I just had a quick question on my telescope I recently purchased. I have a Canon Rebel XTi and I paired that with the William Optics Guide Star 61 Guidescope. The Specs on the Scope are:

  • Guide Scope Aperture 61mm
  • Guide Scope Focal Ratio f/5.9
  • Guide Scope Focal Length 360mm

I believe a lot of people use this for deep sky photography. I was having issues trying to focus on planets, do you guys think this would be able to focus tightly on any planets. Here is the link to what I have.

Thank you so much for you're time, I'm new to this site too!

  • 3
    $\begingroup$ 360 mm focal length is very short for planetary photography. Don't planetary photographers use scopes with focal lengths in the range of 1 to 4 m? Also, 61 mm aperture is fairly small for Deep sky objects. Maybe "alot" of people use this scope to guide their main scope, and the imaging camera is looking through the main scope. $\endgroup$
    – JohnHoltz
    Commented Nov 28, 2021 at 5:02

2 Answers 2


do you guys think this would be able to focus tightly on any planets

No. The focal length of your telescope is too short to effectively see planets.

I have a William Optics GT71 and, with its focal reducer, it is approximately the same focal length as yours. I also use an APS-C format camera and when I attempt to photograph planets, they are tiny white dots. Here is an example from the very useful application Stellarium.

enter image description here

On the other hand, there are many larger deep-sky objects that can be photographed easily with your equipment. An excellent beginner target is the Andromeda galaxy.


Welcome to Astrophotography!

Since you have already purchased your 'scope, I think you should definitely go ahead and try to photograph some planets.

Your results will not be as good as you might hope since as @chili555 points out planets are small, but if you keep an eye on their distances and shoot when they are near their closest distances to Earth, you may have some reasonably interesting results.

You can certainly look for additional things like Jupiter's Galilean moons and their nightly changes, and Saturn's rings, and play with RAW format images and color processing to try to bring out a few bands of clouds, try enhancing color differences for example.

You can also start looking for planetary occultations of stars (ask about that in a new question) or a conjunction between a planet and a star.

And you can amaze yourself by how Venus changes from a small full circle to a large thin crescent while at the same time [managing to maintain an almost constant visual magnitude!

You can also try to photograph a planet or star during the day just for fun

or "video tape" a lunar occultation of a planet

So I'd say go for it! and learn a lot about astrophotography and about astronomy, how to predict things, find them and photograph them, and dream about what your next system will be like if you want to go the route of a much longer focal length and lucky imaging.

About your camera Wikipedia says:

The EOS 400D, called Digital Rebel XTi in North America and EOS Kiss Digital X in Japan, is an entry-level digital single-lens reflex camera introduced by Canon on 24 August 2006.


  • 22.2 mm × 14.8 mm
  • 3,888 × 2,592 pixels

which means the pixel spacing is about 5.7 microns or 0.0057 mm.

Your linked page William Optics Guide Star 61 Guidescope - Gold - M-GS61-GD says that your scope has a focal length of 360 mm.

That means your pixel spacing is 0.0057 / 360 radians or 3.3 arc seconds.

The angular sizes of some planets are

Object       angular size         image size
             (arcseconds)          (pixels)
-------      ------------         ----------
Jupiter       30    to  50        9    to  15
Venus         10    to  66        3    to  18
Saturn        15    to  20        4.5  to   6
Mars           3.5  to  25        1    to 7.6

With an aperture diameter of 61 mm the diffraction limit at a mid-range wavelength $\lambda$ of 550 nm will be 1.1 $\times 10^{-5}$ radians or 2.3 arc seconds so that will contribute a little but not too much additional blurring considering your pixel size is 3.3. arcseconds, but it means that there's only so much benefit to using a longer focal length with this aperture.

Perhaps doubling it will be worth it, but any more than that will be empty magnification unless you get a scope with a larger aperture.

Of course above about 15 cm astronomical seeing will limit your resolution if just taking snapshots, but with work and some software you can try your hand at lucky imaging.

There are other questions and answers both here in Astronomy SE tagged with photography and in Photography SE tagged with astrophotography that address these topics further.

from How (the heck) was this photo of Venus at inferior conjunction (between us and the Sun) taken? which got it from this answer to Planets visible at night which got it from Wikimedia's File:Phases Venus

These images are beautiful. According to the answer there they were taken by an experience amateur using

  • 4.5-inch (Newtonian?) Mizar telescope
  • Nikon Coolpix 750

I don't know the exact specs but with an aperture 114 mm and assuming an f/8 Newtonian, the focal length would be 912 mm and with the Coolpix' sensor' pixel spacing of about 5.97 microns that's 1.35 arcseconds per pixel.

Phases of Venus

  • $\begingroup$ I have a Rebel XTi and a Celestron 8. I stumbled on this thread because I can't get a well focused picture (even of the Sun or the Moon), and I thought it was because the adapter (which I bought for my Canon 35mm SLR) was the wrong length for a DSLR. But from what you're saying it may be a more fundamental problem? $\endgroup$
    – Duston
    Commented Nov 30, 2021 at 20:34
  • $\begingroup$ @Duston the Sun and Moon are huge in apparent size compared to planets (~1800 arcseconds) and a Celestron 8 has a focal length of >2000 mm so while the math I wrote here applies to any case the conclusions certainly don't apply to yours. I recommend you post a new question here and include one or two of your better images and ask if it is as good as it can be. Make sure it has the whole Sun or Moon so the scale can be obtained, and the image should be straight from the camera and not changed in size. $\endgroup$
    – uhoh
    Commented Nov 30, 2021 at 20:58

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