflow.

 

Back Focus - Getting It Right

A Nikon F Mount lens has 46.5 mm of back focus.  This means that you need to get everything installed from the lens so that it is 46.5 mm to the focal plane of your astrophotography camera.

The Nikon Adapter that I bought has a total of 34 mm of length.  This was achieved by using the 5mm spacer that came with the adapter.   My ZWO camera has 12.5 mm of back focus.  Adding these two together achieved the correct 46.5mm of length.  I had no problem at all getting my nebula in focus.

Now if you have a different camera, you can easily make it work by adjusting the spacer as shown here:

This configuration works with cooled cameras with a 17.5mm back focus length

Watch The Tutorial

I’ll show you how I hooked up my ZWO 1178MM camera to my 80-400 mm Nikon Zoom.4

Using 1.25″ Filters

Out of the box, the adapter works with 2″ filters.  You can easily use 1.25″ filter with an adapter.

Workflow Using Narrow Band Filters

For my first test using this adapter, I wanted to capture the Orion Nebula in narrow band.  I first slewed to the subject and centered it using my AsiAir Plus.

Next I removed the adapter from the lens without disturbing the position of the lens.  This has to be done carefully or it will throw the lens off target.  I unscrewed the outer adapter and then installed my H-alpha filter.  This was re-assembled and the the focus was adjusted with the narrow band filter installed.  At this point imaging can begin.

I set the  exposure for 60 Seconds.  I had previously achieved less than 2 arc seconds total error with my polar alightment.  Here is the very first shot I got in narrow band.

I was pleased with the first test especially since I did not use autoguiding due to the fact that I only have one dedicated astronomy camera.

As soon as the weather clears, i’ll shoot this again in the full Hubble Pallette and update this post.