The 2″ Extension Tube 80mm length is the longest of the available extension tube in 2″. Most adapters or eyepieces fit into it snugly. In some cases the barrel of the eyepiece or adaptor may be a little bit too long so that it extends a little bit beyond the extension tube. This does not cause any problems or any loss in mechanical stability.

The “optical length” includes only the length of the extension tube that acually extends out of the focuser. The length of the 2″ barrel of the extension tube is not included in this figure since the barrel normally disappears entirely into the focuser.

Eyepiece Clamp:

The eyepiece is clamped firmly but without scratches through a brass compression ring. The advantage is that eyepieces are held more securely and the risk of the eyepiece being tilted out of the optical axis is minimized.

Quality of Make:

These 2″ Extension Tube – 80mm are machined from solid aluminium. They are black anodized. The 2″ barrel has a safety undercut to prevent the extension tube from dropping out of the focuser accidentally. Of course the barrel also has a 2″ filter thread (M48x0.75) on the telescope side.

Extension Tubes from TS

Different telescopes can have very different back focus distances. In most telescopes the focal plane is deliberately moved further out, allowing users to attach cameras and other accessories more easily.

Refractors always have a rather long back focus since they are usually used with a star diagonal or erecting prism.

The Problem:

As a result of this you often cannot directly focus the telescope if you do not use it with the above accessory. You would have to move the focuser further out, but you reach the end of the focus travel before reaching a focused image.

The Solution:

TS extension tubes bridge the gap between the focuser and the focal plane. They are the ideal solution whenever focus lies further outside than you can normally reach with the focuser.

The post 2″ Extension Tube – 80mm appeared first on Modern Astronomy.

The 2″ Extension Tube 35mm length is the shortest of the available extension tubes in 2″. Most adapters or eyepieces just fit into it snugly. In some cases the barrel of the eyepiece or adaptor may even be a little bit too long so that it extends a little bit beyond the extension tube. This does not cause any problems or any loss in mechanical stability.

The “optical length” includes only the length of the extension tube that acually extends out of the focuser. The length of the 2″ barrel of the extension tube is not included in this figure since the barrel normally disappears entirely into the focuser.

Eyepiece Clamp:

The eyepiece is clamped firmly but without scratches through a brass compression ring. The advantage is that eyepieces are held more securely and the risk of the eyepiece being tilted out of the optical axis is minimized.

Quality of Make:

These extension tubes are machined from solid aluminium. They are black anodized. The 2″ barrel has a safety undercut to prevent the extension tube from dropping out of the focuser accidentally. Of course the barrel also has a 2″ filter thread (M48x0.75) on the telescope side.

Extension Tubes from GSO

Different telescopes can have very different back focus distances. In most telescopes the focal plane is deliberately moved further out, allowing users to attach cameras and other accessories more easily.

Refractors always have a rather long back focus since they are usually used with a star diagonal or erecting prism.

The Problem:

As a result of this you often cannot directly focus the telescope if you do not use it with the above accessory. You would have to move the focuser further out, but you reach the end of the focus travel before reaching a focused image.

The Solution:

Extension tubes bridge the gap between the focuser and the focal plane. They are the ideal solution whenever focus lies further outside than you can normally reach with the focuser.

The post 2″ Extension Tube – 35mm appeared first on Modern Astronomy.

A 1mm version is also available

The post 1.5mm Stainless Steel Fine Tuning Ring for T Thread appeared first on Modern Astronomy.

A 1.5mm version is also available.

The post 1mm Stainless Steel Fine Tuning Ring for T Thread appeared first on Modern Astronomy.

Available sizes are 5mm, 7mm, 17mm, 7.5mm, 20mm, 20.5mm – 30mm variable, 30mm

How To Set the Correct Back Focus Spacing?

To change back focus spacing, you need to add (or remove) spacers, sometimes also called extenders, within your telescope’s imaging train. To do that, you’ll need to physically thread the spacers into the imaging train at some point in between the camera and your telescope. It usually doesn’t matter where, but if you’re using filters or a filter wheel of any kind, you’ll want to keep the filters as close to the camera sensor as possible. If you’re using filters, see the section below on how filters add back focus distance.

How to Use Spacers for Back Focus

Spacers come in many different lengths to accommodate all different back focus spacing needs. They also come in a variety of thread sizes, such as M42 and M48, so be sure to choose the right size threads for your system. To make this easier, many astronomy camera manufacturers (such as ZWO) include the correct spacers to reach 55mm back focus. Spacers can be as short as less than 1mm and as long as necessary.

