Astrophotography Corner- December 2020

Original article appears in GuideStar December, 2020.

By: Don Selle

 

Its December and as the weather cools and the toddies get hotter, all good astronomers thoughts turn to – NEW EQUIPMENT!! Which brings us to our subject for this month’s AP Corner.

A picture containing objectDescription automatically generatedOver the last month, there have been a number of our members seeking input on intended equipment purchases to upgrade their systems, and maybe expanding their options to get into astrophotography. For instance, Jim King asked the HAS list server whether or not he would see enough visual improvement to upgrade to a Celestron Edge HD SCT (on an Alt Az mount), which would also possibly open the door to astrophotography later on.

This type of question has multiple answers, depending on the objective of the person asking the question. As Jeff Lepp responded – “Traditional advice is buy a scope for visual or buy a scope for astrophotography but don’t buy a scope to do both.” He then went on to explain that the state of current technology and cost of astrophotography equipment makes this answer somewhat obsolete. “It all depends on what you want to do” he said.

There are several ways this could go, depending on how important astrophotography is to Jim. So, for the purposes of answering this question, I will start by answering the question as Jim asked, take into account that in astrophotography “there is always an upgrade” then finish with advice to someone who really intends to get into astrophotography. In each case the upgrades are presented in the order from lower to higher cost.

Jim’s primary question is about the quality of the Edge HD optics for visual observing. An SCT on an Alt-AZ is a very good visual telescope. The Alt-Az mount makes for comfortable observing and is easy to automate for GoTo operation. The optics on a standard SCT are designed to be easy to mass produce, with both the primary and secondary mirrors ground to a spherical shape (very forgiving) and the corrector plate designed to eliminate spherical aberration from the mirrors. The result is good images in the center of the FOV, but because the focal plane is curved, modern wide field of view eyepieces will be less focused and other aberrations like coma can be seen at the edges of the image.

Edge HD optics (like Meade’s ACF) change the figure of the secondary mirror to a more hyperbolic shape (and change the corrector plate to match). This flattens out the image plane and eliminates much of the coma and other aberrations seen at the edges of a standard SCT FOV. So, for visual observing, if you plan to or currently use modern wide field eyepieces, upgrading your SCT optics might be worthwhile, but you would do well to check them out on your friends scope with your own eyepieces before you pull the trigger.

But what about astrophotography? An Alt-Az mounted SCT has several less than optimal features that make it difficult to do high quality DSO astrophotography. But that does not mean that good and satisfying work is not possible.

So what are the downsides of an Alt-AZ SCT?

  • The Alt-Az mount tracks the target well enough for visual observation, however for long exposures, the tracking may not be smooth enough and (unless the scope is polar aligned) the image will rotate.
  • The long focal length (typically >= 2000mm) makes tracking critical, as the angular size of each camera pixel records is dictated in part by the focal length. Large focal length means small pixel scale. Small errors in tracking lead to oblong stars perceptible image blurring.
  • The high focal ratio (typically f/10) means sub-exposures take longer to record the same faint detail as compared to the same camera on faster f/6 to f/2 optics.

Are the Edge HD optics worthwhile to have for astrophotography? In a word YES! The flatter image circle and reduced coma and other aberrations that come with this package make the OTA a better optical system for astrophotography than a standard SCT. These OTAs also are fitted to accept a Hyperstar lens system in place of the secondary mirror ( which can be safely removed without taking off the corrector plate) turning the OTA to a lower focal length fast f/2 optic, designed for astrophotography.

So what astrophotography can you do with an Alt-Az fork mounted Edge HD SCT?CPC2.png