To ensure all spacers and components in your imaging train can thread together, you will need to know the size of the threads on the end of your telescope or corrector/reducer. This is most commonly M48 or M42 size threads, but thread size can depend on the corrector, reducer, or telescope. Make sure your t-ring and/or spacers you have can thread together between the telescope, camera, and any accessories in between. ZWO has a helpful back focus guide on how to thread their included adapters together to reach 55mm of back focus.

I Use Filters in My Imaging Train. Does this Change My Back Focus?

Yes, using filters of any kind in your imaging train will alter your back focus slightly. Putting a filter into the optical path always increases the back focus distance. To calculate how much back focus spacing you need to add, take the thickness of the filter and divide it by 3. So, if you have a filter that is 3mm thick, you need to add 1mm of spacing to your imaging train to retain the correct back focus. Therefore, a 55mm back focus with a filter that is 3mm thick added to the imaging train would become 56mm. In practice, however, 1mm off from back focus spacing is usually not very noticeable unless you’re using a large sensor or a very fast optical system.

How do filter wheels/drawers, off-axis guiders, and other accessory in the imaging train affect back focus?

Filter wheels, filter drawers, and off-axis guiders all have thicknesses that will add to the overall length of your imaging train, and these need to be factored in when calculating back focus. Each individual part has their own thickness, which you can usually find in the product manual online. Some manufacturers produce these components with thicknesses that match the spacers included with cooled astronomy cameras, so that you can easily swap out a spacer for an off-axis guider, for example. For that reason, we usually recommend sticking with one brand’s product ecosystem as their components are sometimes interchangeable.

I have a Petzval/Quadruplet Refractor. Do I Need to Adjust Back Focus Spacing?

The benefit of the Petzval/Quadruplet refractor design, such as the William Optics RedCat 51, is that the focusing mechanism moves the entire image train. This means that for these telescopes, all you need to worry about is reaching focus, which is rarely an issue, and your back focus spacing will be correct.

Is Back Focus Spacing Important for Planetary Imaging?

While it depends on the sensor size being used, back focus is not nearly as important to get correct for planetary imaging. The reason for this is because incorrect back focus spacing usually effects off-axis portions of the image like the corners. Since most planetary cameras use very small sensors unlike cooled astronomy cameras for deep sky, back focus is not nearly as critical and the effects of incorrect back focus spacing are usually unnoticeable.

Back Focus Spacing Summary

Back focus spacing is important to get right for nearly any deep sky imaging setup, especially those using large sensor cameras or fast focal ratio telescopes. All optical correctors and reducers require a specific back focus length, so be sure to check the manual or diagrams to find out what that measurement should be. Most often, it is 55mm, which is easily achievable with a T-ring for DSLR/Mirrorless cameras or adapters included with cooled astronomy cameras. However, if it’s other than 55mm, you may need to purchase separate spacers for your setup.

The post 17mm T Spacer appeared first on Modern Astronomy.

The spacer has a male T-thread on one side and a female T-thread on the other. These T2 spacers help achieve a predetermined spacing, e.g. to bring a camera into correct position. By combining different spacers the overall distance can be set quite precisely.

Specifications:
On telescope side: T2 Internal thread (M42 x 0.75)
On camera side: T2 Internal thread (M42 x 0.75)
Length 7.5mm
Black anodised and knurled

Other sizes are available 5mm, 7mm, 20mm

The post Baader 7.5mm T Spacer appeared first on Modern Astronomy.

The spacer has a male T-thread on one side and a female T-thread on the other. These T2 spacers help achieve a predetermined spacing, e.g. to bring a camera into correct position. By combining different spacers the overall distance can be set quite precisely.

Specifications:
On telescope side: T2 Internal thread (M42 x 0.75)
On camera side: T2 Internal thread (M42 x 0.75)
Length 5mm
Clear aperture 39mm
Black anodised

Available sizes are 5mm, 7mm, 17mm, 7.5mm, 20mm, 20.5mm – 30mm variable, 30mm

The post 5mm T Spacer appeared first on Modern Astronomy.

Available sizes are 5mm, 7mm, 17mm, 7.5mm, 20mm, 20.5mm – 30mm variable, 30mm

The post 20mm T Spacer appeared first on Modern Astronomy.

Available sizes are 5mm, 7mm, 17mm, 7.5mm, 20mm, 20.5mm – 30mm variable, 30mm

The post 7mm T Spacer appeared first on Modern Astronomy.

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