  • Eyepiece projection photography – just about any Alt-Az SCT can take decent pictures of lunar, planetary and bright DSO through the eyepiece. Adapters for smartphones are very popular for outreach. But Wait! There’s always an Upgrade!
  • Add the correct adapters, a 2x to 5x barlow a high sensitivity CMOS camera that takes high frame rate video and the right software, and you can take high resolution lunar and planetary images.
  • Add a focal reducer (0.6x – f/6 of about 1200mm focal length) and software that can live stack short (30-60 secs) sub exposures and you have an EAA system (Electronic Assisted Astronomy). Include a light pollution filter, and a second monitor for public viewing, and you have a great public outreach system that will capture astro-images in the city of the brighter DSO’s which you will be happy to show your friends and family. But Wait! There’s always an Upgrade!
  • You could also add an equatorial wedge so that your Alt-Az mount can be polar aligned. You could also add an autoguiding system. You would also need to add a 3D weight system to properly balance your scope. Doing this would allow you to lengthen your sub-exposures an image most any target, regardless of its apparent magnitude to several minutes each. This type of exposure definitely allows you to image many dimmer DSO, but the limited tracking accuracy of the fork mount means you will throw away a significant fraction of your sub-frames. In all honesty though, I would advise against this upgrade and suggest you get an actual German Equatorial Mount (GEM), with at least a 40lb payload rating. De-fork your SCT if you like, or sell it to fund the purchase of a good 80mm refractor.

 

I personally took this path, adding a wedge to my 8 inch Meade LX200 and struggled for a couple of years until I finally got it to work, though I was frustrated that at least 1 out of 3 of my subframes had to be trashed. In a short time thereafter, I got my first GEM and a good refractor. The change in imaging setup significantly increased my imaging efficiency and cut down the labor involved to get the camera started capturing sub-frames. But Wait! There’s always an Upgrade!

  • If you want to stick with your fork mounted SCT, there is an alternative which can help improve your images. Add a Hyperstar lens system and convert your SCT from f/10 to a super-fast f/2 (or about 400-500mm focal length) optical system. This is a bit pricey, but the change in f/ratio from about f/6 to f/2 means you will capture more than eight times the photons in your 30 second sub-exposure as you would with the same camera on an f/6 system. But there is a downside. The very fast focal ratio of the Hyperstar system significantly reduces the Critical Focus Zone (the difference between just inside and just outside of focus) so that most imagers add a focus motor, controller and autofocus software to make focusing easier, less time consuming more precise and easier to repeat. The additional expense of adding the Hyperstar and autofocus system though will set you back about what an adequate GEM mount would cost you, so you make the call.  

If getting into EAA interests you, please be sure to attend our virtual meeting on January 8th, (or watch the video afterwards) as we will have an excellent presentation all about EAA to get you started.

We have taken the EAA track to its logical end, and its now appropriate to discuss what you might consider if astrophotography is really more of your objective. This means that you have seen the many high quality astro-images published in magazines or online, and when you do, you say to yourself “I want to do as good or better than that”.

In this case, I would strongly urge you to bypass the Alt-Az mount and get the best GEM with the highest payload you can afford, and that is light weight enough (not including the counter weights or OTA) for you to handle and lift onto your tripod or pier. A Gem like this will have GoTo capability and will cost more upfront than an Alt-Az fork mount, but the difference in ease of use and performance are generally worth the expense. Add in a wide field refractor, like an 80mm f/6 to f/7 frefractor, and you will have a decent imaging rig which at about a 500mm focal length will be reasonably forgiving, and net you decent images with less time and trouble than a fork mounted SCT will.

Using a GEM will require you to polar align the mount before you use it even for visual use (though for visual use its not as critical to be accurate, just close enough). This is not a major drawback as polar alignment is fairly easy to master, and there are now several technology driven methods that make it even easier to get aligned than the old days of drift alignment. (Polar alignment will be the subject of next month’s AP Corner).

What the GEM does for your astrophotography is:

  • You can mount almost any different type of OTA including the Edge HD SCT. Like different camera lenses for different photographs, these OTAs will have different focal lengths, focal ratios, and field of views with your cameras
  • Balance can be critical to good accurate tracking and autoguiding. Balancing your OTA and camera on the mount is much easier on a GEM.
  • The GEM will typically track more accurately than an SCT fork mount. This means that autoguiding is much easier with a GEM, with fewer sub-frames trashed due to tracking errors.
  • You will be able to take much longer sub-exposures, and longer total exposures, meaning you can accumulate the hours of image data necessary to create a high-quality low noise astro-image.

Astrophotography can be a highly challenging pastime that has high rewards when you get everything working right and produce a great image. It can also become an obsession, and There’s always an Upgrade!  So, having said that remember I have warned you! Please don’t blame me if this article helps you on your way to becoming hooked and obsessed!

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